Scientific Method / Science & Exploration

Tevatron: top quarks may indicate new particle, need for new physics

Pairs of top-antitop quarks produced in Tevatron collisions aren't behaving …

Even as it's slated for retirement, Fermilab's Tevatron particle collider may be providing its successor, the Large Hadron Collider, with directions to some new physics. Recently, researchers with the Tevatron's CDF detector started discussing results that they submitted to the arXiv at the end of last year. The draft paper suggests that top quark pairs that originate in the proton-antiproton collisions at the Tevatron are showing an odd asymmetry, one that might be explained by a particle that should be detectable by the LHC. The big surprise: it's not one of the ones we expected to find there.

Like many other particle physics papers, the key to this one is separating out the relevant events from the background noise. In this case, the events researchers were looking for were collisions that produced top and antitop quarks (the top quark is the heaviest of the six quarks; only the lightest quarks make up the matter we're familiar with). Typically, these quarks tend to leave the collisions with a slight bias: top quarks prefer to travel in the direction of the proton, with the antitop going backwards relative to the proton. The bias is slight, but the Tevatron now has produced sufficient data (5.3 inverse femtobarns) that over 1,200 events that appeared to involve two top quarks were identified.

The authors then calculated the degree of asymmetry predicted by Quantum Chromodynamics. From the detector's perspective, they didn't. The observed asymmetry "has less than 1 percent probability of representing a fluctuation from zero, and is two standard deviations above the predicted asymmetry." But they also ran the calculations against the predictions based on the perspective of the quarks themselves; these showed an enhanced asymmetry, but still within the bounds of error of their measurements. So, based on this analysis, the results were somewhat mixed, but point to something going on.

But there was something else apparent in the data when analyzed from the quarks' reference frame. At low masses (indicating the quarks were part of a lower-energy collision), the data closely matched the calculated asymmetry. However, as the mass rose to closer to the Tevatron's limit, the divergence between the data and predictions increased. At the highest mass, the difference between the two was over three standard deviations, typically considered a significant result in these sorts of experiments. This suggests that whatever is driving this asymmetry, it's only produced in very high energy collisions.

Combined, the data strongly suggests that there's something strange going on with top-antitop quark production in higher energy collisions. The obvious question is what. The leading theoretical candidates aren't the usual sorts of things people talk about finding with the LHC, like a supersymmetric particle or the Higgs boson. Instead, the possibilities seem to be extra dimensions or exotic particles. "A number of theoretical papers suggest interesting new physics mechanisms," the authors note, "including axigluons, diquarks, new weak bosons, and extra-dimensions that can all produce forward-backward top-antitop asymmetries."

We had looked for axigluons, which are part of an alternative to the Standard Model, at the Tevatron previously, and did not come up with any. Diquarks shouldn't exist as independent particles, as far as we know. So, the possible causes of this asymmetry seem to be really exotic physics, pretty far removed from what we're currently focused on.

It will take the LHC a few years to build up to the amount of data that has been produced by the Tevatron, so we're unlikely to be able to detect a similar asymmetry there any time soon. However, LHC collisions are significantly more energetic than those at the Tevatron, meaning that it might be possible to detect the particle directly. In the meantime the Tevatron has more data that has yet to be analyzed, so it may provide a clearer picture soon.

Of course, there's still the chance that these results won't hold up as more data comes in. Still, it would be nice to think that the Universe has a bit of a surprise for us.

Although this paper has yet to go through peer review, the huge number of authors involved suggests that it's likely to be pretty reliable, as are other drafts in the field of particle physics.

Listing image by Particle Zoo

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