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Hopes Fade for Monumental Last-Minute Discovery at U.S. Atom Smasher

10 June 2011 3:19 pm
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D0 Collaboration

Dang it! The two-jet mass plot from the D0 experiment shows the peak from familiar W and Z bosons but no sign of the peak expected for the purported new particle.

It would have been the feel-good science story of the year. Two months ago, physicists working at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, reported hints of a bizarre new particle emerging from the collisions produced by the 25-year-old Tevatron atom smasher. That particle would come as a complete surprise and would mark a triumph for the Tevatron, which will shut down for good this year, having been surpassed by a more-powerful atom smasher in Europe. Alas, an independent team of physicists at Fermilab sees no sign of the particle. That suggests that the first team was misled by some unaccounted "systematic" effect in its analysis and that the particle doesn't exist.

"I would probably be more excited if it were otherwise," says Dmitri Denisov, a physicist at Fermilab and co-spokesperson for the 500 physicists working with the D0 particle detector, the team that reported the new, negative result today. "Basically, our data is perfectly compatible with the standard model [of particle physics]. We don't see any deviations."

From the beginning, the oddball new particle seemed like a long shot. In early April, the 500 researchers working with the CDF particle detector reported that, among the billions of collisions they'd recorded, about 250 seemed to produce a particle weighing about 145 giga-electron volts (GeV), or 160 times as much as a proton, decaying into two sprays of particles called jets. As rare as it appeared to be, that particle nevertheless popped up too often to be anything like some of the highly anticipated new particles that theorists had predicted—such as the long-sought Higgs boson that is the key to physicists' understanding of mass. So the observation triggered a minor media storm and a flurry of efforts to fit the particle into more-exotic theories.

The observation came with a caveat, however. CDF researchers used the energy and momenta of the two jets to infer the mass of the particle that may have produced them. They saw a peak right at 145 GeV, which seemed to be the new particle. But to see the peak at all, the researchers had to subtract off a gargantuan "background" created by random jets in the messy collisions. Physicists worried that if CDF researchers didn't do that subtraction just right, they might inadvertently produce a fake peak.

That's why all eyes turned to researchers working with the D0 detector, which is also fed by the Tevatron. If the D0 team also saw the peak, that would be a strong argument for a new particle. If the team saw no peak, it would seem more likely that CDF researchers had missed some systematic effect in their analysis that produced a spurious peak. Unfortunately for physics fans, D0 researchers do not see a peak, as they report today in a talk at Fermilab and in a paper submitted to the journal Physical Review Letters. The physicists spent 2 months using simulated data to ensure that their analysis was sensitive enough to see the particle if it was there. "That's why [the analysis] took a couple of months," Denisov says. "Otherwise, we could have done it in 3 days."

Robert Roser, co-spokesperson for the CDF team, acknowledges that now the simplest explanation is that CDF has picked up a spurious signal. But it's far from clear exactly why the two experiments disagree, he says. "The fact that they don't see [the peak] means that the situation is muddy and that you have to get down in the mud and wrestle around and figure it out," he says. Fermilab is assembling a task force to go through the details of the two experiments and try to reconcile the results. For many physicists, this mud-wrestling match may prove less interesting than the tantalizing hints that preceded it.

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