No new particle from second detector

In the search for a new elementary particle that would transmit a previously unknown force, it’s been an 11-day roller coaster ride at Fermilab’s Tevatron, the premiere U.S. atom smasher. And the ride’s not over yet.

At a conference in Blois, France, on May 30, Giovanni Punzi of the University of Pisa and the INFN in Italy raised hopes that one of the two particle detectors at the Tevatron, known as CDF, might indeed have discovered a new elementary particle. With the analysis of additional data, Punzi announced that a previously revealed sign of a possible new particle was unlikely to be a statistical fluke.

The team posted its updated analysis in early June. The CDF researchers base their findings on Tevatron collisions producing a small but puzzling excess in the number of pairs of particle jets at an energy of about 150 billion electron-volts.

Although the new analysis still doesn’t meet the criteria for proof, it comes close, Punzi said. But he also cautions that the small excess might simply indicate that his team has underestimated the high background of particle jets that come from ordinary particle physics.

On June 10, researchers using the other detector at the Tevatron, known as DZERO, put a damper on the CDF results. Analyzing the data in the same way that the CDF team had, the DZERO team finds no sign of a jet excess, says one of the experiment’s spokesmen, Stefan SÓldner-Rembold of the University of Manchester in England. The team’s new paper, posted June 10, details the findings.

The DZERO team hasn’t analyzed as many collisions as the CDF team did, but SÓldner-Rembold said he was confident of the DZERO nondetection, especially because he and his collaborators could reproduce the CDF results only if the researchers inserted “fake data” into their analysis.  

The simplest explanation for the discrepancy is that the CDF collaboration did underestimate the signal produced by ordinary, standard-model interactions. But Punzi and his collaborators aren’t ready to throw in the towel. “If we combined our results [with those from DZERO], the statistical significance of the excess will be lowered,” he acknowledges, but adds that it’s unclear how strong the disagreement is and whether the discrepancy falls within reasonable statistical fluctuations.

The teams have agreed to create a task force that will study the issue.

Resolution of the conflict now belongs to the Large Hadron Collider, the world’s most powerful atom smasher, near Geneva, asserts Fermilab theorist Christopher Hill. “If they see no new phenomena that we would expect if the CDF result was real … then it is an error.”

In the meantime, a separate puzzle that might also indicate a new elementary particle remains. The CDF team has found evidence — although not proof — that the standard model of particle physics cannot explain why top quarks travel much more often in the direction of the Tevatron’s proton beam than the antiproton beam. SÓldner-Rembold said the DZERO team expects to weigh in on that puzzle in the next few weeks.