This time I want to report on a new bachelor thesis, which I supervise. In this project we try to understand a little better the foundations of so-called gauge symmetries. In particular we address some of the ground work we have to lay for understanding our theories.
Let me briefly outline the problem: Most of the theories in particle physics include some kind of redundancy I.e., there are more things in it then we actually see in experiments. The surplus stuff is actually not real. It is just a kind of mathematical device to make calculations simpler. It is like a ladder, which we bring to climb a wall. We come, use the ladder, and are on top. The ladder we take again with us, and the wall remains as it was. The ladder made live simpler. Of course, we could have climbed the wall without it. But it would have been more painful.
Unfortunately, theories are more complicated than wall climbing.
One of the problems is that we usually cannot solve problems exactly. And as noted before, this can mess up the removal of the surplus stuff.
The project the bachelor student and I am working on has the following basic idea: If we can account for all of the surplus stuff, we should be able to know whether our approximations did something wrong. It is like preparing an engine. If something is left afterwards it is usually not a good sign. Unfortunately, things are again more complicated. For the engine, we just have to look through our workspace to see whether anything is left. But how to do so for our theories? And this is precisely the project.
So, the project is essentially about listing stuff. We start out with something we know is real and important. For this, we take the most simplest thing imaginable: Nothing. Nothing means in this case just an empty universe, no particles, no reactions, no nothing. That is certainly a real thing, and one we want to include in our calculations.
Of this nothing, there are also versions where some of the surplus stuff appears. Like some ghost image of particles. We actually know how to add small amounts of ghost stuff. Like a single particle in a whole universe. But these situations are not so very interesting, as we know how to deal with them. No, the really interesting stuff happens if well fill the whole universe with ghost images. With surplus stuff which we add just to make life simpler. At least originally. And the question is now: How can we add this stuff systematically? As the ghost stuff is not real, we know it must fulfill special mathematical equations.
Now we do something, which is very often done in theoretical physics: We use an analogy. The equations in question are not unique to the problem at hand, but appear also in quite different circumstances, although with a completely different meaning. In fact, the same equations describe how in quantum physics one particle is bound to each other. In quantum physics, depending on the system at hand, there may be one or more different ways how this binding occurs. You can count the number, and there is a set which one can label by whole numbers. Incidentally, this feature is where the name quantum originates from.
Returning to our original problem, we do the following analogy: Enumerating the ghost stuff can be cast into the same form as enumerating the possibilities of binding two particles together in quantum mechanics. The actual problem is only to find the correct quantum system which is the precise analogous one to our original problem. Finding this is still a complicated mathematical problem. Finding only one solution for one example is the aim of this bachelor thesis. But already finding one would be a huge step forward, as so far we do not have one at all. Having it will probably be like having a first stepping stone for crossing a river. From understanding it, we should be able to understand how to generate more. Hopefully, we will eventually understand how to create arbitrary such examples. And thus solve our enumeration problem. But this is still in the future. For the moment, we do the first step.
Wednesday, September 28, 2016
Tuesday, June 21, 2016
How to search for dark, unknown things: A bachelor thesis
Today, I would like to write about a recently finished bachelor thesis on the topic of dark matter and the Higgs. Though I will also present the results, the main aim of this entry is to describe an example of such a bachelor thesis in my group. I will try to follow up also in the future with such entries, to give those interested in working in particle physics an idea of what one can do already at a very early stage in one's studies.
The framework of the thesis is the idea that dark matter could interact with the Higgs particle. This is a serious possibility, as both objects are somehow related to mass. There is also not yet any substantial reason why this should not be the case. The unfortunate problem is only: how strong is this effect? Can we measure it, e.g. in the experiments at CERN?
We are looking in a master thesis in the dynamical features of this idea. This is ongoing, and something I will certainly write about later. Knowing the dynamics, however, is only the first step towards connecting the theory to experiment. To do so, we need the basic properties of the theory. This input will then be put through a simulation of what happens in the experiment. Only this result is the one really interesting for experimental physicists. They then look what any kind of imperfections of the experiments change and then they can conclude, whether they will be able to detect something. Or not.
In the thesis, we did not yet had the results from the master student's work, so we parametrized the possible outcomes. This meant mainly to have the mass and the strength of the interaction between the Higgs and the dark matter particle to play around. This gave us what we call an effective theory. Such a theory does not describe every detail, but it is sufficiently close to study a particular aspect of a theory. In this case how dark matter should interact with the Higgs at the CERN experiments.
With this effective theory, it was then possible to use simulations of what happens in the experiment. Since dark matter cannot, as the name says, be directly seen, we needed somehow a marker to say that it has been there. For that purpose we choose the so-called associate production mode.
We knew that the dark matter would escape the experiment undetected. In jargon, this is called missing energy, since we miss the energy of the dark matter particles, when we account for all we see. Since we knew what went in, and know that what goes in must come out, anything not accounted for must have been carried away by something we could not directly see. To make sure that this came from an interaction with the Higgs we needed a tracer that a Higgs had been involved. The simplest solution was to require that there is still a Higgs. Also, there are deeper reasons which require that dark matter in this theory should not only arrive with a Higgs particle, but should be obtained also from a Higgs particle before the emission of the dark matter particles. The simplest way to check for this is that there is besides the Higgs in the end also a so-called Z-boson, for technical reasons. Thus, we had what we called a signature: Look for a Higgs, a Z-boson, and missing energy.
There is, however, one unfortunate thing in known particle physics which makes this more complicated: neutrinos. These particles are also essentially undetectable for an experiment at the LHC. Thus, when produced, they will also escape undetected as missing energy. Since we do not detect either dark matter or neutrinos, we cannot decide, what actually escaped. Unfortunately, the tagging with the Higgs and the Z do not help, as neutrinos can also be produced together with them. This is what we call a background to our signal. Thus, it was necessary to account for this background.
Fortunately, there are experiments which can detect, with a lot of patience, neutrinos. They are very different from the one we at the LHC. But they gave us a lot of information on neutrinos. Hence, we knew how often neutrinos would be produced in the experiment. So, we would only need to remove this known background from what the simulation gives. Whatever is left would then be the signal of dark matter. If the remainder would be large enough, we would be able to see the dark matter in the experiment. Of course, there are many subtleties involved in this process, which I will skip.
So the student simulated both cases, and determined the signal strength. From that she could deduce that the signal grows quickly with the strength of the interaction. She also found that the signal became stronger if the dark matter particles become lighter. That is so because there is only a finite amount of energy available to produce them. But the more energy is left to make the dark matter particles move the easier it gets to produce them, an effect known in physics as phase space. In addition, she found that if the dark matter particles have half the mass of the Higgs their production became also very efficient. The reason is a resonance. Just like two noises amplify each other if they are at the same frequency, so such amplifications can happen in particle physics.
The final outcome of the bachelor thesis was thus telling us for the values of the two parameters of the effective theory how strong our signal would be. Once we know these values from our microscopic theory in the master project, we know whether we have a chance to see these particles in this type of experiments.
The framework of the thesis is the idea that dark matter could interact with the Higgs particle. This is a serious possibility, as both objects are somehow related to mass. There is also not yet any substantial reason why this should not be the case. The unfortunate problem is only: how strong is this effect? Can we measure it, e.g. in the experiments at CERN?
We are looking in a master thesis in the dynamical features of this idea. This is ongoing, and something I will certainly write about later. Knowing the dynamics, however, is only the first step towards connecting the theory to experiment. To do so, we need the basic properties of the theory. This input will then be put through a simulation of what happens in the experiment. Only this result is the one really interesting for experimental physicists. They then look what any kind of imperfections of the experiments change and then they can conclude, whether they will be able to detect something. Or not.
In the thesis, we did not yet had the results from the master student's work, so we parametrized the possible outcomes. This meant mainly to have the mass and the strength of the interaction between the Higgs and the dark matter particle to play around. This gave us what we call an effective theory. Such a theory does not describe every detail, but it is sufficiently close to study a particular aspect of a theory. In this case how dark matter should interact with the Higgs at the CERN experiments.
With this effective theory, it was then possible to use simulations of what happens in the experiment. Since dark matter cannot, as the name says, be directly seen, we needed somehow a marker to say that it has been there. For that purpose we choose the so-called associate production mode.
We knew that the dark matter would escape the experiment undetected. In jargon, this is called missing energy, since we miss the energy of the dark matter particles, when we account for all we see. Since we knew what went in, and know that what goes in must come out, anything not accounted for must have been carried away by something we could not directly see. To make sure that this came from an interaction with the Higgs we needed a tracer that a Higgs had been involved. The simplest solution was to require that there is still a Higgs. Also, there are deeper reasons which require that dark matter in this theory should not only arrive with a Higgs particle, but should be obtained also from a Higgs particle before the emission of the dark matter particles. The simplest way to check for this is that there is besides the Higgs in the end also a so-called Z-boson, for technical reasons. Thus, we had what we called a signature: Look for a Higgs, a Z-boson, and missing energy.
There is, however, one unfortunate thing in known particle physics which makes this more complicated: neutrinos. These particles are also essentially undetectable for an experiment at the LHC. Thus, when produced, they will also escape undetected as missing energy. Since we do not detect either dark matter or neutrinos, we cannot decide, what actually escaped. Unfortunately, the tagging with the Higgs and the Z do not help, as neutrinos can also be produced together with them. This is what we call a background to our signal. Thus, it was necessary to account for this background.
Fortunately, there are experiments which can detect, with a lot of patience, neutrinos. They are very different from the one we at the LHC. But they gave us a lot of information on neutrinos. Hence, we knew how often neutrinos would be produced in the experiment. So, we would only need to remove this known background from what the simulation gives. Whatever is left would then be the signal of dark matter. If the remainder would be large enough, we would be able to see the dark matter in the experiment. Of course, there are many subtleties involved in this process, which I will skip.
So the student simulated both cases, and determined the signal strength. From that she could deduce that the signal grows quickly with the strength of the interaction. She also found that the signal became stronger if the dark matter particles become lighter. That is so because there is only a finite amount of energy available to produce them. But the more energy is left to make the dark matter particles move the easier it gets to produce them, an effect known in physics as phase space. In addition, she found that if the dark matter particles have half the mass of the Higgs their production became also very efficient. The reason is a resonance. Just like two noises amplify each other if they are at the same frequency, so such amplifications can happen in particle physics.
The final outcome of the bachelor thesis was thus telling us for the values of the two parameters of the effective theory how strong our signal would be. Once we know these values from our microscopic theory in the master project, we know whether we have a chance to see these particles in this type of experiments.
Tuesday, May 3, 2016
Digging into a particle
This time I would like to write about a new paper which I have just put out. In this paper, I investigate a particular class of particles.
This class of particles is actually quite similar to the Higgs boson. I. e. the particles are bosons and they have the same spin as the Higgs boson. This spin is zero. This class of particles is called scalars. These particular sclars also have the same type of charges, they interact with the weak interaction.
But there are fundamental differences as well. One is that I have switched off the back reaction between these particles and the weak interactions: The scalars are affected by the weak interaction, but they do not influence the W and Z bosons. I have also switched off the interactions between the scalars. Therefore, no Brout-Englert-Higgs effect occurs. On the other hand, I have looked at them for several different masses. This set of conditions is known as quenched, because all the interactions are shut-off (quenched), and the only feature which remains to be manipulated is the mass.
Why did I do this? There are two reasons.
One is a quite technical reason. Even in this quenched situation, the scalars are affected by quantum corrections, the radiative corrections. Due to them, the mass changes, and the way the particles move changes. These effects are quantitative. And this is precisely the reason to study them in this setting. Being quenched it is much easier to actually determine the quantitative behavior of these effects. Much easier than when looking at the full theory with back reactions, which is a quite important part of our research. I have learned a lot about these quantitative effects, and am now much more confident in how they behave. This will be very valuable in studies beyond this quenched case. As was expected, there was not many surprises found. Hence, it was essentially a necessary but unspectacular numerical exercise.
Much more interesting was the second aspect. When quenching, this theory becomes very different from the normal standard model. Without the Brout-Englert-Higgs effect, the theory actually looks very much like the strong interaction. Especially, in this case the scalars would be confined in bound states, just like quarks are in hadrons. How this occurs is not really understood. I wanted to study this using these scalars.
Justifiable, you may ask why I would do this. Why would I not just have a look at the quarks themselves. There is a conceptual and a technical reason. The conceptual reason is that quarks are fermions. Fermions have non-zero spin, in contrast to scalars. This entails that they are mathematically more complicated. These complications mix in with the original question about confinement. This is disentangled for scalars. Hence, by choosing scalars, these complications are avoided. This is also one of the reasons to look at the quenched case. The back-reaction, irrespective of with quarks or scalars, obscures the interesting features. Thus, quenching and scalars isolates the interesting feature.
The other is that the investigations were performed using simulations. Fermions are much, much more expensive than scalars in such simulations in terms of computer time. Hence, with scalars it is possible to do much more at the same expense in computing time. Thus, simplicity and cost made scalars for this purpose attractive.
Did it work? Well, no. At least not in any simple form. The original anticipation was that confinement should be imprinted into how the scalars move. This was not seen. Though the scalars are very peculiar in their properties, they in no obvious way show confinement. It may still be that there is an indirect way. But so far nobody has any idea how. Though disappointing, this is not bad. It only tells us that our simple ideas were wrong. It also requires us to think harder on the problem.
An interesting observation could be made nonetheless. As said above, the scalars were investigated for different masses. These masses are, in a sense, not the observed masses. What they really are is the mass of the particle before quantum effects are taken into account. These quantum effects change the mass. These changes were also measured. Surprisingly, the measured mass was larger than the input mass. The interactions created mass, even if the input mass was zero. The strong interaction is known to do so. However, it was believed that this feature is strongly tied to fermions. For scalars it was not expected to happen, at least not in the observed way. Actually, the mass is even of a similar size as for the quarks. This is surprising. This implies that the kind of interaction is generically introducing a mass scale.
This triggered for me the question whether the mass scale also survives when having the backcoupling in once more. If it remains even when there is a Brout-Englert-Higgs effect then this could have interesting implications for the mass of the Higgs. But this remains to be seen. It may as well be that this will not endure when not being quenched.
This class of particles is actually quite similar to the Higgs boson. I. e. the particles are bosons and they have the same spin as the Higgs boson. This spin is zero. This class of particles is called scalars. These particular sclars also have the same type of charges, they interact with the weak interaction.
But there are fundamental differences as well. One is that I have switched off the back reaction between these particles and the weak interactions: The scalars are affected by the weak interaction, but they do not influence the W and Z bosons. I have also switched off the interactions between the scalars. Therefore, no Brout-Englert-Higgs effect occurs. On the other hand, I have looked at them for several different masses. This set of conditions is known as quenched, because all the interactions are shut-off (quenched), and the only feature which remains to be manipulated is the mass.
Why did I do this? There are two reasons.
One is a quite technical reason. Even in this quenched situation, the scalars are affected by quantum corrections, the radiative corrections. Due to them, the mass changes, and the way the particles move changes. These effects are quantitative. And this is precisely the reason to study them in this setting. Being quenched it is much easier to actually determine the quantitative behavior of these effects. Much easier than when looking at the full theory with back reactions, which is a quite important part of our research. I have learned a lot about these quantitative effects, and am now much more confident in how they behave. This will be very valuable in studies beyond this quenched case. As was expected, there was not many surprises found. Hence, it was essentially a necessary but unspectacular numerical exercise.
Much more interesting was the second aspect. When quenching, this theory becomes very different from the normal standard model. Without the Brout-Englert-Higgs effect, the theory actually looks very much like the strong interaction. Especially, in this case the scalars would be confined in bound states, just like quarks are in hadrons. How this occurs is not really understood. I wanted to study this using these scalars.
Justifiable, you may ask why I would do this. Why would I not just have a look at the quarks themselves. There is a conceptual and a technical reason. The conceptual reason is that quarks are fermions. Fermions have non-zero spin, in contrast to scalars. This entails that they are mathematically more complicated. These complications mix in with the original question about confinement. This is disentangled for scalars. Hence, by choosing scalars, these complications are avoided. This is also one of the reasons to look at the quenched case. The back-reaction, irrespective of with quarks or scalars, obscures the interesting features. Thus, quenching and scalars isolates the interesting feature.
The other is that the investigations were performed using simulations. Fermions are much, much more expensive than scalars in such simulations in terms of computer time. Hence, with scalars it is possible to do much more at the same expense in computing time. Thus, simplicity and cost made scalars for this purpose attractive.
Did it work? Well, no. At least not in any simple form. The original anticipation was that confinement should be imprinted into how the scalars move. This was not seen. Though the scalars are very peculiar in their properties, they in no obvious way show confinement. It may still be that there is an indirect way. But so far nobody has any idea how. Though disappointing, this is not bad. It only tells us that our simple ideas were wrong. It also requires us to think harder on the problem.
An interesting observation could be made nonetheless. As said above, the scalars were investigated for different masses. These masses are, in a sense, not the observed masses. What they really are is the mass of the particle before quantum effects are taken into account. These quantum effects change the mass. These changes were also measured. Surprisingly, the measured mass was larger than the input mass. The interactions created mass, even if the input mass was zero. The strong interaction is known to do so. However, it was believed that this feature is strongly tied to fermions. For scalars it was not expected to happen, at least not in the observed way. Actually, the mass is even of a similar size as for the quarks. This is surprising. This implies that the kind of interaction is generically introducing a mass scale.
This triggered for me the question whether the mass scale also survives when having the backcoupling in once more. If it remains even when there is a Brout-Englert-Higgs effect then this could have interesting implications for the mass of the Higgs. But this remains to be seen. It may as well be that this will not endure when not being quenched.
Wednesday, April 27, 2016
Some small changes in the schedule
As you may have noticed, I have not written a new entry since some time.
The reasons have been twofold.
One is that being a professor is a little more strenuous than being a postdoc. Though not unexpected, at some point it takes a toll.
The other is that in the past I tried just to keep a regular schedule. However, that often required of me to think hard about a topic as there was no natural candidate. At other times, I had a number of possible topics, which where then stretched out rather than to be written when they were important.
As a consequence, I think it is more appropriate to write entries when something happens that is interesting to write about. This will be at least any time we put out a new paper, so that I will still update you on our research. I will also write something whenever somebody new starts in the group, or otherwise we start a new project. Also, some of my students want to also contribute, and I will be very happy to give them the opportunity to do so. Once in a while, I will also write some background entries, such that I can offer some context for the research we are doing.
So stay tuned. It may be in a different rhythm, but I will keep on writing about our (and my) research.
The reasons have been twofold.
One is that being a professor is a little more strenuous than being a postdoc. Though not unexpected, at some point it takes a toll.
The other is that in the past I tried just to keep a regular schedule. However, that often required of me to think hard about a topic as there was no natural candidate. At other times, I had a number of possible topics, which where then stretched out rather than to be written when they were important.
As a consequence, I think it is more appropriate to write entries when something happens that is interesting to write about. This will be at least any time we put out a new paper, so that I will still update you on our research. I will also write something whenever somebody new starts in the group, or otherwise we start a new project. Also, some of my students want to also contribute, and I will be very happy to give them the opportunity to do so. Once in a while, I will also write some background entries, such that I can offer some context for the research we are doing.
So stay tuned. It may be in a different rhythm, but I will keep on writing about our (and my) research.
Friday, February 5, 2016
More than one Higgs means more structure
We have published once more a new paper, and I would like again to outline what we did (and why).
The motivation for this investigation started out with another paper of mine. As described earlier, back then I have taken a rather formal stand on proposals for new physics. It was based on the idea that there is some kind of self-similar substructure of what we usually call the Higgs and the W and Z bosons. In this paper, I speculated that this self-similarity may be rather exclusive to the standard model. As a consequence, this may alter the predictions for new physics models.
Of course, speculating is easy. To make something out of it requires to do real calculations. Thus, I have started two projects to test them. One is on the unification of forces, and still ongoing. Some first results are there, but not yet anything conclusive. It is the second project which yielded new results.
In this second project we had a look at a theory where more Higgs particles are added to the standard model, a so-called 2-Higgs-doublet model, or 2HDM for short. I had speculated that, besides the additional Higgs particles, further additional particles may arise as bound states. I. e., as states which are made from two or more other particles. These are not accounted for by ordinary methods.
In the end, it now appears that this idea is not correct, at least not in its simplest form. There are still some very special cases left, where this may still be true, but by and large not. However, we have understood why the original idea is wrong, and why it may still be correct in other cases. The answer is symmetry.
When adding additional Higgs particles, one is not entirely free. It is necessary that we do not alter the standard model where we have already tested it. Especially, we cannot easily modify the symmetries of the standard model. However, the symmetries of the standard model then induce a remarkable effect. The additional Higgs particles in 2HDMs are not entirely different from the ones we know. Rather, they mix with it as a quantum effect. In quantum theories, particles can change into each other under certain conditions. And the symmetries of the standard model entail that this is possible for the new and the old Higgses.
If the particles mix, the possibilities to distinguish them diminish. As a consequence, the simplest additional states can no longer be distinguished from the states already accounted for by ordinary methods. Thus, they are not additional states. Hence, the simplest possible deviation I speculated about is not realized. There may still be more complicated ones, but to figure this out is much more complicated, and has yet to be done. Thus, this work showed that the simple idea was not right.
So what about the other project still in progress? Should I now also expect this to just reproduce what is known? Actually no. The thing we learned in this project was why everything fell into its ordinary places. The reason is the mixing between the normal and the additional Higgs particles. This possibility is precluded in the other project, as there the additional particles are very different from the original ones. It may still be that my original idea is wrong. But it has to be wrong in a different way than in the case we investigated now. And thus we have also learned something more about a wider class of theories.
This shows that even disproving your ideas is important. From the reasons why they fail you learn more than just from a confirmation of them - you learn something new.
The motivation for this investigation started out with another paper of mine. As described earlier, back then I have taken a rather formal stand on proposals for new physics. It was based on the idea that there is some kind of self-similar substructure of what we usually call the Higgs and the W and Z bosons. In this paper, I speculated that this self-similarity may be rather exclusive to the standard model. As a consequence, this may alter the predictions for new physics models.
Of course, speculating is easy. To make something out of it requires to do real calculations. Thus, I have started two projects to test them. One is on the unification of forces, and still ongoing. Some first results are there, but not yet anything conclusive. It is the second project which yielded new results.
In this second project we had a look at a theory where more Higgs particles are added to the standard model, a so-called 2-Higgs-doublet model, or 2HDM for short. I had speculated that, besides the additional Higgs particles, further additional particles may arise as bound states. I. e., as states which are made from two or more other particles. These are not accounted for by ordinary methods.
In the end, it now appears that this idea is not correct, at least not in its simplest form. There are still some very special cases left, where this may still be true, but by and large not. However, we have understood why the original idea is wrong, and why it may still be correct in other cases. The answer is symmetry.
When adding additional Higgs particles, one is not entirely free. It is necessary that we do not alter the standard model where we have already tested it. Especially, we cannot easily modify the symmetries of the standard model. However, the symmetries of the standard model then induce a remarkable effect. The additional Higgs particles in 2HDMs are not entirely different from the ones we know. Rather, they mix with it as a quantum effect. In quantum theories, particles can change into each other under certain conditions. And the symmetries of the standard model entail that this is possible for the new and the old Higgses.
If the particles mix, the possibilities to distinguish them diminish. As a consequence, the simplest additional states can no longer be distinguished from the states already accounted for by ordinary methods. Thus, they are not additional states. Hence, the simplest possible deviation I speculated about is not realized. There may still be more complicated ones, but to figure this out is much more complicated, and has yet to be done. Thus, this work showed that the simple idea was not right.
So what about the other project still in progress? Should I now also expect this to just reproduce what is known? Actually no. The thing we learned in this project was why everything fell into its ordinary places. The reason is the mixing between the normal and the additional Higgs particles. This possibility is precluded in the other project, as there the additional particles are very different from the original ones. It may still be that my original idea is wrong. But it has to be wrong in a different way than in the case we investigated now. And thus we have also learned something more about a wider class of theories.
This shows that even disproving your ideas is important. From the reasons why they fail you learn more than just from a confirmation of them - you learn something new.
Wednesday, January 20, 2016
More similar than expected
Some while ago I have written about a project a master student and myself have embarked upon: Using a so-called supersymmetric theory - or SUSY theory for short - to better understand ordinary theories.
Well, this work has come to fruition, both in the form of the completion of the master project as well as new insights written up in a paper. This time I would like to present these results a little bit.
To start, let me briefly rehearse what we did, and why. One of the aims of our research is to better understand how the theories work we are using to describe nature. A particular feature of these theories is redundancy. This redundancy makes many calculations possible, but at the same time introduces new problems, mainly about how to get unique results.
Now, in theories like the standard model, we have all problems at the same time: All the physics, all the formalism, and especially all the redundancy. But this is a tedious mess. It is therefore best to reduce the complexity and solve one problem at a time. This is done by taking a simpler theory, which has only one of the problems. This is what we did.
We took a (maximal) SUSY theory. In such a theory, the supersymmetry is very constraining, and a lot of features are exactly known. But the implications of redundancy are not. So we hoped that by applying the same procedures we use to deal with the redundancy in ordinary theories to this theory, we could check whether our approach is valid.
Of course, the first, and expected, finding was that even a very constraining theory is not simple. When it comes to technical details, anything interesting becomes a hard problem. So it required a lot of grinding work before we got results. I will not bore you with the details. If you want them, you can find them in the paper. No, here I want to discuss the final result.
The first finding was a rather comforting one. Doing the same things to this theory that we do to ordinary theories did not do too much damage. Using these approximations, the final results were still in agreement with what we do know exactly about this theory. This was a relief, because this lends a tiny amount of support more to what we are usually doing.
The real surprise was, however, a very different one. We knew that this theory shows a very different kind of behavior than all the theories we are usually dealing with. So we did expect that, even if our methods work, the results will still be drastically different from the other cases we are dealing with. But this was not so.
To understand better what we have found, it is necessary to know that this theory is similar in structure to a conventional theory. This conventional one is a theory of gluons, but without quarks to make the strong interactions complete. In the SUSY theory, we also have gluons. In addition, we have further new particles, which are needed to get.
The first surprise was that the gluons behaved unexpectedly similar to their counterparts in the original theory. Of course, there are differences, but these differences were expected. They came from the differences of both theories. But where they could be similar, they were. And not roughly so, but surprisingly precisely so. We have an idea why this could be the case, because there is one structural property, which is very restricting, and which appears in both theories. But we know that this is not enough, as we now other theories where this still is different, despite also having this one constraining structure. Since the way how the gluons are very similar is strongly influenced by the redundancy features of both theories, we can hope that this means we are treating the redundancy in a reasonable way.
The second surprise was that the new particles mirror the behavior of the gluons. Even though these particles are connected by supersymmetry to the gluons, the connection would have allowed many possible shapes of relations. But no, the relation is an almost exact mirror. And this time, there is no constraining structure which gives us a clue why, out of all possible relations, this one is picked. However, this is again related to redundancy, and perhaps, just speculating here, this could indicate more about how this redundancy works.
In total, we have learned quite a lot. We have more support for what we doing in ordinary theories. We have seen that some structures might be more universal than expected. And we may even have a clue in which direction we could learn more about how to deal with the redundancy in more immediately relevant theories.
Well, this work has come to fruition, both in the form of the completion of the master project as well as new insights written up in a paper. This time I would like to present these results a little bit.
To start, let me briefly rehearse what we did, and why. One of the aims of our research is to better understand how the theories work we are using to describe nature. A particular feature of these theories is redundancy. This redundancy makes many calculations possible, but at the same time introduces new problems, mainly about how to get unique results.
Now, in theories like the standard model, we have all problems at the same time: All the physics, all the formalism, and especially all the redundancy. But this is a tedious mess. It is therefore best to reduce the complexity and solve one problem at a time. This is done by taking a simpler theory, which has only one of the problems. This is what we did.
We took a (maximal) SUSY theory. In such a theory, the supersymmetry is very constraining, and a lot of features are exactly known. But the implications of redundancy are not. So we hoped that by applying the same procedures we use to deal with the redundancy in ordinary theories to this theory, we could check whether our approach is valid.
Of course, the first, and expected, finding was that even a very constraining theory is not simple. When it comes to technical details, anything interesting becomes a hard problem. So it required a lot of grinding work before we got results. I will not bore you with the details. If you want them, you can find them in the paper. No, here I want to discuss the final result.
The first finding was a rather comforting one. Doing the same things to this theory that we do to ordinary theories did not do too much damage. Using these approximations, the final results were still in agreement with what we do know exactly about this theory. This was a relief, because this lends a tiny amount of support more to what we are usually doing.
The real surprise was, however, a very different one. We knew that this theory shows a very different kind of behavior than all the theories we are usually dealing with. So we did expect that, even if our methods work, the results will still be drastically different from the other cases we are dealing with. But this was not so.
To understand better what we have found, it is necessary to know that this theory is similar in structure to a conventional theory. This conventional one is a theory of gluons, but without quarks to make the strong interactions complete. In the SUSY theory, we also have gluons. In addition, we have further new particles, which are needed to get.
The first surprise was that the gluons behaved unexpectedly similar to their counterparts in the original theory. Of course, there are differences, but these differences were expected. They came from the differences of both theories. But where they could be similar, they were. And not roughly so, but surprisingly precisely so. We have an idea why this could be the case, because there is one structural property, which is very restricting, and which appears in both theories. But we know that this is not enough, as we now other theories where this still is different, despite also having this one constraining structure. Since the way how the gluons are very similar is strongly influenced by the redundancy features of both theories, we can hope that this means we are treating the redundancy in a reasonable way.
The second surprise was that the new particles mirror the behavior of the gluons. Even though these particles are connected by supersymmetry to the gluons, the connection would have allowed many possible shapes of relations. But no, the relation is an almost exact mirror. And this time, there is no constraining structure which gives us a clue why, out of all possible relations, this one is picked. However, this is again related to redundancy, and perhaps, just speculating here, this could indicate more about how this redundancy works.
In total, we have learned quite a lot. We have more support for what we doing in ordinary theories. We have seen that some structures might be more universal than expected. And we may even have a clue in which direction we could learn more about how to deal with the redundancy in more immediately relevant theories.
Friday, December 18, 2015
Touching dark matter with the Higgs
Some time ago, I have written about my growing interest in dark matter. Now, both a master student and a bachelor student have actually started working on this topic. Thus, I want to describe this time what they are exactly working on.
As told, we know actually very little about what dark matter is. Especially, we know very little about how it interacts with the rest of the universe, except for gravity. If you do not want to assume that this is the only way dark matter shows its there, you have therefore to guess. Luckily, the number of guesses is somewhat limited by experiment and observation.
One interesting possibility is that dark matter is actually only interacting additionally with the Higgs. Theories of this type are called Higgs-Portal models, because the Higgs is the portal through which we see dark matter. Such models have some nice features. Probably the nicest is that there is a good chance that the LHC will be able to access through this portal dark matter. This idea has received much more attention since we know that there is a Higgs. Thus, a lot of investigations have been performed already. So what do we want to add?
Here enters a second observation about dark matter. Today, we have become pretty good at observing dark matter indirectly through its gravitational action on galaxies. From this, people have indirectly deduced that dark matter can interact with itself. Especially, it seems quite possible that it interacts very strongly with itself. Thus, while dark matter is very reclusive, it still forms in its reclusion a very active world.
Now, comes the new part. Essentially all of the previous investigations of Higgs-portal models assumed that dark matter is not strongly self-interacting. Therefore they used perturbation theory, which is then the adequate language. To capture the effects of strong interactions requires a different method. We will employ numerical simulations to deal with them. However, we will reduce, for the sake of computing time, the problem somewhat. We keep only the Higgs, the W and Z, and the dark matter particle. This is still a formidable problem.
The topic of the master thesis is now to perform these simulations. The goal of them are the following: How much do the strong interactions of the dark matter particle imprint on the Higgs and the W and the Z? Are their properties changed? If yes, how strong can the dark matter self-interaction be before they are changed too strongly, i.e. before they do no longer agree with our experimental knowledge? What are the properties of the dark matter particles? How strongly can the Higgs and the dark matter particle communicate through the portal before the Higgs becomes changed? In this context, how is the structure of the Higgs affected? These are the most important questions, which need answers.
However, with this we will very much understand the theoretical aspects. But this is not enough. If there is some interesting effect in principle, this by no means guarantees that we can see it in an experiment. On the one hand, there is also still the rest of the standard model. Do they interfere? And then, if they do not, are the effects of dark matter reduced so strongly in a real experiment that we ca no longer see it? Especially, can we still see anything of the strong self-interaction?
Herein lies the goal of the bachelor student. We can, unfortunately, not simulate the whole standard model and the experiment. But we can encode it into an effective theory, which we then can treat sufficiently well. This is again a combination of methods which I do so often. Using this effective model, and a toolbox created by other people, a so-called Monte-Carlo generator, she can make predictions for an actual experiment. This can be either the LHC, or one of the planned next experiments. That should give us at least a rough idea, whether we can see something of the dark matter. Or, if we are lucky, a very good idea.
This also demonstrates how different projects, and the work of several people, feed into each other. I am quite curious what will come out, and what we will learn about strongly-interacting dark matter.
As told, we know actually very little about what dark matter is. Especially, we know very little about how it interacts with the rest of the universe, except for gravity. If you do not want to assume that this is the only way dark matter shows its there, you have therefore to guess. Luckily, the number of guesses is somewhat limited by experiment and observation.
One interesting possibility is that dark matter is actually only interacting additionally with the Higgs. Theories of this type are called Higgs-Portal models, because the Higgs is the portal through which we see dark matter. Such models have some nice features. Probably the nicest is that there is a good chance that the LHC will be able to access through this portal dark matter. This idea has received much more attention since we know that there is a Higgs. Thus, a lot of investigations have been performed already. So what do we want to add?
Here enters a second observation about dark matter. Today, we have become pretty good at observing dark matter indirectly through its gravitational action on galaxies. From this, people have indirectly deduced that dark matter can interact with itself. Especially, it seems quite possible that it interacts very strongly with itself. Thus, while dark matter is very reclusive, it still forms in its reclusion a very active world.
Now, comes the new part. Essentially all of the previous investigations of Higgs-portal models assumed that dark matter is not strongly self-interacting. Therefore they used perturbation theory, which is then the adequate language. To capture the effects of strong interactions requires a different method. We will employ numerical simulations to deal with them. However, we will reduce, for the sake of computing time, the problem somewhat. We keep only the Higgs, the W and Z, and the dark matter particle. This is still a formidable problem.
The topic of the master thesis is now to perform these simulations. The goal of them are the following: How much do the strong interactions of the dark matter particle imprint on the Higgs and the W and the Z? Are their properties changed? If yes, how strong can the dark matter self-interaction be before they are changed too strongly, i.e. before they do no longer agree with our experimental knowledge? What are the properties of the dark matter particles? How strongly can the Higgs and the dark matter particle communicate through the portal before the Higgs becomes changed? In this context, how is the structure of the Higgs affected? These are the most important questions, which need answers.
However, with this we will very much understand the theoretical aspects. But this is not enough. If there is some interesting effect in principle, this by no means guarantees that we can see it in an experiment. On the one hand, there is also still the rest of the standard model. Do they interfere? And then, if they do not, are the effects of dark matter reduced so strongly in a real experiment that we ca no longer see it? Especially, can we still see anything of the strong self-interaction?
Herein lies the goal of the bachelor student. We can, unfortunately, not simulate the whole standard model and the experiment. But we can encode it into an effective theory, which we then can treat sufficiently well. This is again a combination of methods which I do so often. Using this effective model, and a toolbox created by other people, a so-called Monte-Carlo generator, she can make predictions for an actual experiment. This can be either the LHC, or one of the planned next experiments. That should give us at least a rough idea, whether we can see something of the dark matter. Or, if we are lucky, a very good idea.
This also demonstrates how different projects, and the work of several people, feed into each other. I am quite curious what will come out, and what we will learn about strongly-interacting dark matter.
Tuesday, December 1, 2015
An international perspective
This time, I would like to write a little bit about a very important part of our work: Being international.
Right now, in our complete particle physics group, we have with about 25 people about 12 nationalities. So, being international is a very basic part of our daily life. This yields a long list of effects. It starts from using a common language so that everyone one can speak to everyone (which is today English, but has been a different one in the past, and may again be a different one in the future. English as the language of science is only there since less than a century). Thus, we need to educate also all our students in this language somehow. And this not only pertains to normal speaking, but also the specialized vocabulary of our topic.
Being international also requires us to pay attention to many more administrative aspects, which appear due to the existence of different nations. The question of who can represent our work in which country, because it is possible to get a visa, is not an entirely simple problem. Being from the European Union myself puts me in a privileged position, as I can get into most countries with little or no effort. But this is not true for many other people, giving us often headaches and requires long-range planning, if we want someone particular to go somewhere. Furthermore, when students come from abroad, they may have learned different things, and therefore have a different background, which needs to be leveled, so that everyone can talk to everyone. And, finally, this may also manifest on how to incorporate different cultures and habits. This does not only touch upon the personal, but can very much also affect the way how we work together. In some areas of the world, it is still usual that less experienced people accept that what more experienced people do without questioning, probably since childhood, as an example. This does not help in doing science: Everyone has to speak open, and also criticize to find out errors. None of us is error free, and therefore everyone must contribute in nailing errors.
This list can be continued almost indefinitely.
Why do we put up with this? It appears a lot of extra work, just to do science.
But here comes into play how science today operates: On a global scale. And this is very good for two reasons.
One is that the problems we have to deal with becomes more and more specialized, and thus a smaller and smaller percentage of scientists can work on them. To still have a sizable workforce requires therefore to include as many people as possible. Otherwise, too specialized subgroups may loose contact, and become adrift, with no possibility to regain the overarching picture. This could also be put the other way around: Today's problems are far too complex that any single country, even the largest ones, could have enough scientific workforce to deal with them. Everyone is needed. And this not even touches upon having enough resources to do certain kinds of research.
The other is that we need diversity. The different educational, cultural, and habitual backgrounds also play an important role in science. Everyone has learned in school and during studies something in a particular style. Everyone has adopted certain view points, and certain strategies. But science lives in the unknown. There is no gold-plated way how to deal with the unknown. Therefore, there is no special preparation which is the best way to be prepared for doing science. We need many different minds, vastly different minds, such that we can get many perspectives. We need people with different backgrounds, with a different lookout on everything, to find new angles how to deal with problems. We need all ways of seeing things, even those which at first may look not intuitive to ourselves. But we have to learn and listen to all the view points. Thus, everyone who is willing to support the scientific process, the ever turning wheel of creating a theory and putting it through myriads of experimental tests, can provide a new point of view. Thus, diversity is essential for us. New problems need different points of view.
This is one of the points which also explains the many travels scientists do, often for years. Every new surrounding, every new group of peoples, provides a new perspective. Changing one's perspective by traveling, or by bringing many different peoples to our homes, helps us in broadening our view, in giving us the opportunity to learn adopt to take new perspectives. This is demanding for the individual, as it implies being around the world rather than at home, but our understanding profits from being used to seeing things from many perspectives.
The ability to see from different perspectives is not only supported by talking to other scientists. But experiencing different cultures, different surroundings in general, and trying to understand them, gives us the ability. So, diversity is essential to our ability to understand.
This is why being international is so extremely important for modern research, and why diversity counts so much for basic research.
And this also implies that already living in a diverse culture in a single spot will already help us in becoming better in understanding. If we are used to experience the new, and trying to understand it, in everyday live, it prepares us also to face the new at the boundary of our knowledge.
Right now, in our complete particle physics group, we have with about 25 people about 12 nationalities. So, being international is a very basic part of our daily life. This yields a long list of effects. It starts from using a common language so that everyone one can speak to everyone (which is today English, but has been a different one in the past, and may again be a different one in the future. English as the language of science is only there since less than a century). Thus, we need to educate also all our students in this language somehow. And this not only pertains to normal speaking, but also the specialized vocabulary of our topic.
Being international also requires us to pay attention to many more administrative aspects, which appear due to the existence of different nations. The question of who can represent our work in which country, because it is possible to get a visa, is not an entirely simple problem. Being from the European Union myself puts me in a privileged position, as I can get into most countries with little or no effort. But this is not true for many other people, giving us often headaches and requires long-range planning, if we want someone particular to go somewhere. Furthermore, when students come from abroad, they may have learned different things, and therefore have a different background, which needs to be leveled, so that everyone can talk to everyone. And, finally, this may also manifest on how to incorporate different cultures and habits. This does not only touch upon the personal, but can very much also affect the way how we work together. In some areas of the world, it is still usual that less experienced people accept that what more experienced people do without questioning, probably since childhood, as an example. This does not help in doing science: Everyone has to speak open, and also criticize to find out errors. None of us is error free, and therefore everyone must contribute in nailing errors.
This list can be continued almost indefinitely.
Why do we put up with this? It appears a lot of extra work, just to do science.
But here comes into play how science today operates: On a global scale. And this is very good for two reasons.
One is that the problems we have to deal with becomes more and more specialized, and thus a smaller and smaller percentage of scientists can work on them. To still have a sizable workforce requires therefore to include as many people as possible. Otherwise, too specialized subgroups may loose contact, and become adrift, with no possibility to regain the overarching picture. This could also be put the other way around: Today's problems are far too complex that any single country, even the largest ones, could have enough scientific workforce to deal with them. Everyone is needed. And this not even touches upon having enough resources to do certain kinds of research.
The other is that we need diversity. The different educational, cultural, and habitual backgrounds also play an important role in science. Everyone has learned in school and during studies something in a particular style. Everyone has adopted certain view points, and certain strategies. But science lives in the unknown. There is no gold-plated way how to deal with the unknown. Therefore, there is no special preparation which is the best way to be prepared for doing science. We need many different minds, vastly different minds, such that we can get many perspectives. We need people with different backgrounds, with a different lookout on everything, to find new angles how to deal with problems. We need all ways of seeing things, even those which at first may look not intuitive to ourselves. But we have to learn and listen to all the view points. Thus, everyone who is willing to support the scientific process, the ever turning wheel of creating a theory and putting it through myriads of experimental tests, can provide a new point of view. Thus, diversity is essential for us. New problems need different points of view.
This is one of the points which also explains the many travels scientists do, often for years. Every new surrounding, every new group of peoples, provides a new perspective. Changing one's perspective by traveling, or by bringing many different peoples to our homes, helps us in broadening our view, in giving us the opportunity to learn adopt to take new perspectives. This is demanding for the individual, as it implies being around the world rather than at home, but our understanding profits from being used to seeing things from many perspectives.
The ability to see from different perspectives is not only supported by talking to other scientists. But experiencing different cultures, different surroundings in general, and trying to understand them, gives us the ability. So, diversity is essential to our ability to understand.
This is why being international is so extremely important for modern research, and why diversity counts so much for basic research.
And this also implies that already living in a diverse culture in a single spot will already help us in becoming better in understanding. If we are used to experience the new, and trying to understand it, in everyday live, it prepares us also to face the new at the boundary of our knowledge.
Thursday, October 29, 2015
Being formal
One of the topics I am working on is about basic properties of so-called gauge symmetries. I just have published a new paper on it, and here I want to describe what it is about.
A gauge symmetry is, very roughly stated, a useful tool for which we pay the price of a very redundant description. Pictorially speaking, we can say the same thing with very many different words. This may sound awful. However, in practice, it seems to work just like a charm. So what is there still to investigate?
Well, knowing how to use something in one way, and understanding it fully are two very different things. And actually, we are not, on a very strict and formal level, absolutely sure that we know how to use gauge symmetry. Though this is likely the case. But the situation is nonetheless for two reasons not really satisfactory.
The first is more a question of approach. When we use something, we would really like to know what we are actually doing. The second is that if we would understand it better, there may very well be ways to use it much better than we currently do. So there are reasons for understanding it better.
But what is it what I actually want to do?
It all starts when going back to the meaning of symmetries. Symmetries introduce redundant directions, meaning that when you have a symmetry you have more directions to point then there are actually. That is in general very helpful on a technical level.
But here enters the problem. If we have directions, we should be able to say 'go in this direction'. To do so, we introduce coordinate systems. Now comes the catch: For a local symmetry this is easier said than done, especially when it comes to the gauge symmetries of the standard model.
The problem is somewhat abstract. When you think about directions, then usually you think about left, right, up or down, and so on. This is true if you think about our usual space around us. But not everything has the same geometry. Especially, symmetries can also have a direction of bending. This is still not a major issue. But, there are some symmetries where the bending of directions becomes so strong, that some directions bend back on themselves or meet others, when going too far. And this is a problem. If they bend back, or even worse, bend on a different direction, what is direction anyway? I can start walking in one direction, and then I am in another direction. Sounds like a catastrophe, right?
Well, the reason it sounds like that is that we insisted to define directions once and for all. This is what we are used to. A direction is a direction is a direction. Unfortunately, not everything is so straight and something, especially gauge symmetries, have additional directions which are warped, and can intersect each other. The problem then arises how to orient oneself, if directions change. The answer to this is that it is necessary to give up directions which are always the same. Rather, you need to define directions only in some area around you, and when you move, you may need to change them.
The aforementioned paper now investigates this bending of directions. In a sense, it tries to map how far it is possible to go in a fixed direction, before this direction changes. Finally, it attempts to draw a map of where these directional changes occur. That sounds now pretty graphical, but the reality is once more mathematically involved. But in the end, this map hopefully will help to setup useful collections of coordinate systems, and a dictionary telling you where to use which coordinate system to get your directions.
The details are pretty involved. But the rough outline of what I did was to put myself at many points, create there coordinate systems, follow the directions they give and check when they started to make no more sense - when they hit other directions or themselves. Then I got a list of collisions, and where they occurred. And from this I could get a map of collisions. What I did not yet do is to make something useful out of the map. That comes next.
A gauge symmetry is, very roughly stated, a useful tool for which we pay the price of a very redundant description. Pictorially speaking, we can say the same thing with very many different words. This may sound awful. However, in practice, it seems to work just like a charm. So what is there still to investigate?
Well, knowing how to use something in one way, and understanding it fully are two very different things. And actually, we are not, on a very strict and formal level, absolutely sure that we know how to use gauge symmetry. Though this is likely the case. But the situation is nonetheless for two reasons not really satisfactory.
The first is more a question of approach. When we use something, we would really like to know what we are actually doing. The second is that if we would understand it better, there may very well be ways to use it much better than we currently do. So there are reasons for understanding it better.
But what is it what I actually want to do?
It all starts when going back to the meaning of symmetries. Symmetries introduce redundant directions, meaning that when you have a symmetry you have more directions to point then there are actually. That is in general very helpful on a technical level.
But here enters the problem. If we have directions, we should be able to say 'go in this direction'. To do so, we introduce coordinate systems. Now comes the catch: For a local symmetry this is easier said than done, especially when it comes to the gauge symmetries of the standard model.
The problem is somewhat abstract. When you think about directions, then usually you think about left, right, up or down, and so on. This is true if you think about our usual space around us. But not everything has the same geometry. Especially, symmetries can also have a direction of bending. This is still not a major issue. But, there are some symmetries where the bending of directions becomes so strong, that some directions bend back on themselves or meet others, when going too far. And this is a problem. If they bend back, or even worse, bend on a different direction, what is direction anyway? I can start walking in one direction, and then I am in another direction. Sounds like a catastrophe, right?
Well, the reason it sounds like that is that we insisted to define directions once and for all. This is what we are used to. A direction is a direction is a direction. Unfortunately, not everything is so straight and something, especially gauge symmetries, have additional directions which are warped, and can intersect each other. The problem then arises how to orient oneself, if directions change. The answer to this is that it is necessary to give up directions which are always the same. Rather, you need to define directions only in some area around you, and when you move, you may need to change them.
The aforementioned paper now investigates this bending of directions. In a sense, it tries to map how far it is possible to go in a fixed direction, before this direction changes. Finally, it attempts to draw a map of where these directional changes occur. That sounds now pretty graphical, but the reality is once more mathematically involved. But in the end, this map hopefully will help to setup useful collections of coordinate systems, and a dictionary telling you where to use which coordinate system to get your directions.
The details are pretty involved. But the rough outline of what I did was to put myself at many points, create there coordinate systems, follow the directions they give and check when they started to make no more sense - when they hit other directions or themselves. Then I got a list of collisions, and where they occurred. And from this I could get a map of collisions. What I did not yet do is to make something useful out of the map. That comes next.
Friday, September 18, 2015
Something dark on the move
If you browse either through popular science physics or through the most recent publications on the particle physics' preprint server arxiv.org then there is one topic which you cannot miss: Dark matter.
What is dark matter? Well, we do not know. So why do we care? Because we know something is out there, something dark, and its moving. Or, more precisely, it moves stuff around. When we look to the skies and measure how stars and galaxies move, then we find something interesting. We think we know how these objects interact, and how they therefore influence each other's movement. But what we observe does not agree with our expectations. We think we have excluded any possibility that we are overlooking something known, like that there are many more black holes, intergalactic gas and dust, or any of the other particles we know filling up the cosmos. No, it seems there is more out there than we can detect right now directly and have a theory for.
Of course, it can be that we miss something about how stars and galaxies influence each other, and this possibility is also pursued. But actually the simplest explanation is that out there is a new type of matter. A type of matter which does not interact either by electromagnetism or the strong force, because otherwise we would have seen it in experiment. Since there is no interaction with electromagnetism, it does not reflect or emit light, and therefore we cannot see it using optics. Hence the name dark matter. Because it is dark.
It certainly acts gravitationally, since this is how stars and galaxies are influenced. It may still be that it either interacts by the weak interaction or with the Higgs. That is something which is currently investigated in many experiments around the world. Of course, it could also interact with the standard model particles by some unknown force we have yet to discover. This would make it even more mysterious.
Because it is so popular there are many resources on the web which discuss what we already know (or do not know) about dark matter. Rather than repeating that, I will here write why I start to be interested in it. Or at least in some possible types of it. Because dark matter which only interacts by gravitation is not particularly interesting right know, as we will likely not learn much about in the foreseeable future. So I am more interested in such types of dark matter which couple by some other means to the standard model. Until they are excluded by experiments.
If it should interact with the standard model by some new force then this new force will look likely at first just like a modification of the weak interactions and/or of the Higgs. This would be an effective description of it. Given time, we would also figure out the details, but we have not yet.
Thus, as a first shot, I will concentrate on the cases where it it could interact with the weak force or just with the Higgs. Interacting with the weak force is actually quite complicated if it should fulfill all experimental constraints we have. Modifications there, though possible, are thus unlikely. Leaves the Higgs.
Therefore, I would like to see how dark matter could interact with the Higgs. Such models are called Higgs portal models, because the Higgs act as the portal through which we see dark matter. So far, this is also pretty standard.
Now comes the new thing. I have written several times that I work on questions what the Higgs really is. That it could have an interesting self-similar structure. And here is the big deal for me: The presence of dark matter interacting with the Higgs could influence actually this structure. This is similar to what happens with other bound states: The constituents can change their identity, as we investigate in another project.
My aim is now to bring all these three things together: Dark matter, Higgs, and the structure of the Higgs. I want to know whether such a type of dark matter influences the structure of the Higgs, and if yes how. And whether this could have a measurable influence. The other way around is that I would like to know whether the Higgs influences the dark matter in some way. Combining these three things is a rather new idea, and it will be very fascinating to explore it. The best course of action will be to do this by simulating the Higgs together with dark matter. This will be neither simple nor cheap, so this may take a lot of time. I will keep you posted.
What is dark matter? Well, we do not know. So why do we care? Because we know something is out there, something dark, and its moving. Or, more precisely, it moves stuff around. When we look to the skies and measure how stars and galaxies move, then we find something interesting. We think we know how these objects interact, and how they therefore influence each other's movement. But what we observe does not agree with our expectations. We think we have excluded any possibility that we are overlooking something known, like that there are many more black holes, intergalactic gas and dust, or any of the other particles we know filling up the cosmos. No, it seems there is more out there than we can detect right now directly and have a theory for.
Of course, it can be that we miss something about how stars and galaxies influence each other, and this possibility is also pursued. But actually the simplest explanation is that out there is a new type of matter. A type of matter which does not interact either by electromagnetism or the strong force, because otherwise we would have seen it in experiment. Since there is no interaction with electromagnetism, it does not reflect or emit light, and therefore we cannot see it using optics. Hence the name dark matter. Because it is dark.
It certainly acts gravitationally, since this is how stars and galaxies are influenced. It may still be that it either interacts by the weak interaction or with the Higgs. That is something which is currently investigated in many experiments around the world. Of course, it could also interact with the standard model particles by some unknown force we have yet to discover. This would make it even more mysterious.
Because it is so popular there are many resources on the web which discuss what we already know (or do not know) about dark matter. Rather than repeating that, I will here write why I start to be interested in it. Or at least in some possible types of it. Because dark matter which only interacts by gravitation is not particularly interesting right know, as we will likely not learn much about in the foreseeable future. So I am more interested in such types of dark matter which couple by some other means to the standard model. Until they are excluded by experiments.
If it should interact with the standard model by some new force then this new force will look likely at first just like a modification of the weak interactions and/or of the Higgs. This would be an effective description of it. Given time, we would also figure out the details, but we have not yet.
Thus, as a first shot, I will concentrate on the cases where it it could interact with the weak force or just with the Higgs. Interacting with the weak force is actually quite complicated if it should fulfill all experimental constraints we have. Modifications there, though possible, are thus unlikely. Leaves the Higgs.
Therefore, I would like to see how dark matter could interact with the Higgs. Such models are called Higgs portal models, because the Higgs act as the portal through which we see dark matter. So far, this is also pretty standard.
Now comes the new thing. I have written several times that I work on questions what the Higgs really is. That it could have an interesting self-similar structure. And here is the big deal for me: The presence of dark matter interacting with the Higgs could influence actually this structure. This is similar to what happens with other bound states: The constituents can change their identity, as we investigate in another project.
My aim is now to bring all these three things together: Dark matter, Higgs, and the structure of the Higgs. I want to know whether such a type of dark matter influences the structure of the Higgs, and if yes how. And whether this could have a measurable influence. The other way around is that I would like to know whether the Higgs influences the dark matter in some way. Combining these three things is a rather new idea, and it will be very fascinating to explore it. The best course of action will be to do this by simulating the Higgs together with dark matter. This will be neither simple nor cheap, so this may take a lot of time. I will keep you posted.
Thursday, August 20, 2015
Looking for one thing in another
One of the most powerful methods to
learn about particle physics is to smash two particles against each
other at high speed. This is what is currently done at the LHC, where
two protons
are used. Protons have the advantage that they are simple to handle
on an engineering level, but since they are made up out of quarks andgluons,
these collisions are rather messy.
An alternative are colliders using
electrons and positrons.
There have been many of these used successfully in the past. The
reason is that the electrons and positrons appear at first sight to
be elementary, but are technically much harder to use. Nonetheless,
there are right now two large projects planned to use them, one in
Japan and one in China. The decision is still out, whether either, or
both, will be build, but they would both open up a new view on
particle physics. These would start, hopefully, in the (late) 2020ies
or early 2030ies.
However, they may be a little more
messier than currently expected. I have written previously several
times about our research on the structure of the Higgs particle.
Especially that the Higgs may be much less simpler than just a singleparticle.
We are currently working on possible consequences of this insight.
What has this to do with the
collisions? Well, already 35 years ago
people figured out that if the statements about the Higgs are true,
then this has further implications. Especially, the electrons as we
know them cannot be really elementary particles. Rather, they have to
be bound states, a combination of what we usually call an electron
and a Higgs.
This is confusing at first sight: An
electron should consists out of an electron and something else? The
reason is that a clear distinction is often not made. But one should
be more precise. One should first think of an elementary 'proper'
electron. Together with the Higgs this proper electron creates a
bound state. This bound state is what we perceive and usually call an
electron. But it is different from the proper electron. Thus, we
should therefore call it rather an 'effective' electron. This chaos
is not so much different as what you would get when you would call a
hydrogen atom also proton, since the electron is such a small
addition to the proton in the atom. That you do not do so has a
historical origin, as it has in the reverse way for the (effective)
electron. Yes, it is confusing.
Even after mastering this confusion,
that seems to be a rather outrageous statement. The Higgs is so much
heavier than the electron, how can that be? Well, field theory indeed
allows to have a bound state of two particles which is much, much
lighter than the two individual particles. This effect is called a
mass defect. This has been measured for atoms and nuclei, but there
this is a very small effect. So, it is in principle possible. Still,
the enormous size of the effect makes this a very surprising
statement.
What we want to do now is to find some
way to confirm this picture using experiments. And confirm this
extreme mass defect.
Unfortunately, we cannot disassemble
the bound state. But the presence of the Higgs can be seen in a
different way. When we collide such bound states hard enough, then we
usually have the situation that we actually collide from each bound
states just one of the elements, rather than the whole thing. In a
simple picture, in most cases one of the parts will be in the front
of the collision, and will take the larger part of the hit.
This means that
sometimes we will collide the conventional part, the proper electron.
Then everything looks as usually expected. But sometimes we will do
something involving the Higgs from either or both bound states. We
can estimate already that anything involving the Higgs will be very
rare. In the simple picture above, the Higgs, being much heavier than
the proper electron, mostly drags behind the proper electron. But
'rarer' is no quantitative enough in physics. Therefore we have to do
now calculations. This is a project I intend to do now with a new
master student.
We want to do this, since we want to
predict whether the aforementioned next experiments we will be
sensitive enough to see that some times we actually collide the
Higgses. That would confirm the idea of bound states. Then, we would
indeed find something else than people were originally been looking
for. Or expecting.
Friday, July 10, 2015
Playing indirectly
One of my research areas are neutron stars. To understand them requires to understand how the strong interactions behave when the matter is enormously densely packed. A new PhD student of mine has now started to work on this topic, and I would like to describe a little bit what we will be looking at.
I have already written in the past that this type of situation is very hard to deal with, because we cannot just do simulations. This is unfortunate, since simulations have been very successful in uncovering what happened in the early universe. In that case, the system is hot rather than dense. Though the reason for the problem is 'just' technical, chances are not too bright to resolve it in the near future.
Hence, I had already quite some time ago decided that a possibility is to play indirectly. The basic idea is that there are other methods, which would work. The price we have to pay is that we need to make approximations in these methods. But we would like to check these approximations, ideally against simulations. But we cannot, because there are no. So how to break the circle?
To escape this problem, we can again use a detour. We did this once, because we hoped that we will learn more about the qualitative features. That we can get some insight into this type of physics. Now, we have a much more quantitative approach. We use theories, which are very similar to the strong interactions (also called QCD), but are not QCD, but which can be simulated. And these we will use to break the circle.
Why is it possible to perform simulations for this type of theories? Well, the main reason is the difference between particles and anti-particles. In QCD, a quark and an anti-quark are fundamentally very different objects. Hence, a large density can mean two things. A large density could be having many more quarks than anti-quarks, but still have plenty of both. Or it could be just to have many of one type. For a neutron-star both situations are relevant. And thus, there may be actually many more particles present then we would think, just many of them anti-particles. This is at the heart of the problem, that there is so much more than just the superficial number of particles.
This problem is evaded by using a theory instead where there are no anti-quarks. To be more precise, a theory in which anti-quarks are the same as quarks. There exists a number of such theories. However, such a change is very drastic. It thus may happen that the so changed theory is so radically different from QCD that any comparison becomes meaningless. Thus, it is necessary to ensure that the theory is close enough to the original.
Two candidates for such theories have been identified so far. One is the so-called G2QCD, of which I talked about previously. Another one is very close to QCD, but instead of three color charges is has just two different ones. Both cases have their own merits. The first is closer to QCD. In this theory there are protons and neutrons. The latter does not have these, but it is very cheap to simulate. Both theories are hence quite different, but actually share both also many other traits with QCD.
It therefore stands to reason that whatever approximation describes both well will also work for QCD. Thus, we will now use the simulations of both theories to test the approximations made in the other methods. Especially, we will look at the properties of the quarks and gluons. We will then use the insights gained to improve the approximations. Until we describe both theories well enough. Then we will translate the approximations back to QCD. And if everything works out, we will have then an acceptable description of a piece of neutron star matter.
I have already written in the past that this type of situation is very hard to deal with, because we cannot just do simulations. This is unfortunate, since simulations have been very successful in uncovering what happened in the early universe. In that case, the system is hot rather than dense. Though the reason for the problem is 'just' technical, chances are not too bright to resolve it in the near future.
Hence, I had already quite some time ago decided that a possibility is to play indirectly. The basic idea is that there are other methods, which would work. The price we have to pay is that we need to make approximations in these methods. But we would like to check these approximations, ideally against simulations. But we cannot, because there are no. So how to break the circle?
To escape this problem, we can again use a detour. We did this once, because we hoped that we will learn more about the qualitative features. That we can get some insight into this type of physics. Now, we have a much more quantitative approach. We use theories, which are very similar to the strong interactions (also called QCD), but are not QCD, but which can be simulated. And these we will use to break the circle.
Why is it possible to perform simulations for this type of theories? Well, the main reason is the difference between particles and anti-particles. In QCD, a quark and an anti-quark are fundamentally very different objects. Hence, a large density can mean two things. A large density could be having many more quarks than anti-quarks, but still have plenty of both. Or it could be just to have many of one type. For a neutron-star both situations are relevant. And thus, there may be actually many more particles present then we would think, just many of them anti-particles. This is at the heart of the problem, that there is so much more than just the superficial number of particles.
This problem is evaded by using a theory instead where there are no anti-quarks. To be more precise, a theory in which anti-quarks are the same as quarks. There exists a number of such theories. However, such a change is very drastic. It thus may happen that the so changed theory is so radically different from QCD that any comparison becomes meaningless. Thus, it is necessary to ensure that the theory is close enough to the original.
Two candidates for such theories have been identified so far. One is the so-called G2QCD, of which I talked about previously. Another one is very close to QCD, but instead of three color charges is has just two different ones. Both cases have their own merits. The first is closer to QCD. In this theory there are protons and neutrons. The latter does not have these, but it is very cheap to simulate. Both theories are hence quite different, but actually share both also many other traits with QCD.
It therefore stands to reason that whatever approximation describes both well will also work for QCD. Thus, we will now use the simulations of both theories to test the approximations made in the other methods. Especially, we will look at the properties of the quarks and gluons. We will then use the insights gained to improve the approximations. Until we describe both theories well enough. Then we will translate the approximations back to QCD. And if everything works out, we will have then an acceptable description of a piece of neutron star matter.
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