So where is all the SUSY?
4 October 2013 | By
Supersymmetry (SUSY) is one of the most loved, and most hated, theories around that works as an extension of our beloved Standard Model. It's loved because it has some very nice features: it can explain dark matter, it has some very suggestive features when it comes to the possibility of unifying the forces (a "grand unified theory"), it can explain why the Higgs mass is so light (though the Higgs is a bit on the heavy side for some versions of SUSY), and it has some other very nice theoretical features that are perhaps a bit technical. It's hated, for the most part, because the full version has around 300 free parameters. That means SUSY can predict just about anything that the LHC might see, and it also means that SUSY is almost impossible to rule out - even the simple versions!
There's been quite a bit of ink spilled lately over where SUSY might be hiding in the LHC data. Tomasso Dorigo has written about the slowly dissolving faith in finding SUSY at the LHC and how an increasing number of theorists think that some of the apparent problems that SUSY solves might not be problems at all, but just our misunderstanding of fundamental physics. Peter Woit has written quite a bit about the trouble with SUSY at the moment and the apparent crisis in theory (quite interesting, but I would quickly be out of my depth in a discussion of that, so I leave it to the professionals).
Nevertheless, the SUSY group in ATLAS hasn't given up! Quite the opposite: we've produced 22 Conference Notes (notes describing physics analyses that aren't quite ready for publication, often because for publication we like to test a lot of additional signal models that are not determinent in the question of "was there a new particle in the data") and 2 papers on the 2012 dataset, and have a large number of new searches and papers waiting in the wings. Why bother, and what are all those analyses doing?
Well, as I said, SUSY is a theory that can give a lot of different results. One version might give us heavy (1000-1500 times heavier than the proton) objects called "squarks" and "gluinos" (yes, all SUSY particles have funny names) that decay to quarks, which show up in our detector as things called "jets." So one set of searches should look for jets coming from heavy objects. In their decays they can also produce electrons, muons, and taus (called leptons) so we also want a search with one, or two, or three, or even four leptons in the final state. Each one of those is a separate analysis, so now we're starting to get a longer list. Another version of SUSY might give us lighter particles (300-600 times heavier than the proton) objects that decay to bosons like the W, the Higgs, or the Z. Those give us events with lots of leptons, but without lots of jets - so we should have a different set of searches for those. There is a lot of interest in SUSY making extra top and bottom quarks right now (for reasons that go under the heading "naturalness"). So we also want a set of searches looking for traces of bottom and top quarks in the events. That's a lot of variety!
Of course, the nice thing about having such a diverse range of searches is that they aren't only sensitive to SUSY. There are a number of other theories out there that could predict things that our SUSY searches are sensitive to. So even though they're carried out by the SUSY group, and even though we talk about SUSY models that we set limits on with each of these analyses, they are a great way to cover a huge range of models of new physics. There are a lot of strange models out there (Hephalons?), so searching for things that SUSY predicts provides a nice starting point for covering that enormous set of models (of course, the exotics group on ATLAS is doing some nice work to cover other models!! I'm just a bit biased).
Even if SUSY doesn't show up in our current data, I still think there's a decent chance that we will find it - or something like it - in the high energy run set to start in 2015. The SUSY searches could find the first bump, but that doesn't mean it's SUSY - we'd have our work cut out for us to understand what we'd found! In the meantime, we can look through strange SUSY models to make sure that we haven't potentially missed something interesting in our results.