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.
I’m struck by how wide the mass range of the bump seems to be. The elevation is about the same in the data points from 120 GeV to 160 GeV. It almost looks like a composite of a double or triplet of similar particles decay sequences with similar but not identifical masses (a bit like the W+, W- and Z at ca. 80 Gev and 90 Gev).
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Most 3 sigma results “go away” and “turn out to be nothing” and now, Sean, you are essentially saying the same thing about ~5 sigma results. Not long ago I thought 5 sigma was enough to claim a discovery?
All of this talk of sigma this or sigma that seems almost meaningless at this point since you guys are saying there could always be some confounding factor that could creep into an experiment at any level of sigma. If some subtle systematic effect can always creep in when can you ever really claim a discovery in science: SOME SKEPTIC COULD SAY THAT WHAT YOU ARE SEEING IS DUE TO SOME OTHER EFFECT!
The importance of a 5 sigma result is just that at that point other experimenters take you very seriously and look hard at debunking or confirming your claims. Until your “discovery” has been independently verified in different places by different people with different techniques then, yes, skeptics can say what they will.
So a 5 sigma signal is just a chance to put other experiments on notice that there is something very interesting worth looking at. In contrast, a 3 sigma result is something that you as an experimentalist need to work harder to resolve. Theorists might get excited, but that’s a low bar. Discovery takes a while; 3 sigma and 5 sigma are important milestones on that road, but by no means the end of the process.
@forester: You need 5 sigma to rule out “it’s due to chance”. You need independent confirmation to rule out “it’s due to systematic errors.”
In other words 5 sigma is necessary for discovery, but not sufficient.
147(5)GeV – Excellent! This shifts the center of the data to my 10+ year old prediction (148 GeV) in eq. 18 of http://theoryofeverything.org/TOE/JGM/ToE.pdf.
Eccezionale
Well, this is still same old wrong Jet Energy Corrections. If you will add more data with same crappy corrections, you will get larger bump – garbage in, garbage out.
I have a conspiracy theory here – FNAL looking for ways to get back the funding which is lost when Tevatron was scheduled to stop at the end of this year. I remember same thing at LEP when it was less than a year before closing. Everybody looked for Higgs and found it with 3 sigma at 115-118 GeV – remember Wu?
So, relax and don’t hold your breath. This is a battle for the money. It has nothing to do with physics or science.
invariant mass released today : http://www-cdf.fnal.gov/physics/ewk/2011/wjj/7_3.html
for relaxed #35 : see Fig 5
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