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.

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.