In April, the Collider Detector at Fermilab (CDF) researchers reported hints that particles not predicted by the standard model of particle physics cropped up in collisions that produced a W boson — a particle associated with the weak nuclear force — and two jets of other particles. Adding more data strengthened their case, the researchers reported in May, setting the particle physics blogosphere abuzz and sending theorists scrambling frantically for their pencils.
But now, researchers at the Tevatron’s DZero experiment say they see no sign of CDF’s signal.
“We copied their experiment as closely as possible, to remove any potential bias,” said DZero spokesperson Dmitri Denisov at Fermilab. “Our result is no, we do not confirm it. We don’t require any new particles to describe what we see.”
The disagreement is over a possible particle weighing 145 gigaelectronvolts, or GeV (because Einstein’s famous E=mc2 equation says mass and energy are interchangeable, particle physicists often talk about particles’ masses in terms of their corresponding energies).
When the CDF collaboration analyzed collisions between a proton and an antiproton, they saw an unexpected ‘bump’ in the data above a relatively smooth background. They interpreted the bump as a particle with a mass of 145 GeV that decayed into two jets.
|CDF's data describing a W boson and two jets showed|
an unexpected 'bump' in the data (blue line) around
145 GeV, hinting at the existence of a new particle.
Such a particle has no place in the standard model of particle physics, which has been wildly successful in most of its predictions.
“There are no known particles with mass of 145 GeV, and that’s why everyone got excited,” Denisov said.
Denisov thinks the discrepancy could come from differences in the way each experiment detects jets, or in the way they simulate the standard model. The researchers can’t compare their results to the standard model directly, he explained — they have to use so-called generators that approximate the standard model.
“What we really see is disagreement between the generators,” he said. “It might be that something is not perfect in the CDF modeling here.” The DZero results (.pdf) were released online today and were submitted to Physical Review Letters.
CDF and DZero have only disagreed a handful of times over the past 10 years, Denisov said. In more than 500 joint papers, he could think of only two or three significant disagreements. The two teams plan to form a joint task force to figure out exactly what went wrong with the 145 GeV bump.
The Tevatron is scheduled to shut down in September, so this may be one of the aging particle collider’s last shots at finding something spectacular. But Denisov remains hopeful.
“The end of the Tevatron data operations is not the end of Tevatron results,” he said. Data analysis will continue for at least five years after the collider shuts down. “We have a huge pile of data, billions of events, and they’re unique events…. From that point of view, I’m still very hopeful that there will be more exciting results from Tevatron that will be confirmed.”