Higgs Predictions and Results

Tomorrow at 14:00 CET CERN will start to unveil the results of this year’s LHC Higgs search, see here. Jester has just posted details consistent with what I’ve been hearing for the past couple weeks, although his numbers are slightly different. The big news is that both experiments are seeing something that looks exactly like a Standard Model Higgs in the Higgs -> gamma-gamma channel around 125 GeV. Some details about this signal:

  • I’ve heard that ATLAS sees this with significance a bit less than 3 sigma, 3.5 if you combine results from other channels. To take into account the look-elsewhere effect, the significance of seeing such a fluctuation anywhere in the range studied is 2.2 sigma. Jester has

    The combined significance is around 3 sigma, the precise number depending on statistical methods used, in particular on how one includes the look-elsewhere-effect.

  • What makes this something to take very seriously is that CMS is also seeing much the same thing in the gamma-gamma channel around the same mass. I’ve heard their significance is 2.5 sigma. Jester has

    All in all, the significance at 125 GeV in CMS is only around 2 sigma.

  • Another channel that might start to have some sensitivity to a Higgs of this mass is the “golden channel”, where you look for 4 leptons reconstructed as coming from 2 Z’s. Here Jester says that ATLAS has three events near 125 GeV, contributing to the higher significance in the combined result. CMS also sees an excess involving three events, but they see this in the range 117-121 GeV, which should be too low to come from a 125 GeV Higgs. They also see a 2.1 sigma bump in gamma-gamma their combination at 119 GeV [due to the 4 lepton excess, they see nothing in gamma-gamma at 119 GeV] .
  • In this business, for each experiment a 2 sigma bump is not worth taking seriously, around 3 sigma is where it gets serious (and 5 sigma is the conventional standard to claim a discovery, they’re definitely not there yet). But a 3 sigma bump from one experiment and a 2.5 sigma bump from the other, at the same place, is serious evidence indeed. It is still quite conceivable though that this kind of signal could disappear with more data (for this we’ll have to wait until mid-2012). I think Jester has it about right:

    There is a good chance we’re finally looking at the real thing, I’d say 50% based on the data alone and 80% adding our sincere convictions that Higgs must really be in that mass range.

    One thing that can be predicted with certainty is a flood of papers from theorists claiming that their favorite model predicts this particular Higgs mass. Something to keep in mind when evaluating such claims is that for more than ten years we have known from LEP that the Higgs mass is above 114 GeV, and from precision electroweak measurements that it can’t be too much above that value. Preliminary data early this summer showed some indications of a signal around 140 GeV (leading some to claim this as vindication of the multiverse, see here). By the end of the summer this was gone, with the combined CMS+ATLAS data excluding a Higgs down to 140 GeV or so at 95% confidence level, around 130 at 90% (tomorrow’s data should have each experiment excluding down to 130 GeV at 95%, with a Philip Gibbs combination result sure to follow soon). Rumors of the gamma-gamma signal at 125 GeV have been circulating for at least the past two weeks.

    We’ll soon move from rumors to results, and I’ll add anything new or different we hear tomorrow to this posting. Surely there will be a press release and a lot of versions of the results coming out of CERN, covered by the media and many bloggers. For some good recent blog postings explaining what to look for tomorrow, see Matt Strassler and Tommaso Dorigo.

    Update: One correction. Besides the 124-125 bump in gamma-gamma, there’s another bump in the CMS combination at low mass, around 119 GeV, but this is due just to the “golden channel” excess there, they see nothing there in gamma-gamma.

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    8 Responses to Higgs Predictions and Results

    1. petergreat says:

      I read that a SUSY Higgs can be indistinguishable from a SM Higgs, at least during the initial stage of discovery. Is the same true for composite Higgs models?

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    3. Bob S says:


      Nice props for “blogger” Peter Woit. Heck, the article even states string theory is “contraversial”

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