Friday, 2 December 2011

The status of the Higgs boson in 2011

The Higgs boson is the last piece of the Standard Model of particle physics which has yet to be verified by experiment. As everybody knows, discovering this particle (or perhaps ruling out its existence) is one of the primary goals of the LHC.

Last month, the two big collaborations focussed on discovery of new particles at the LHC — the ATLAS and CMS teams — released combined results from their searches for the Higgs; the punchline is the following plot:

If you don't understand these "Brazil band" plots, I refer you to a blog entry by Tommaso Dorigo (I actually stole the picture from another of his blog posts; I hope he doesn't mind). Roughly speaking though, we conclude that the Higgs does not exist at mass values where the data falls below the red line. (One thing which has confused me about this plot is the fact that the data sits well above the expected exclusion limits over a large mass range, and is in fact around $2\sigma$ or more adrift all the way from 130 GeV to 170 GeV. This suggests to me some poorly-understood systematic errors, but nobody seems concerned by this, so that must be wrong. If somebody reading this can shed some light on the issue for me, I would be grateful.)

The two experiments have now collected a lot more data, and in less than two weeks time, there is due to be a public seminar at CERN at which they will announce updated results. There is a sense of nervous excitement, and already rumours are circulating that perhaps there is now some evidence for the Higgs, instead of just better exclusion limits. Certainly, there was already a nice-looking bump (of no statistical significance, of course) in the plot above at around 120 GeV. Or is it 118 GeV? Or maybe 119 GeV …? I, among many others, would be quite happy if that bump has grown, because a Higgs boson in this low-mass region is highly favoured by supersymmetric extensions of the standard model, so this would be a hint, however inconclusive, that supersymmetry really might be realised in nature, and be discoverable at the LHC within the next couple of years.

Another possibility, of course, is that there is still no evidence for the Higgs, and that the mass window in which it might be hiding has shrunk significantly yet again. If this is the case, and continues to happen next year, it will lead to a lot of head-scratching — if there is no Higgs, what the hell should we replace it with? I am not aware of any compelling models which predict that the LHC won't see some form of Higgs boson.

Whatever ATLAS and CMS have to tell us on the 13th, there will be a lot of people listening very carefully.

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