The Sciences

Anomaly at the Tevatron Might Be Something Real?

Cosmic VarianceBy Sean CarrollMay 30, 2011 7:26 PM

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The Tevatron, Fermilab's mighty but ancient (as these things go) particle accelerator, is scheduled to be shut down at the end of this year. But the old beast might have a trick or two left yet. Way back in April we talked about a couple of lingering anomalies in the Tevatron data that had risen to the level where theorists were intrigued enough to start building models. One of these -- a forward/backward asymmetry in top-quark interactions -- had been around for a while, and was taken seriously by a number of people. The other -- a tiny bump near 150 GeV in the total number of events that produce a W boson and two jets -- was relatively new, and was greeted by a bit of scoffing. The bump credibility took another hit when it was pointed out that it could be explained away by a simple (although completely hypothetical) systematic error -- a miscalibration of the jet energies. Bump-hunting is hard, and experiments near the end of their lifetimes are more willing to share their anomalies than they would be if they knew they were going to keep going, since there's little hope that new data will solve the problem. But there's some hope. The real reason to be patient rather than excited by the bump at 150 GeV was that it was a 3-sigma effect, in a game where most 3-sigma effects go away. In particle physics, we generally take a solid 3-sigma result as "evidence for" something, and require 5 sigma -- a much greater deviation from the expected numbers -- to declare something a "discovery." More data are now in! This is from the CDF experiment at Fermilab, as reported in a conference talk by Giovanni Punzi (pdf), and shared worldwide by Jester at Résonaances. There's a reason why I mentioned Résonaances among the physics blogs above -- it's unquestionably the go-to place for new results in particle physics. And the anomaly is now -- almost five sigma! It didn't go away with more data, it became more prominent. It would be very hard at this point to simply attribute it to an energy miscalibration or something like that; if it is a systematic error, it's a subtle one. But it doesn't look like an error; it looks like a signal.

Of course, it's still very possible that it will go away. These things usually do. But when an interesting result is pushing five sigma, it's perfectly okay to get a bit excited and start wondering what's going on. One of the nice things about this bump is that it's not very hard to come up with models that can explain it -- all you need is a neutral boson, similar to the well-known Z boson of the weak interactions, with a mass near 150 GeV. This kind of idea is so well-known in the trade that it already has a name -- the Z' boson, imaginatively enough. Except it's not that simple, of course -- where would be the fun? When you start mindlessly adding new particles to the Standard Model, you have to check consistency with all sorts of experimental constraints. In particular, a naive Z' boson would sometimes decay into leptons as well as quarks (the jets mentioned above). In that case, it would have been seen long ago in LEP, the electron-positron collider at CERN that previously lived in what is now the LHC's tunnel. So what you really need is a "leptophobic" Z', one that decays into quarks but not into leptons. Or something along those lines, or something completely different. See Résonaances once again for the lay of the theoretical land. Yes, there are possible explanations within supersymmetry; and yes, there are explanations that have nothing to do with supersymmetry. If this is real -- still a very, very, big if -- it's the beginning of the "beyond the Standard Model era" in collider particle physics. Things aren't going to snap into place overnight; there will be false starts, mysteries, and sudden epiphanies. That's where the real fun is in science. Update: Note that the very preliminary word from the LHC is that they don't yet see the same bump that CDF does. But from a glance at the figure it doesn't look like they have nearly as much data yet, so that's probably not surprising. The LHC has seen incredible jumps in luminosity recently, however, so they should be able to do a proper check before too long.

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