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.

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