Physicists sifting through data generated by the Tevatron particle collider in Illinois have uncovered a signal that neither they nor the long-standing Standard Model of particle physics can explain.
The international team of researchers work with data from CDF, one of the two Tevatron detectors where protons and their antimatter counterparts collide at nearly light speed. The wreckage of those high-energy collisions produces a variety of short-lived particles, which allows physicists a fleeting glimpse into the inner workings of the physical world. The Tevatron, at Fermi National Accelerator Laboratory, is the second-most powerful particle collider in the world after the Large Hadron Collider outside Geneva, Switzerland.
Examining a very specific kind of outcome when protons and antiprotons collide inside the CDF detector, the researchers noticed an unexplained blip in their signal that could be explained by a previously undiscovered elementary particle—but not the Higgs boson, the hotly pursued particle that is theorized to imbue other particles with mass.
The researchers reported their perplexing but unconfirmed new finding in a study posted online April 4 at the physics preprint Web site arXiv.org The researchers have also submitted their results for publication in Physical Review Letters.
The CDF team found that the Tevatron was a bit more prolific than it should be in terms of collisions that yield a heavy elementary particle known as the W boson plus a pair of particulate jets. "What we see is that there a region between 120 and 160 GeV (giga-electron volts) where there is an excess," CDF physicist Viviana Cavaliere of the University of Illinois at Urbana–Champaign explained to a packed Fermilab auditorium April 6. (A giga-, or a billion, electron volts is a unit of particle mass or energy.)
The result is compatible, Cavaliere said, with the collisions producing a W boson plus a hitherto unknown—and even heavier—particle with a mass of about 150 GeV. But that particle appears not to be the Higgs boson, which would be expected to emerge from the collisions alongside a W boson with far less frequency. If CDF has uncovered a new elementary particle, it would be the first such discovery since the tau neutrino was observed at Fermilab in 2000. But in the case of the tau physicists had predicted the particle's existence and had gone out looking for it.
As theorists scramble to figure out just what CDF has found, experimentalists will be working to verify that the detector has found anything at all. The new analysis claims that the data disagree with existing theory to better than three standard deviations, or 3 sigma. Assuming the analysis is correct, that means that there is just a fraction of a 1 percent chance that the effect is a mere statistical glitch. But extraordinary claims demand stronger proof.
"Five sigma is our gold standard," says Brookhaven National Laboratory physicist Sally Dawson, adding that the physics community has seen 3-sigma effects come and go. "If it's true, and if it holds up, it is of course very exciting, because it's completely unexpected," Dawson says. "If it persists, it's very hard to explain theoretically."
"We will learn pretty soon whether it's true or not," says Fermilab theorist Bogdan Dobrescu, who did not contribute to the new study. "This is pretty credible at this stage." If the results hold up, theorists will need to figure out what kind of new particle could fit the bill. "It would be a major breakthrough, especially because this is a particle that no one really predicted to the best of my knowledge," Dobrescu says. "We can try to invent some new particles and see if they have the appropriate properties that we see, but none of the answers are very expected."
The Tevatron, which is slated to shut down for good in the fall, is still collecting data that could strengthen the case for a new particle—or sink it. Cavaliere said that the new analysis began more than a year ago and does not include the latest data from CDF. The team already has already logged a good deal more collisions that await analysis, but Cavaliere cautioned that the expanded data set would not be enough to vault the discovery into the 5-sigma range.
But the physics community will not have to wait long before the new particle gets a reality check. Physicists working with the other detector at the Tevatron, known as DZero, are now replicating the CDF analysis with their own voluminous data set, says Fermilab physicist and DZero co-spokesperson Dmitri Denisov. "We expect we will be able to clarify this topic on a timescale of a few weeks," he says.