A joint Fermilab/SLAC publication

Scientists continue to see puzzling behavior in top quarks, reaffirm strength of Tevatron experiments


Fermilab's CDF detector. Image: Fermilab

The Tevatron may be shut down for good, but – as evidenced by the catalogue of results presented at this week’s Rencontres de Moriond conference – the collider’s experiments still have plenty to say.

In some areas, the Fermilab experiments still hold the advantage over those at the higher-powered Large Hadron Collider at CERN.

Fermilab’s CDF experiment announced today that physicists continue to see possible signs of new physics in their ongoing study of the heaviest type of quarks, top quarks. Last year, scientists at CDF and its sister experiment, DZero, detected a difference between the ways top quarks and their antiparticles behaved when springing from collisions at the Tevatron. Top quarks preferred to propel in the forward direction, and anti-top quarks preferred the opposite. The level of asymmetry they found went against current predictions from the Standard Model of particle physics.

“It forces people to rethink this kind of physics and the way these predictions are made,” said Giovanni Punzi, co-spokesperson of the CDF experiment.

After about doubling the dataset they were using, CDF scientists repeated the study. The results held fast. The asymmetry in question measured at 0.296 ± 0.067 percent, when theory predicted it should fall at 0.100.

Although CDF physicists will continue to refine their research in this area, “this will be the central number from the CDF experiment,” said physicist Sandra Leone of Italian laboratory INFN, who presented the result.

“I think it is interesting,” Leone said. “It’s not a statistical fluctuation. In general, there is good consistency between the two [Tevatron] experiments.”

DZero scientists plan to update their study in 2012.

The CDF and DZero experiments hold the world title in making this type of measurement because of the way particles collided in the Tevatron. There are two ways to produce a top quark and an anti-top quark in an accelerator like the Tevatron or the LHC, but only one of these ways leads to forward-backward asymmetry. The Tevatron uses the right recipe 85 percent of the time; the LHC, only 15 percent.

Tevatron experiments showed their strength earlier in the week presenting their latest measurement of the mass of the W boson, a result they first announced in a press release. Measuring the mass of the W boson, more than any other elementary particle, gives scientists important insight into the properties of the coveted Higgs boson. It also puts limits on a multitude of new physics models. In an effort that took about five years to complete, CDF and DZero identified the mass of the particle with 0.02 percent precision.

The Tevatron experimentalists gained their clout in making this particular measurement from their thorough understanding of their detectors. Unlike scientists at LHC experiments, those on the DZero and CDF experiments have had decades to learn their detectors inside and out. The detectors are less complicated than those at the LHC, and collisions at the Tevatron weren't as messy as proton-proton collisions in the LHC. It is unclear whether experiments at CERN will be able to make such a precise measurement in the future.

Last but not least, the Tevatron experiments contributed to the search for the Higgs boson this week by announcing possible hints of the particle in a range compatible with results from the LHC.

Overall, LHC experiments hold the advantage in collider searches, and they represent the future of the energy frontier. But in the years to come, Tevatron scientists will continue to contribute their distinctive voices to the conversation.

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