The Higgs boson remains ephemeral (no surprise)

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Since the big news in the physics world is this morning’s presentation of the Higgs search results from the LHC, it’s only appropriate that I comment on it here, even though every physics blog in the world will be doing the exact same thing so there will be no shortage of Higgs information out there ;-) In summary: no, they haven’t really found it, but there is a bump around \(\SI{126}{GeV}\) that could represent detection of a Higgs boson. It will take another year’s worth of data to be confident either way.

Here are the plots that were released this morning by ATLAS and CMS, respectively:

ATLAS combined Higgs production limits CMS combined Higgs production limits

The quantity being plotted here is the cross section for candidate Higgs events, which is denoted \(\sigma\), relative to a theoretical prediction, \(\sigma_{SM}\). In other words, the thing on the vertical axis is related to the fraction of collisions in which something that looks like a sign of a Higgs boson comes out. (I’ll perhaps post on this in more detail later; for now see Matt Strassler’s post on Higgs production for a good explanation.) But not everything that looks like a sign of a Higgs boson actually means that a Higgs particle was produced. Quantum field theory allows us to predict how often these events should occur without a Higgs boson being involved, which (after being run through the detector) gives the yellow and green bands. In theory, any excess above those bands is possibly caused by the Higgs. ATLAS has posted a fantastic explanation of these plots that goes into more detail about this.

Naturally, the thing that is piquing everybody’s interest about these plots, and which generated all the buzz leading up to today’s press conference, is the region between about \(\SI{121}{GeV}\) and \(\SI{128}{GeV}\) where the observed number of events (solid line) is greater than the theoretical prediction (yellow band). That’s the potential sign of the Higgs boson being observed. But it’s not very far above the yellow band, which means that it could just as well be a random fluctuation.

Even though a lot of people will be excited about this (and to be fair, it does represent an impressive amount of work on the part of the ATLAS and CMS experiments), I’m not getting too hopeful yet. You may remember that the physics community found itself in a very similar situation about 5 months ago, when ATLAS and CMS released news of a small excess around \(\SI{140}{GeV}\). At the time, I posted the production limit plots, and if you go back and look at them (go on, I’ll wait), they showed nothing more than a small excess of the observed number of canididate Higgs events (the solid line) above the theoretical prediction (the yellow band) — in other words, they looked remarkably similar to the newly released plots shown above. As we know, that excess disappeared a month later. So there’s a very reasonable chance that the same thing might happen this time. But the whole process is statistical, so you never know! I expect that when ATLAS and CMS release their next round of data next year, it will either show that this was a statistical fluke, or it will put the current result on a stronger footing. That’s when it will may be time to get excited.