Posts have been a bit rare due to overwork and travel, but I have a few things to say about the search for the Standard Model Higgs particle (the simplest possible form of the Higgs) at CMS, one of the two major Large Hadron Collider [LHC] experiments. Last week I focused on the big news from the Tevatron and from ATLAS (the other major LHC experiment involved in the Higgs search), because the changes in their results were much larger than those from CMS, partly because CMS had already analyzed all their data for all of the different types of Higgs decays, and also the changes at CMS were, on the face of it, rather small.
However, the results at CMS have a significant effect on the overall picture, both negative and positive, and so deserve comment.
The main change from the December and January CMS two-photon Higgs search is that a more powerful and sophisticated technique has been applied to the same data. The results are roughly consistent with the previous search, but show important differences (which tells you how sensitive the current evidence is to how you slice the data.) Here are the consequences of the new result compared to the old, illustrated in the two figures late in the post:
- A new exclusion region, not only above 125 GeV/c2 as they had before, but now also below it, from about 117.5 GeV/c2 to 120.5. Again, this is very good news if you think there is a Higgs somewhere around 125 GeV/c2, because it is a lot more persuasive that you see a Standard Model-like Higgs at 125 GeV/c2 if you can say more convincingly that you don’t see it anywhere else.
- The significance of the Higgs hint in the data actually stayed almost the same with the improved techniques, whereas if there had been a real signal there before with the older techniques, you would have expected an increase in the significance. We now see 2.9 standard deviations [or “sigmas”] locally (i.e., without look-elsewhere effect) and 1.6 with the look-elsewhere effect (versus the previous values of 3.0 locally and 1.8 with look-elsewhere accounted for.) But this is because the size of the excess went down a little.
- The collisions with two photons were divided, as was the case in the CMS January update, into those that have two jets and those that don’t. (Actually, to be precise, the division separates out those that have jets that are consistent with the process p p > q q H [“vector boson fusion”] — see this article on Higgs production — and you must remember that a large and extremely uncertain fraction of these are actually from p p > H [“gluon fusion”] with two radiated jets.) The new result shows that the majority of the 2.9 sigma excess is now coming from the handful of two-photon-two-jet events; this makes the significance subject to large fluctuations with future data, so don’t be surprised if it moves around a lot in the next update.
- BUT the new CMS analysis puts its best value for the Higgs mass at 125 GeV/c2, whereas previously it was at 123.5. The ATLAS result has put its best value a bit above 126. Such shifts are not unexpected, and we’ve been waiting for them, as I have emphasized (here and here). Of course a shift could go in either direction, and one reason for my caution about the LHC results has been that it was perfectly possible the shifts would go in the wrong direction, pushing the ATLAS and CMS results apart and causing the entire case for the Higgs at 125 to collapse. But this shift is in the right direction. The ATLAS and CMS results are now in more accord, favoring values of the Higgs mass that are closer together than before.And that too is good news for the hypothesis of a 125 GeV two-photon Higgs signal.
So I think one can say that the concern was justified; a large shift was indeed quite possible. But fortunately for the evidence, the current seismic shift at CMS created stronger, not weaker, alignment with ATLAS.
And so, even though the evidence at ATLAS went down somewhat, despite more analyses being added, and the evidence at CMS didn’t increase as it might have been expected to, given the better two-photon analysis — the evidence is overall a bit more plausible in my book. While each experiment sees reduced evidence, collectively their evidence is more consistent than before with a Standard Model Higgs at 125 GeV — and they have improved evidence against a Standard Model Higgs appearing anywhere else.
To this one must add the new Tevatron data, with care, since DZero and CDF don’t see the same thing, and since the significance isn’t large, and since (see the Fermilab talk on the CDF result, which has far more information about this than the Moriond talk — note especially the pages around page 56) the CDF excess is driven larger than expected by a small number of events. My point of view is that one should simply say that Tevatron data roughly supports the LHC evidence and leave it at that.
All in all, my confidence in there actually being something real at 125 GeV has gone up. I have been heard complaining about the lack of exclusion below the 125 GeV region and the fact that ATLAS and CMS were far enough apart that a shift of their best-estimate of the putative Higgs mass might easily make them inconsistent. Now we have some nascent exclusions from both experiments, and the best estimate of the Higgs mass at CMS has indeed shifted toward ATLAS. So — the game isn’t over, but it looks a bit more like a winner to me now than it did before.