Tuesday, 10 July 2012

A Few More Thoughts on the Higgs

It's been almost a week since ATLAS and CMS announced their discoveries.  Even within the field, not much has changed.  What has is mostly a perspective; we have a Higgs, probably, so it becomes an input to our models instead of an output.  There have been a lot of new papers on the arXiv, especially this week, but the biggest splurge came with the hints last December.  So far I haven't seen anything really notable, though I'll admit I'm a bit behind on my reading.

What do we know about the discovery?  It looks very much like a we would expect.  The nice thing about the Standard Model is that when it comes to the Higgs, it is highly predictive.  All the relevant couplings relevant were known long ago; only the mass and the Higgs self-coupling were not predicted.  Now that we know the mass, we also know the latter value too, but this cannot be measured at the LHC.  When we compare those predictions to the data, everything more or less agrees.  Only the Higgs decay to two photons is not within the error bars, but the difference is still small.  The most interesting hint is that the decay to W bosons is small, but the Higgs-W coupling is also relevant for Higgs production, and the evidence there is consistent.  Of course, the fact that the differences are small won't and hasn't stopped people writing papers!

The LHC was initially going to shut down at the end of the year for eighteen months.  This would allow the experiment to be upgraded and checked, so that when it resumed running it could operate at the design energy.  (Currently, the LHC has been working at half that.)  The discovery has persuaded CERN to run for three months next year, in the hope that they can pin down the couplings better.  I was surprised to hear this, as I thought that contracts had been signed preventing this.  It's most likely for the best, however.  The shut down will see pretty wild speculation in the theory community, so better data to tame that is a good idea.

The discovery of the Higgs as in the Standard Model is a triumph of the predictability of science.  We said "this is what we expect to find", looked for it and found it.  This is an important part of what makes science different to other claimed paths to knowledge, such as religion.  Within particle physics alone, the Higgs follows the top, W, Z, gluon, tau neutrino and bottom quark as objects predicted before they were found.  They all pretty much ended up with the mass we expected, too, though the top was initially something of a surprise.

Still, as a theorist early in my career it would be terrible to learn that we have already finished!  So I'm hoping that as we get more Higgs data, there will be some discrepancy that won't go away.  (Or we could just find something else; that works too!)

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