I have been attending the conference called “Stringy reflections on LHC”. This has been put together by the Clay Mathematics Institute, and here is the link. So with that wonderful title, you might be wondering what are we up to. What will follow is a quasi-technical discussion of what I have seen so far.
You might or might not be aware of that there is this branch of theoretical physics labeled `string theory’. It is an attempt to write a consistent theory of gravity that is compatible with the standard model, and in the initial days (make that years) there was a lot of optimism that one would be able to derive the standard model of particle physics from first principles and calculate all of the coupling constants of the standard model from scratch. This turned out to be a harder problem than was initially thought and after a couple of years we have a lot of constructions that look like the standard model of particle physics, but we do not have The Standard Model of particle physics. More to the point, there seem to be too many constructions that might give the standard model, so that one does not have a unique model of physics beyond the standard model, but rather a large collection of options to sort through.
However, the LHC is going to be turned on pretty soon, and we would like to see if there is a prediction that can be made on general ground of what we would expect to see. Barring that, given new data for physics beyond the standard model, we would like to have a roadmap that discards constructions (or scenarios) to tell us who is left standing so that we can concentrate our efforts there. In essence, we are asking what should we be looking into once we have data. Various people have different ideas and it is good to get a picture of all of the approaches that are available out there.
The conference has been dedicated to figuring out how we can address various interesting phenomenological questions given various such construction. Statistically, supersymmetric models seem to dominate the discussion, mostly because those are the only one we understand sufficiently well to say something useful about them. Supersymmetry, if it is a symmetry of nature, is broken spontaneously. It predicts that for every particle we have seen, there is a partner particle with different spin, and the interactions of these partner with matter are controlled to a large extent by this symmetry.
If one just tries to make the standard model supersymmetric, it does not break supersymmetry on it’s own, so the supersymmetry breaking has to be communicated to the standard model somehow.
In the past two years there has been a lot of progress in understanding a lot better how supersymmetry breaking might be communicated to the standard model. In particular, there is a scenario called gauge mediation that seems to do a pretty nice job of it and it has a couple of predictions for physics beyond the standard model that make the scenario attractive. In particular, it does not introduce problems with flavor physics, and if one is given some additional discrete symmetries, one can guarantee that one does not produce anomalously large CP violating phases ( we would have seen these in precision experiments already, for example at Belle and Babar), so it is a good candidate for how to go beyond the standard model. We heard a lot of talks about these quantum field theory problems, with very nice talks by Intriligator, Seiberg, Shih, and people are trying to accommodate these quantum field theory scenarios in models for physics beyond the standard model arising from string theory. Moreover, we heard of what predictions are made by these scenarios that would let us know if there is gauge mediated supersymmetry breaking in nature or not.
I don’t want to review all the talks (mostly I can’t remember all the details), but there are obviously various other discussions on how to get reasonable supersymmetry breaking setups in string theory where we can still get some numbers out at the end of the day. I have had a lot of fun in the conference.
When it was my turn, I spoke about how to test if we might live on a D-brane with a low string scale (meaning that we could see a few string states at the LHC, but we might not have enough energy to reach the string scale). Surprisingly, these models have not been ruled out yet , despite their inherent `theoretical ugliness’, so we should try to look for distinctive signals that would give us a hint that these are possible. I went through some of those possibilities and some can be quite interesting as far as experimental searches are concerned.
You can read all the talks once they are online if you want to. Since these are all technical talks, I don’t expect that everyone would be able to read them. We have also had some quite spirited discussions that I can’t reproduce here. As a whole there is no consensus on what will come next, but we have a lot of possibilities to sort through.