Published online 23 July 2011 | Nature | doi:10.1038/news.2011.436

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Particle collisions hint at existence of undiscovered gluon.

Fermilab's DZero detector has seen some top quarks acting strangely. Reidar Hahn / Fermilab

Newly released observations of the top quark — the heaviest of all known fundamental particles — could topple the standard model of particle physics. Data from collisions at the Tevatron particle accelerator at Fermilab in Batavia, Illinois, hint that some of the top quark's interactions are governed by an as-yet unknown force, communicated by a hypothetical particle called the top gluon. The standard model does not allow for such a force or particle.

The results, presented1 today at the Europhysics Conference on High-Energy Physics in Grenoble, France, could help researchers to understand the origins of mass. According to one theoretical interpretation, a top quark bound by to its anti-matter partner, the antitop, would act as a version of the elusive Higgs boson, conferring mass on other particles.

Regina Demina, a physicist at the University of Rochester in New York, and her colleagues sifted through eight years' worth of particle-collision data recorded by one of the Tevatron's two detectors, known as DZero. Top quarks produced during collisions can fly off in the direction of the accelerator's proton beam or its antiproton beam; Demina and her team discovered that more travel towards the proton beam than is predicted in the standard model of physics. A different model would seem to be needed to explain the discrepancy.

Paired particles

One possible model has been suggested by Christopher Hill, a theorist at Fermilab who 20 years ago proposed how a top quark and its antiparticle could impart mass to the W and Z bosons, particles that carry the weak nuclear force responsible for radioactive decay. The work, updated in 20032, draws heavily on an analogy with some types of low-temperature superconductors, materials that have no electrical resistance at temperatures just a few degrees above absolute zero.

In some superconductors, electrons pair up, bound by particle-like vibrations in the material. The bound electrons limit the range over which the electromagnetic force can act within the material, an effect that in turn imparts an effective mass to nearby photons — particles of light, which carry the long-range electromagnetic force and are normally weightless.

In a similar way, Hill suggests, top quarks and anti-top quarks might pair up throughout the cosmos, bound by a force carried by an as-yet undiscovered particle dubbed the top gluon. "It's as if the entire universe was a special kind of superconductor," says physicist Matthew Schwartz of Harvard University in Cambridge, Massachusetts. The theory explains the origin of mass throughout the universe as a team effort, First, the top gluon would act to make both the top quark and the antitop heavy, just like the force binding electrons in a superconductor makes nearby photons heavy. Then, the top–anti-top pair would itself explain the origin of mass throughout the rest of the universe, conferring mass, for instance, on the W and Z bosons, the carriers of the weak nuclear force. The relatively heavy mass acquired by the W and Z particles limits the range of the weak force, breaking the symmetry between this force and the long-range electromagnetic force that theorists believe exists at very high energies.

In a study posted online3 on 16 June, Schwartz and his Harvard colleagues show that Hill's model could also account for the top-quark asymmetry observed at the Tevatron. The details have to do with the way the up quark, a component of the proton, couples with the top quark in the new theory.

Independent confirmation

The asymmetry observed at DZero is not certain enough to constitute proof of the existence of the top gluon, but it does independently match findings reported4 earlier this year by researchers at the Tevatron's other detector, the Collider Detector at Fermilab (CDF).

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Dmitri Denisov, a spokesman for the DZero experiment, agrees that the results are similar to the directional preference of the top quark seen with CDF. He cautions, however, that the standard model of particle physics is so complicated that it is difficult to accurately describe with equations. The observed top-quark asymmetry is being compared to an imperfect surrogate for the true standard model, so the supposed discrepancy might fall within the uncertainty of the model.

Schwartz's theory is easily testable. The top gluon has a predicted energy within the current range of the world's most powerful particle collider — the Large Hadron Collider (LHC) near Geneva, Switzerland — so it could be found within a year, says Schwartz.

A research team working with the LHC's Compact Muon Solenoid detector reported5 on 21 July that they see no evidence of the top-quark asymmetry. But Schwartz notes that the asymmetry is much harder to see at the LHC than at the Tevatron, because the LHC starts with an intrinsically symmetrical setup — smashing a proton beam into another proton beam — so it's more difficult to discern if the top quark has a directional preference at the LHC than at the Tevatron. "I suspect that you can't rule out anything with this data," he says, "and it doesn't negate any models."

Dan Hooper, a theoretical physicist at Fermilab, notes that the top-quark asymmetry is just one of many cracks in the standard model of particle physics. And although Schwartz agrees that it is unlikely that any one theory will explain all the defects, he says that accounting for the odd behaviour of the top quark would be a promising start.