LHC Passes Milestone

Over the weekend, the Large Hadron Collider [LHC] passed a significant milestone: the amount of data collected so far in 2012 now exceeds the amount of data collected in 2011.  (The LHC is also running very efficiently right now.)  As you may recall, the data is being taken at a slightly higher energy than last year — 8 TeV instead of 7 TeV for each proton-proton collision — and among the benefits of this change are slightly higher probabilities of making Higgs particles.  In short, more Higgs particles (assuming they exist) have now been made this year than last, and if last year’s preliminary evidence of a Standard Model (or Standard-Model-like) Higgs particle [the simplest possible type of Higgs particle] with a mass of about 125 GeV/c² turns out not to be a mirage, then the number of Higgs particles produced so far this year is about 100,000 for the CMS experiment and a similar number for the ATLAS experiment.  Most of these Higgs particles decay in ways that make them indistinguishable from other processes, but the dozens that decay to two photons and the handful that decay to two lepton/anti-lepton pairs will be the most important in this year’s search.

When will we see results from the new data on the search for the Higgs particle?  (Remember we’re still at the end of Phase 1 of this search, in which the goal is to find or exclude the Standard Model Higgs particle.) This will happen in about four weeks, at the ICHEP conference in Melbourne, Australia in early July.  Results from this year’s 8 TeV collision data will not be combined with last year’s 7 TeV data, but rather will be presented as a separate analysis.  You can think of this as something like a do-over of last year’s study, but with the look-elsewhere effect essentially removed.  That’s because the results from 2011 have eliminated the possibility of a Standard-Model-like Higgs particle in most of the previously allowed range.  Last year we had to ask: Is there a Standard Model Higgs particle of any mass between 115 and 800 GeV/c²?  This year, taking into account the ATLAS and CMS results on the 2011 data, we already know a Standard Model Higgs is excluded unless it has a mass in a narrow range of a few GeV on either side of 125, or lies above 600.  So we are asking only: is there a Standard Model Higgs particle near 125 GeV/c², and (separately) is there a Standard Model Higgs particle between 600 and 800 GeV/c^2?  The narrower the question, the less data you need to answer it — so if there really is a Standard Model Higgs at 125, there’s a good chance (though it depends on how the random fluctuations go) that we’ll see fairly convincing evidence of this in early July.  The issue may not be fully settled, however, until the year is over, when there should be 3 to 4 times as much data, so don’t be entirely surprised if the July results are still inconclusive.

The Higgs search is priority one, so my sense is that all efforts (and in particular, all the computers, which are needed to simulate all the processes that occur in 8 TeV collisions) are directed toward this primary goal.  For this reason I think it is doubtful that by July there will have been enough time for there to be any other important analysis of the 2012 data.   But I think we can still expect some interesting new studies of 2011 data to appear at the ICHEP conference.