Anomalies at Fermilab

By Sean Carroll | April 6, 2011 10:47 am

The Tevatron accelerator at Fermilab is shutting down soon, for some unavoidable reasons (the LHC is taking over) and some frustrating ones (we’re out of money). But there may be life in the old beast yet; a couple of intriguing anomalies have particle theorists raising their eyebrows in charmingly understated excitement.

Two different anomalies are getting attention right now. One, which has been around for a while but doesn’t seem to be going away, is a forward-backward asymmetry in top quark production. Unlike the LHC, which just smashes protons together, the Tevatron has a proton beam and an antiproton beam. Intriguingly, when collisions produce a top-antitop pair, they seem to be preferentially produced in the direction of the protons rather than the antiprotons. If you want a popular-level account, here’s Ron Cowen at Science News, while Jester gives you the technical details at R├ęsonaances. I’ll just show you a pretty picture; the horizontal axis is “forward” cross section, while the vertical axis is the “backward” cross section, both in units of the Standard Model expectation. The center of the plot is where we should be, and as you can see we are just a bit off.

A completely different anomaly seems to have just cropped up, this time in collisions that produce a W boson as well as two “jets” (particle-physics speak for “bunches of particles typically associated with the production of strongly-interacting stuff). Once again we have explanations in the MSM and on the blogs: Dennis Overbye at the NYT, and Flip Tanedo at US LHC Blogs. What happens here is that you just measure how many events you see as a function of how much energy is in the jets. Then you look for a “bump,” which as John has taught us is often a signature of a new particle that has been produced and then quickly decayed. Do you see the bump?

The colorful histograms are the various expected Standard Model backgrounds, while the bars are the data. The new bump is just to the right of the red peak labeled “WW+WZ.” (This means that we produced one W boson, as well as another W or Z that decayed into two quarks that became jets.) Not easy for the human eye to pick out, but if we subtract off everything but that red peak, here’s what we’re left with:

See it? In case your eye needs guiding, the CDF collaboration has helpfully provided a version with a blue histogram representing something unknown and bumplike (i.e., that blue thing is not predicted by the Standard Model).

(Thanks to Michael in comments for pointing out the existence of the top version, which I didn’t notice at first, having just stolen the plot from Flip rather than directly from the paper.) It’s a 3-sigma effect, which is where people begin to get excited, although most 3-sigma effects go away. Probably this one will go away, but … you never know until it does, and you wouldn’t want to dismiss the possibility that it won’t.

What could it be? I’m not the one to ask. It’s at 150 GeV, which is an interesting place to have new particles crop up. It is not the ordinary Higgs boson — the bump is far too large. I guess we’ll just wait and see; happily a giant particle accelerator in Geneva is collecting data as we speak.

There is a special seminar at Fermilab this afternoon at 4:00 p.m. Central time to talk about the result. Not sure if there will be anything there that isn’t in the paper.

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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