A Discrepancy to Keep an Eye On

Today (as I sit in a waiting room for jury service) I’ll draw your attention to something that has been quite rare at the Large Hadron Collider [LHC]: a notable discrepancy between prediction and data.  (Too rare, in fact — when you make so many measurements, some of them should be discrepant; the one place we saw plenty of examples was in the search for and initial study of the Higgs particle.)  It’s not big enough to declare as a definite challenge to the Standard Model (the equations we use to describe the known particles and forces), but it’s one we’ll need to be watching… and you can bet there will be dozens of papers trying to suggest possibilities for what this discrepancy, if it is real, might be due to.

The discrepancy has arisen in the search at the CMS experiment for “multileptons”: for proton-proton collisions in which at least three charged leptons — electrons, muons and (to a degree) taus — were produced. Such events are a good place to look for new phenomena: very rare in the Standard Model, but in the context of some speculative ideas (including the possibility of additional types of Higgs particles, or of superpartner particles from supersymmetry, or new light neutral particles that decay sometimes to lepton/anti-lepton pairs, etc.) they can be produced in the decays of some unknown type of particle.

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Theory Killers at the HCP conference

There were many interesting results presented yesterday at the HCP conference in Kyoto, and they were both too numerous and too detailed for me to completely absorb as yet — a follow-up will clearly be needed.  But a few are obviously so important that I want to point them out now.

First, both ATLAS and CMS, the two general purpose experiments at the Large Hadron Collider [LHC], produced important new results on “multileptons”.  Based on a significant fraction of their 2012 data, they looked for signs of new phenomena that would appear as proton-proton collisions that produce at least three leptons or anti-leptons, or even (in unusual combinations and/or along with other unusual things) two leptons or anti-leptons.  (I’ll just summarize this class of studies as “multileptons” for the purpose of this brief post and be more specific at a later date.) ATLAS used about 50% more data than CMS, but CMS had a more intricate analysis of their data, so I believe the results were similar where they can be compared.  [By the way, the CMS result was approved to be shown at this conference under extreme conditions; at least two of the major players in the analysis had no power or internet for over a week following Hurricane Sandy!]

The bottom line is simple: neither CMS nor ATLAS sees any significant deviation from what is predicted by the Standard Model.  And this now kills off another bunch of variants of many different speculative ideas.  The details are extremely complicated to describe, but essentially, what’s dead is any theory variant that leads to many proton-proton collisions containing

  • two or more top quark/anti-quark pairs
  • multiple W and Z particles
  • two or more as-yet unknown moderately heavy particles that often decay to muons, electrons and/or their anti-particles
  • new moderately heavy particles that decay to many tau leptons

and probably a few others I’m forgetting. While multilepton searches (especially those for 3 or more leptons) are often touted as a great way to look for supersymmetry in particular, that description vastly understates their power — they are a great way to look for many different types of phenomena not predicted in the Standard Model.  (This is something that a number of scientists at Rutgers University have been emphasizing in talks and papers.)  And both experiments have demonstrated this with various interpretations of their results; CMS has over a dozen of them!

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