The Sciences

Science! It Marches On

Cosmic VarianceBy Sean CarrollDec 13, 2011 1:39 PM


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The news from Geneva this morning is in. Essentials: what we're seeing is pretty consistent with the existence of a Higgs boson around 123-126 GeV. The data aren't nearly conclusive enough to say that it's definitely there. But the LHC is purring along, and a year from now we'll know a lot more. It's like rushing to the tree on Christmas morning, ripping open a giant box, and finding a small note that says "Santa is on his way! Hang in there!" The LHC is real and Santa is not, but you know what I mean. Here are the technical write-ups from ATLAS and CMS. For stories and some live-blogs, check out Philip Gibbs, Matt Strassler, Aidan Randle-Conde, Ken Bloom, or Jester. Or if you just want the bottom line sigmas, Jim Rohlf provides them. ATLAS gives 3.6 sigma local significance, 2.3 sigma global significance; CMS gives 2.6 sigma local significance, 1.9 sigma global significance (although CMS points to about 124 GeV, while ATLAS points to about 126, which might be important). The difference between "local" and "global" is that the first asks "if I were only looking at this one point in parameter space, how surprising would the result be?", while the latter asks "what is the chance I would find this kind of deviation somewhere in parameter space?" Nominally the global significance is obviously more relevant, although one could argue that we have good reasons to expect that the Higgs is actually lurking right there, so the local significance isn't completely cheating. Let's put it this way: if we were testing a theory that everyone thought was wrong, rather than one that everyone thinks is right, nobody would take these results as strong indications that the idea was correct. We have a strong theoretical bias that the Higgs exists and is somewhere close to this mass range, so it's completely reasonable to think that we are seeing hints (tantalizing ones!) that it's there, but wait-and-see is still the right attitude. Here are the simplest plots I could find. First the full analysis from ATLAS (zoomed in on the interesting region), via Philip Gibbs's blog.:

Then from CMS, via Ken Bloom:

These plots are complicated because they're trying to tell you two things at once. The black curve is the data, the green/yellow bands are the expected ranges of the data at 1 sigma and 2 sigma. If all you want to do is ask whether we can exclude the Higgs in a certain range, just check whether the black band is below the value 1. But if you want to say you have evidence for the Higgs, you need the black line to wander above the yellow band (or higher, if you want more than 2 sigma [and you do]). So ATLAS sees something at 126 GeV, CMS is at least consistent with 123-124 GeV (although it doesn't see much at 126). As Sarah Kavassalis puts it, the real message today is that the LHC is working great. 2012 will bring another year of data, hopefully at even higher luminosity (so many more total events). The Higgs has been around for 13.7 billion years, it will still be there tomorrow.

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