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[personal profile] maxwells_daemon
Today was an exciting day. Finally we can see our our new Higgs search results public, and we can see what CMS have seen and compare. As everyone keeps emphasising, we haven't discovered the Higgs. ATLAS has seen a fairly large "excess" and CMS have a couple of moderate excesses, one of which matches ours at a mass of 126 GeV.

You might want to stop reading here, as it's all science (too much science for LJ?) from here on in...

The two most telling plots are these:

which give the significance (actually (im)probability that there is no Higgs at each mass) of the ATLAS and CMS results, separated into different channels (ways the Higgs can decay) and combined (in black) within each experiment. Those plots are the ones to look at to answer the question "did we see anything?", so probably most interesting at this stage (nothing to do with the fact that I worked most on this one for ATLAS, though I was a small cog in an enormous machine).

If this is a hint of the Higgs, then ATLAS was rather lucky to get such a large signal (though it is still not so large to be suspicious). If this is a statistical fluctuation, then ATLAS was really unlucky (as our spokesperson put it "if it's background, it will be really difficult to kill"). CMS's smaller signal was neither lucky or unlucky.

These numbers have to be corrected for the "look-elsewhere effect" (the fact that we are looking for the Higgs at many different masses and if we look often enough we are bound to see a statistical fluctuation) - unfortunately this was not included in all the internet rumours that prefigured the announcements, so today may have been a bit of a let-down for some people. There is actually a philosophical problem with that correction: should we count everywhere we looked, or just the area that we are still looking (and haven't excluded). ATLAS and CMS actually took different options in their conclusions (probably because it doesn't make much difference for ATLAS, so they can be conservative, while CMS have nothing very exciting to say if they follow us).

So ATLAS/CMS see a significance of 2.5/1.9 sigma (0.6%/3% probability that this is just chance, not a Higgs) or 2.3/0.6 sigma (1%/27%) for a Higgs at around 124/126 GeV, depending on how you calculate the look-elsewhere effect. It is quite incorrect to try to combine these numbers without detailed study (several of the uncertainties are correlated between ATLAS and CMS - it took months for ATLAS and CMS to combine their previous Higgs results), but that hasn't stopped the blogosphere. If you were to naively combine the larger set of numbers you reach 3.1 sigma (0.1%), which in our field is sufficient to claim "evidence" (still well short of 5 sigma required for an "observation"). But I couldn't possibly comment.

For me, the fact that we see an excess at the same place in so many channels (3 channels in ATLAS, 1-3 in CMS, not to mention various sub-channels), makes me quite hopeful that this is something real. Each channel looks for different things (and the two experiments have different detectors and analysis techniques), so it is unlikely to be a mistake. That means it can probably only be a fluke if this isn't really a Higgs. Next year we hope to have lots more data (and at a higher energy), so we should be able to pin it down soon.
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April 2013

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