Wednesday, May 30, 2012


Last time, I described our reasons and attempts to go beyond the standard model. Among the myriads of possibilities of extensions of the standard model there is one, on which I work. Let me describe it first. Then, I will tell you, why I find especially this one interesting.

This extension of the standard model is called technicolor. It is one of the bottom-up approaches. It was mainly conceived to cure problems we have with the Higgs particle. One particular annoying problem with the Higgs is that its mass is not constrained in the standard model. Well, this is not yet really a problem, since neither are the masses of any of the other particles. But the Higgs mass is in so far different as it is very sensitive to the rest of the standard model. If we change the properties of the standard model by a factor of, say, a hundred, the electron mass will only change by a factor of two, or so. But the Higgs mass will change by a factor ten thousand. Both statements are, of course, very rough, but you should get the idea: To get the Higgs mass to the value we will, hopefully soon, measure, we have to be very careful with the standard model. We have to tune it very finely. This is called the fine-tuning problem, or also the hierarchy problem. It is actually a perceived problem only. Nature may be "just so". But so far, whenever nature seemed to be "just so", there was actually a deeper reason behind it.

Thus, people have set out to find an explanation for this extreme sensitivity of the Higgs mass. Well, actually, people rather have searched for an alternative to the standard model, where this is not the case.

One of the possibilities, which has been conceived, was technicolor. The rather poetic name comes from the historical fact that QCD with its colors has been a role model for technicolor. However, in the decades after its original inception in the 1970ies, technicolor has changed a lot. Today, this theory has only remote similarities to QCD, but, of course, the name stuck once more.

So how does technicolor works? Actually, technicolor is not really a single theory. Since we do not yet have any experimental information about the Higgs, we are not yet able to restrict such extensions of the standard model very well. Thus, there are many theories, which could be called technicolor. It is more an idea than a strict mathematical concept, and it can come in many disguises. The basic idea is that the Higgs is not an elementary particle. Rather, it is made out of other particles, which are called techniquarks. These are held together by exchanging technigluons - once more you see how QCD has been inspirational to the naming.

You may ask what one wins by making the Higgs a more complicated object. The answer is that the Higgs mass, once made out of other (fermionic) particles, is no longer so sensitive, but behaves like the other masses in the standard model. Thus, this solves the problem one has set out to solve.

Of course, as long as we do not yet have found the Higgs, we cannot yet tell whether it is really made up out of other particles. And even when we find it, it will be very complicated to determine whether it is, and it will take a lot of time and a lot of experiments.

But it turns out that things do not come so freely. As you may remember, we needed the Higgs also to provide mass to the other particles in the standard model. And this becomes much more complicated once the Higgs is made out of techniquarks. The reason is that the standard model Higgs is rather flexible, and we can adjust its interactions with the other particles so that the right mass of, say, the electron comes out. This is no longer easily possible once the Higgs has a substructure. This has led in the 1980ies to the believe that technicolor cannot work.

In 1990ies, and now after 2000, however, clever people found a way to make it work. The original problem came about because in the beginning technicolor was made too much alike to QCD. Once one is satisfied with a theory being more different from QCD, and being willing to add a few more particles, the situation improves. Unfortunately, without guidance by experimental results it becomes somewhat ambiguous how to solve the problem, but it seems feasible after all.

So far, this is the status of technicolor. Why am I interested in it?

If you want to make the Higgs from two techniquarks and at the same time simulate the Higgs effect, it is necessary to introduce a new mechanism to the theory. When you look at QCD, its strength depends on the energy. Actually, it does so in a very violent way, and QCD changes from strongly interacting to weakly interacting very quickly. To make the technicolor idea work, this had to change. Technicolor needs to change from strongly interacting to weakly interacting very slowly. To make this distinction more vivid, QCD is called a running theory while technicolor is called a walking theory. This means that technicolor is strongly interacting for a large range of energies. As with any strongly interacting theory, its description is very complicated. Since we know such theories only since a couple of years, we are really at the beginning of understanding them, even the very basic mechanisms of them. But this is what we need to do, if we ever want to make a real quantitative comparison to experiment. And that interests me. I want to understand how such theories work at a very fundamental level. They are so strange compared to the rest of the standard model, and to many other proposal beyond the standard model, that they are a very fascinating topic. And that is, why I am studying them.

Wednesday, May 9, 2012

Above and beyond

Before I can introduce my last research topic, I have first to go above and beyond the standard model. Given that I am calling this blog a tourist guide to the standard model, this is a bit like trespassing. But that is how my research went over the last years. So let me keep the name, but nonetheless venture beyond.

With keeping the name, I am using a very important concept in scientific naming: For historic reasons. For historic reasons, this blog was named, but then reality overtook it. This is something very often happening in physics. In the beginning, we encounter a phenomena. To not always have to describe it, we give it a name, based on what we have encountered so far. Very often, when we really understand what is going on, this name would no longer be appropriate. But at this time, the name stuck, and thus we stick with it. You should keep this in mind, when you think that the name of something seems to have nothing to do with the thing. Then the name is just there for historic reasons. And that is something one will also encounter beyond the standard model.

So let us go beyond the standard model. As I told you, the standard model is just that: A model. It has its limits. We know today, mostly for reasons of mathematical consistency and for astronomical observations that the standard model cannot be the end. We know that with what we have we cannot explain what happens at very high energies. We also cannot explain with the standard model why the outer rims of the galaxies rotate faster than they should. We do not know why the universe is expanding faster and faster. We do not know why the particles have precisely the masses they have. We cannot tell, why nature is such that the sun can shine: Given the standard model, we can explain how the sun shines. But we cannot explain why the standard model is such that this is possible. And there are many other things.

However, irritatingly, all our experiments here on earth have been in accordance with the standard model.

This is very unsatisfactory, to say the least. As a consequence, almost since the birth of the standard model in its present form people have started to consider extensions of it. This is commonly known as beyond-the-standard-model models, or BSM for short. Unfortunately, the observations listed above are not pointing all in the same direction. Actually, most just point somewhere, for the amount of knowledge we have of them. Thus, we are, right now, very much in the dark when it comes to figure out how we should extend the standard model.

This lead to very many proposals. Even listing all conceptual proposals would easily fill a couple of months worth of blog entries. But they can be broadly distinguished by their approach. Some have a top-down approach. They try to envisage the underlying theory, which will solve all (or many) of the known problems in one strike. You may have heard of (super)string theory. This is one example of this approach. The other is bottom-up. Here, one just tries to resolve a single problem. The ones I am working on belong to the latter category.

Now, what is the state of affairs? Top-down approaches are often rather complicated theories, and it is often very complicated to calculate anything at all. Thus, progress on this side is naturally slow. The bottom-up approaches are often more tractable. It is often not too complicate to design a theory, which solves the problem one was setting out to solve. However, in doing so one usually gets for a completely different thing a result which disagrees with the known experiments. Thus, one is forced to modify the model to sate this problem. But then the next springs up, and one finds oneself adding bits and pieces to the model, until it becomes rather baroque.

You may say now: Well, if things are either too complicated or too much mingling, maybe you are on the wrong track. And I would not really disagree with you. Of course, nothing prevents nature from being really complicated or baroque. Just because both versions do not look aesthetically pleasing to us does not mean it cannot be. But this way of thinking has never been right in the history of physics. When something got complicated, and any amendment made it worse, then we were on the wrong track.

So, why do we not abandon everything we did, take the standard model as the basis, and start over from scratch? This has actually been done many times, and so far was not successful. On the other hand, going back to the complicated theories, one hope is that by making the theory more complex by making more and more things agree with experiment, we can hope that at some point a pattern emerges. This has occurred in the past, and is thus a real possibility.

Thus, today, all of these possibilities are followed. We try to imagine solution to the problems, test them against experiments, both old and new, and reiterate. A spectacular new observation at any experiment would greatly help. That is why any ever so slight deviation from the standard model expectation is greeted with great enthusiasm by the theorist. Even if we know that in most cases this will be a coincidental fluke, which will go away when we keep looking more carefully.

Where will this lead us to? We do not know yet. That is the really exciting part of particle physics: We try to push the boundaries of knowledge, and we can only speculate what we will find there.

Thursday, May 3, 2012

The Higgs beyond the obvious

It is very likely that you have heard in the media of my next research topic: It is the Higgs particle. Well, actually it is not just the Higgs particle alone. In isolation, without its connection to the weak interactions and all the rest of the standard model, the Higgs is actually a very boring particle. In fact, the Higgs particle in isolation is almost every particle theorists first encounter with particle physics. Because it is so boring and simple. Just that it is then not called the Higgs particle, but usually phi, and the theory is called the phi-to-the-fourth theory. Much less fancy.

I guess from all the attention the Higgs has received you may be assuming that many people are working on it. That is true. So, what is particular about my research? What is the specific twist of this thing that I am interested in?

Most people nowadays are interested in either of two aspects of the Higgs: What are its properties? More precisely, what is its mass? And how does it play along with the rest of the bunch? Or: Where does it come from? Is there some theory where the standard model is just a special case of? And if yes, what role does the Higgs plays there?

Me, I am more interested in some more subtle questions. Actually, in three.

The first question concerns the validity of our theoretical description of Higgs physics. As you may remember, the standard model is only valid up to a certain amount of energy. We assume that there will be another theory including it and resolving all (or at least a fair amount of) our questions. Until we figure out this theory, we try to hide our ignorance. For a theory like QCD, this works very well. There we can hide our ignorance just in a few numbers, which we can determine unambiguously in an experiment. After that, we are fine, and we can make predictions. With the Higgs, this is different. And the difference is here in the word 'unambiguous'.

The theory with only Higgs particles I mentioned before is actually a sad story. It turns out that it has the same problems as the standard model, but we cannot hide them unambiguously. And thus our predictions for it are flawed. We can only prevent this by switching off all the interactions. But then the theory is boring, because nothing happens. That is what we call a trivial theory.

The question I am interested in is, what happens when we put the Higgs in the standard model. Does it remain ambiguous? Or do the interaction with the other particles cure this problem in some fancy way? Right now, the judge is still out. If it cannot be cured, the search for the new theory becomes even more important. Because then the standard model would be much more flawed. Deciding what is the case is one of the questions I am looking into.

To even ponder the other two questions, I assume that the combination of the Higgs and the weak interactions do not have such a problem. Or that the resolution of the problem is not altering the answer to the two questions substantially. This may seem a bit like cheating or evading the problem. But we quite often come across problems not immediately solvable. These maybe so hard that we will take a long time to solve them. To get not totally stuck, we do often assume such solutions do exist, and carry on. Of course, we do not forget these questions, but work on them further, to have eventually an answer. This has the risk that some of what we may do then becomes invalid once we solved the problem. In fact, this has happened often. But as often it has also resolved the original problem, or gave us fundamental new insights. That is one thing we have to do in science: Always explore all the possibilities. Since we do not know what lies ahead, this is the only way to eventually find the right answer.

But after this detour, back to the two questions.

One is again rather fundamental. The theory with the Higgs is very peculiar. If you would switch off the Higgs effect, you would end up with a theory like QCD. Especially, the weak interactions would confine the Higgs particles, just like the strong interaction confines quarks. In fact, both phenomena could even be linked in a very abstract fashion. Merely two sides of the same coin. This is something which has been noticed already back in the 1970s. We do not yet understand what this is really about. One of the reasons was that back then confinement was not well understood. We understood much more about confinement since then, especially in the last ten years. Armed with this knowledge, I reinvestigate this connection. And try to clarify what is going on.

The last question concerns bound states. That is right! We have not yet discovered the Higgs alone, but I am already try to understand how two Higgs particles could come together and form something like a Higgs meson. That is a rather new problem I came across recently. There are some fascinating consequences of it. Some people speculate that we can observe such objects at the LHC. I am currently trying to understand, whether we have a realistic chance of seeing them. And if yes, how they are build up.