Next week I am giving a talk a the SUSY 2010 conference in Bonn.
It is a bit weird that there have been seventeen of these annual SUSY (for “SUperSYmmetry”), meetings, even though there is as yet no experimental evidence for SUSY. Perhaps it’s excusable. SUSY is still the best way I’m aware of to improve the Standard Model of particle physics.
To me the three biggest arguments in its favour are: One, it plugs an important hole in the theory. Two, it sort-of-predicts Dark Matter. Three, it looks nice.
The first is to do with why the Higgs boson is not millions of times heavier than it is. Given we don’t know whether there is a Higgs yet, that’s a pretty forward-looking concern, but it is a real worry for the credibility of the theory. Basically without it, the Standard Model looks like a coincidence on the level of one in ten-thousand-million-million (1016). This is 100 times less likely than winning the lottery jackpot two weeks running if you buy a single ticket each week. SUSY introduces some quantum cancellations which make the Higgs mass much more stable, and therefore plausible. Still, maybe the universe got lucky. Some string theorists might say we should be glad it’s not one in 10500.
The second is the most compelling to me, since astronomical observations tell us there is probably some Dark Matter out there (or else we really do not understand gravity) and many SUSY models predict a particle which would be an ideal candidate for Dark Matter. It may be right behind you. When two different branches of science have problems which seem to converge on the same solution, look out for progress.
The third point is arguable and I may argue it later, but not now.
Another feature of SUSY is its flexibility. It can appear in many different guises in an experiment, to the extent that almost any weird event we see could (and will, I bet you) be interpreted as a “hint of SUSY”.
For example, a big part of my doctoral thesis involved simulating a SUSY process which we might have seen at the electron-proton collider, HERA. When you whack protons and electrons together, one thing which might happen is that the quarks in the proton stick to the electron. This would be a “leptoquark” (because electrons are leptons) and would be a sign of the unification of the strong, weak and electromagentic interactions, so-called “Grand Unification”. Very exciting stuff.
Just before we switched on, JoAnne Hewitt realised that the signature of a leptoquark also looked like a particular form of SUSY. Herbi Dreiner, then a postdoc at Oxford (and now organiser of SUSY2010) had realised that if so, there would other ways it could decay, and he calculated them.
I remember his calculation being given me on a napkin, but my memory may be embellishing here. Anyway, I wrote a program predicting how the results would look in our detector, so we could search for them.
Sadly they never showed up, though we did have a bit of a false alarm at one point.
Not-so-coincidentally, SUSY is one of the things we might also find at the LHC. In fact I have even written a couple of papers on some possible signatures. (Which is why I am talking at SUSY10). Basically I had a new experimental technique, and was looking for applications. SUSY provides some.
This flexibility makes SUSY a good test-case for experimentalists to make sure we aren’t missing anything. If we are alert to all possible SUSY processes, we are alert to a very wide range of weird stuff.
However, when weird stuff doesn’t show up, as so far it has not, that unfortunately does not disprove SUSY, it just rules out a given subset of SUSY models. This can be frustrating.
Still, to its credit, if there is no low mass Higgs, SUSY loses much of its attraction. It would not quite be ruled out, but it would certainly be relegated down the ranks of speculative theories. Conversely, if we do find a Higgs, the search for SUSY will become much more compelling.
Meanwhile, I had better write my talk.