By Eugenie Samuel Reich of Nature magazine
Research groups at the Tevatron, the proton-antiproton collider at Fermilab in Batavia, Illinois, have reached starkly different conclusions about a possible sighting of new particles beyond what is expected under the standard model of particle physics.
In April, researchers on the Collider Detector at Fermilab (CDF) experiment reported tentative evidence that particles not predicted by the standard model had surfaced in collisions that produced a W boson--a particle of the weak nuclear force--and jets of other particles. In May, they released data strengthening the case for the novel particles, and theorists have submitted at least a dozen articles to the online preprint server arXiv trying to explain them.
But today, researchers on the independent D0 experiment, also at Fermilab, announced that their data do not confirm the signal. "The result is not good for the CDF. We are not confirming the signal. We just see nothing," says Dmitri Denisov, spokesman for D0, which released its results online today.
Disagreement between the CDF and D0 is rare. Denisov estimates that of the roughly 500 papers produced by the two experiments over the past decade, there have been only two or three significant disagreements. As spokesman for D0, he's naturally more confident in its result, and he suspects that something may be wrong with the way the CDF modeled background events from which its signal was extracted.
But Rob Roser, a spokesman for the CDF, says the collaboration used state-of-the-art techniques for their analysis and held a careful review, as he believes D0 also did. The next step will be for the two teams to sit down together to compare each other's analyses, plot by plot, he says. "This is a hard problem and it will take a while to tease out," he adds.
Although the Tevatron is due to shut down in September, data analysis will continue, and Denisov says that there should already be enough data to resolve the conflict by comparing the analyses.
The anomaly seen by the CDF was an excess in the number of events in which proton-antiproton collisions produced a W boson and two jets of other particles. The excess, evident as an unexpected bump in the energy spectrum of the jets (blue bump in graph), pointed to the production of new particles, not predicted by the standard model, that decayed to produce the extra jets. The statistical significance of the excess is about 4.1 sigma, only just short of the 5 sigma needed to claim a discovery. D0's spectrum in the same energy range shows no bump, Denisov says, which corresponds to a 4.3 sigma negative result.
Teams on the Large Hadron Collider at CERN, Europe's particle-physics lab near Geneva, Switzerland, have also been checking their data, and have so far failed to see anything to support the CDF's sighting. But Guido Tonelli, the spokesman for CERN's Compact Muon Solenoid (CMS) experiment, cautions that the amount of data collected by the LHC is still too small for a definitive statement to be made. "We want to be sure we will have significance," he says. He expects the CMS to have enough data to test the CDF sighting by the end of June, and it will report its results at the European Physical Society's conference on high-energy physics in Grenoble, France, at the end of July.
In the meantime, the conflict between the CDF and D0 is likely to create headaches for theorists, says Pran Nath, a theoretical particle physicist at Northeastern University in Boston, Massachusetts, who has proposed an extension of the standard model to explain the CDF result. "If experiments are ambiguous we cannot rely on them," he says. Nath adds that, if the CDF result does turn out to be wrong, he will be disappointed. "You might say wild goose chase is the right way to put it."
Scott Thomas, a theoretical particle physicist at Rutgers University in Piscataway, New Jersey, says that he will be looking closely at the D0 result. He has already looked at the CDF's analysis and thought the team did a careful job. "It's a very difficult measurement, but it seemed like they did everything they could," he says.
He adds that even if the CDF result turns out to be wrong, the theoretical work done to try to explain it has highlighted several new ways that physics beyond the standard model can be tested in collider data, which might come in useful at the LHC. "It's still been worthwhile," he says.
For his part, Denisov thinks that it is time to put theoretical speculations on hold and to let the experiments work out their differences. "It's unreasonable to talk about new physics now," he says. "It's back to the drawing board."
This article is reproduced with permission from the magazine Nature. The article was first published on June 10, 2011.