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.