Remodeling the standard model

Fermilab findings suggest new elementary particle may soon emerge

The Large Hadron Collider, the world’s most powerful atom smasher, may be only months away from finding a new elementary particle — a sign of a new force in nature — recent studies suggest.

The studies focus on the top quark, the heaviest of the six quarks, which are the fundamental building blocks of nature. Top quarks appear to behave badly when they are produced during proton-antiproton collisions at a lower-energy particle accelerator, the Fermilab’s Tevatron in Batavia, Ill. Compared with what the standard model of particle physics predicts, these quarks fly off too often in the direction of the proton beam and not enough in the antiproton direction.

The Tevatron finding was first reported in 2008, but the results could have been due to chance. A recent report, using additional data, boosts confidence in the result, says Dan Amidei of the University of Michigan in Ann Arbor, a member of the Tevatron’s CDF experiment. For energies above 450 billion electron volts, 45 percent of the top quarks travel along the path of the proton beam while only 9 percent are expected to do so, the CDF team reported online January 3 at Online March 10, the team reported additional evidence of the top quark’s puzzling directional preference after examining the paths of quarks generated by a different set of particle interactions.

There’s only about a 0.04 percent chance, for energies greater than 450 billion electron volts, that the top quark’s apparent directional preference is a fluke, Amidei notes. Although that percentage still doesn’t meet the threshold for what physicists consider proof, the Tevatron’s other experiment, DZero, has recently found hints of the same asymmetry, using different data and techniques.

Assuming the effect is real, the directional preference suggests the existence of a new elementary particle, not predicted by the standard model. The particle could be the messenger of a new type of force that interacts with top quarks — along with their antiparticles — in such a way as to cause the asymmetry.

Researchers are truly abuzz about the possibility that the proposed particle could be within the grasp of the Large Hadron Collider, near Geneva. If the asymmetry arises from some new type of particle just slightly too massive for the Tevatron to produce, the LHC could produce the particles directly, possibly by the end of 2011, says Markus Schulze of Johns Hopkins University.

Finding such a particle, Schulze says, “would be a beautiful and delicate signal of physics beyond the standard model.”

Theorist Moira Gresham of the University of Michigan says many explanations for the Tevatron asymmetry predict new particles that could be seen at the LHC — even though it is now running at only half its maximum energy. She and her colleagues posted a paper on the proposed properties of several such particles at on March 18, one of a flurry of articles that have appeared in response to the Tevatron finding.

Among the possible particles is a heavy cousin to the massless gluon, the particle that binds quarks together. Another idea, noted in a paper posted at on April 1, posits the existence of a particle called the Z’ boson, a heavier version of the Z boson, a messenger particle for the weak interaction. The Z’ boson would transforms one type of quark into another, leading to the asymmetry.  

“Particle physicists know that the standard model is incomplete,” Gresham says. “We’ve been waiting a long time to get some more concrete hints.”

It is indeed possible that the LHC might find a new particle to explain the asymmetry this year, says Aleandro Nisati of the Istituto Nazionale di Fisica Nucleare in Rome, a member of one of the main experiments at the LHC.  “If the machine performs as well as I hope,” he says.

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