Quick post: the CMS experiment at the Large Hadron Collider [LHC] has updated its measurement of the rate for Higgs particles to be produced and then decay to two photons. We’ve been waiting for this result with considerable interest. Recall the history: in July, both CMS and its cousin, the ATLAS experiment, found this process to be in excess of the prediction of the Standard Model [the equations we use to describe the known elementary particles and forces]. Indeed, these excesses were part of why the Higgs particle was discovered a few months earlier than was widely expected. Although it was exciting that both experiments saw something amiss, the statistical significance of these excesses wasn’t that high, so more data to confirm the excesses was needed before we could take them very seriously.
Then ATLAS updated their results, first in November and then last week — the latter measurement based on the full 2011-2012 data set. Each time the excess has remained, though it has become a bit smaller each time, and therefore the statistical significance of the result has not really increased. That’s an unfortunate sign, if one is hoping the excess isn’t just a statistical fluke. [Note that, by contrast, the statistical significance of the evidence for the Higgs particle’s very existence has gone up each time we’ve seen new data.]
Meanwhile CMS did not update their results for this process in November. Rumors as to why swirled around, and there were some comments about this by CMS spokesman Joe Incandela in January; but we can forget about the rumors now, because the result based on the full 2011-2012 data set has now appeared at the Moriond conference. And the result basically agrees with the Standard Model. (Details to follow in a future post.)
To repeat: with more data, CMS does not confirm the excess that they saw in July, and does not confirm the excess seen currently by ATLAS. There’s nothing unusual about this, unfortunate as it may be; the results are all consistent with the overall measurement uncertainties that have been quoted at each stage by the two experiments.
So if, in fact, it is really the case that Higgs particles are produced and then decay to two photons more often than the Standard Model predicts, we will not see convincing evidence of this until well past 2015, when the LHC starts running again and much more data is collected by ATLAS and CMS. Sorry, but if there’s anything about this Higgs particle that is dramatically different from a Standard Model Higgs (the simplest possible type of Higgs particle), we’ll have to look elsewhere in the 2011-2012 data to find it. Or we’ll have to wait til 2015.