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 Résonaances. 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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  • RememberTheHERAhighQsquaredExdess

    That is not “a bump”. It is a tail of a distribution that falls more slowly than the simulation says it should. A proper bump (as I believe as HEP physicists would call it) would be an unassailable feature that goes up and back down again before performing a background subtraction. (Note that the subtracted background is ~10 times bigger than the candidate signal). Let’s just call it “an enhancement”.

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  • Steven Colyer

    New scientific results are always cool, but let’s never forget that 3-sigma results are not 5-sigma, and that even though all 5-sigma started out as 3-sigma, the vast majority of 3-sigma results went nowhere. They can gussy up these results all they want, but it remains sad to see the Tevatron at Fermilab “fishing” if you will, if that’s all this turns out to be.

    Careers are in jeopardy here, and unemployment lines are open, by fiat and by law.

    I didn’t vote for Bush. Don’t blame me.

  • jery

    I’m looking forward to the end of 2011 when hopefully Fermilab will stop bothering us with more of these death rattles.

    It’s pathetic trying to steal LHC’s thunder with such lame tricks…

  • Michael

    Hi Sean,

    the CDF paper *does* include a subtracted mass spectrum without the blue bump. See the upper right-hand plot in Figure 1. 😉


  • Sean

    Michael, you’re completely right, I was being sloppy. I’ll fix it.

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  • Steven Colyer

    @#4 jerry – I hear you and agree with your passion, but IF true, this would in fact be a very significant “find.” Correct me if I’m wrong, but the Higgs is pretty much expected among 90% of Theoretical Physicists. It wouldn’t surprise me if Tevatron found it years ago at Fermilab, and years of data analysis means it is just coming to light now.

    But the real reason we have the LHC is two-fold, first to find new Physics, should such new Physics exist, and second, to find supersymmetric (SUSY) particles, should they exist. Higgs is what is sold to the public. It’s almost a given we will find them at the LHC, should they exist. Yes?

    Or maybe we’ve hit “the desert”, and the LHC will do nothing more than confirm Tevatron’s many wonderful results to date, which would be an accomplishment in itself. And the VLHC and SVLHC, should they ever be built, may do no better.

    Have a nice day. 😉

  • Joseph Smidt

    Let’s hope this is real. I think the best thing theorists could hope for is something unsuspected.

  • Chris

    I’d like to hear John Conway’s view on this.

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  • Adam


    That’s a ridiculous comment. These aren’t “fishing” expeditions – this is how science works. The Tevatron has generated tremendous amounts of data which ought to be analyzed, and any anomalies must be documented and described. No one at CDF is claiming a discovery. They are claiming an anomaly in the dijet invariant mass at the 3.2 sigma level that has been verified against all reasonable measurement pathologies suggested over the last year and a half.

    After a year and half of verifications and cross-checks the group performing the analysis has produced an impressively robust result. I could definitely still be a statistical fluctuation, but this was not a quick and dirty analysis.

    In addition, this analysis was presented over the first ~4 fb^-1 of data, and CDF has ~7 fb^-1 of data recorded. Between now and the shutdown (assuming roughly September), CDF should record a decent bit more. The analysis will definitely be updated with roughly twice the data, so if it is a statistical fluctuation, it should be a bit clearer. At the seminar today, Viviana claimed that they will begin working on the current dataset soon, and that they should be able to finish the update on the order of weeks. Beyond that, a representative from D0 said that they would also be able to produce an analysis with similar cuts within a few weeks.

    I’m certain that CMS and ATLAS will also have their first attempt at examining this final state very soon. Their initial attempts will have relatively low statistics, so they may not be able to give independent verification immediately. Also, if the bump really is a new particle, the production cross section at the LHC isn’t automatically much better than the Tevatron; it all depends on whether or not the new particle would be created via quark-quark annihilation or gluon interactions.

    I’m personally impressed with this analysis. Many experimental physicists seem to have a bit of fear about excesses because finding one means at least a year of verifications before you can even think about publishing. When this group found this anomaly in the course of doing a routine diboson measurement, they could have easily found a way quash it, but they instead did the scientifically honest thing by really following it to the bitter end.

  • Adam

    @Steven Colyer:

    It’s fairly clear that this excess can not be a Higgs, at least not one that is normally talked about. The Higgs on that mass would decay mostly to b quarks. They checked the b-tag rates of the jets in the bump region and compared those rates to those in the sideband regions, and they found them to be statistically consistent with each other. In addition, there have been WH searches that look in a similar region but with cuts optimized for finding jets resulting from a Higgs decay, and they turned up nothing.

    The options are pretty much statistical fluctuation or exotic physics of a non-standard variety. It sounds like the major idea being thrown around was technicolor since technirhos and technipions decay preferentially to light flavor quarks. I’m sure that the theory community will think of plenty of possibilities.

  • Manmohan Dash

    A few things are wrong here with some remarks (and misunderstanding) I point out one misunderstanding: the bump is a tail moving slowly as per Monte Carlo.

    “The bump is clearly a Gaussian so we can’t say it’s a tail that’s moving slowly because of simulation”

    1. A Gaussian is never a tail and a tail is never a Gaussian. Gaussian can happen on top of tails, they just like to sit anywhere they are created. If an Australian baby is borne in woods it may just sit on the top of a Kangaroo’s tail.
    2. To say the bump is not predicted by standard model is to say this bump is not included in Monte Carlo. SO why would MC have a say here. It’s all experimental data.
    3. A 3-sigma effect is not something we wish “go away” (as pointed out) It could be real, we need more data and more detailed study. Period. It’s not just that a 3-sigma effect would grow into a 5-sigma effect; it’s also that it’s never going to go below. SO why would anyone say 3-sigma is not important? It’s just that most people would put it under carpet and never bother again.
    4. This bump could be an enigma, playing hide and seek with “slice and dice” not necessarily getting cut off.

    Now I would like to point out that the bump is actually a double Gaussian not a single. See, one data point is off from the Gaussian fit at the same level of the peak of the Gaussian. SO it’s better fitted with two Gaussians, which would just mean that the bump has two resolutions, a usual detector effect. But there is also one more thing to note here the peak of the green; it’s slightly to the left of the mean of the red peak.

    And about discovering a new fundamental force of nature, are we prepared theoretically or experimentally for such a scenario? I had pointed out we may have more 4 fundamental forces of nature, in an article of mine 3 years ago. That’s the real power of LHC.

  • Steven Colyer

    @14 – Adam

    Thanks, that was the answer I was looking for. Technicolor, eh? Very cool and intriguing if so. What is the current state of the art on technicolor? Is it the “walking” or “minimal walking” version, or something else?

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  • J Gregory Moxness

    Very exciting, I have been waiting for these results for over 10 years. Please see equation 18 in my paper which predicts a Higgs sector boson at 148 GeV.

  • Brian137

    It sounds like the major idea being thrown around was technicolor since technirhos and technipions decay preferentially to light flavor quarks.

  • Chris


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  • Mark

    Shouldn’t there be a Viagra ad on this page somewhere?

    The nuclear physics community caught heck from the high energy community in the wake of the (personally deplored) pentaquark fiasco. I guess it’s HE’s turn to get some back.

    At least no one has uttered the phrase, “God particle” here. Yet.

  • DocPossible

    Oh dear :

    Just what the HEP community needs. More breathless reporting of 3 sigma bumps, with the caveat that it’s 99.9% probable to be a real result.

    This seems appropriate somehow :

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  • chungdingyu

    The new elementary particle of Fermilab at 144 GeV is the hidden lepton outside of the three lepton families in the standard model. Unable to exist alone, the hidden lepton exists in the pair mixture of the hidden and the anti-hidden leptons, resulting in producing the dijet of leptons along with W boson as observed.

  • Marion Delgado

    Aren’t they recreating the Big Bang to find the God Particle™?

    (Someone always has to ask that, surely :) )

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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] .


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