The Large Hadron Collider may be up and running outside Geneva, but the particle accelerator it supplanted as top dog in the particle physics community appears to have a few surprises left to deliver. Data from the workhorse Tevatron collider at Fermilab in Illinois show what appears to be a preference for matter over antimatter in the way an unusual kind of particle decays, according to a new analysis in a Tevatron research collaboration.
Physicists and cosmologists seek such mechanisms to help explain why matter prevailed over antimatter in the early universe, when both should have been created in equal parts, yielding a storm of mutual annihilation and not the stable material structures—galaxies and the like—that fill the universe.
Some properties of high-energy physics have been shown to be fundamentally asymmetrical, producing matter more often than antimatter, but in quantities too small to explain the relative dearth of antimatter in the universe. The new mechanism observed at the Tevatron's DZero detector appears to work on a much larger scale, says Fermilab staff scientist Dmitri Denisov, co-spokesperson for the DZero collaboration, but whether it can explain the preponderance of matter today remains to be seen. In any event, the asymmetry does not fit with the long-reigning Standard Model of particle physics, suggesting that some hitherto unknown particle or interaction may be at play.
The DZero collaborators analyzed more than seven years of proton–antiproton collisions in the new study, which the group submitted to Physical Review D and published online May 16. As the exotic, short-lived particles produced in the collisions progressively decayed to more stable particles such as electrons, a collision product known as a neutral B meson appeared to decay more often into muons—unstable particles that exist for roughly two millionths of a second before decaying further—than into antimuons.
"While colliding protons and antiprotons, which creates neutral B mesons, we would expect that when they decay we will see equal amounts of matter and antimatter," Denisov says. "For whatever reason, there are more negative muons, which are matter, than positive muons, which are antimatter." According to DZero member Gustaaf Brooijmans, a physicist at Columbia University, "We observe an asymmetry that is close to 1 percent."
Brooijmans notes that other experiments have used B mesons to expose fundamental asymmetries in physics but that the results of those experiments have adhered more closely to the Standard Model's predictions. So-called B factories have been built to explore the properties of the unusual particles, but in a more limited scope than that available at the Tevatron. "There is one big difference" between the DZero result and those of the B factories, Brooijmans says. "We have access to the Bs meson, and B factories have access mostly to Bd."
Both Bs and Bd mesons (so named because they contain a strange quark or a down quark, respectively) are short-lived, decaying away in roughly 1.5 picoseconds, or 1.5 trillionths of a second. They are known as neutral mesons because they carry no net electric charge. In their brief lifetimes, they can oscillate between two forms, each the antiparticle of the other, Denisov explains. The difference is that Bs mesons oscillate much faster, giving them more flexibility to change from a matter progenitor to an antimatter progenitor, or vice versa. "Neutral B mesons are really interesting because they can basically go back and forth between matter and antimatter, to simplify things a bit, and we would have thought that they would spend an equal time as each," Denisov says. "What we're measuring now, it looks like they prefer matter."
Even within the halls of Fermilab, the new result from the tight-lipped DZero group came as a surprise, says theoretical physicist Bogdan Dobrescu, a staff scientist at the lab. "It's very exciting," Dobrescu says. "This kind of important announcement is not made too often." All the same, he says, the result must check out in other experiments before it can gain much traction. "It needs to be confirmed before we change the textbooks," he says.
Dobrescu says it is too early to speculate on how much of a player the new mechanism might be in establishing matter's prevalence in the universe. "However, all this notion of explaining matter–antimatter asymmetry should not be the central aspect of this discussion," he says. "We are up against something more important, which is, what are the laws of physics? The matter–antimatter asymmetry is just one implication of that."
It is fairly simple to put down on paper a new particle that could explain the asymmetry in B meson decay, Dobrescu says, but it is more difficult to reconcile those hypothetical particles with what is already known. "Most of the time, if you are careful, you will see that your choice is already ruled out by other experiments," he says.
If it turns out that a new particle is in fact responsible for the odd tendency of B mesons to favor matter over antimatter, it might be unmasked in the unprecedented high-energy collisions at the Large Hadron Collider, or LHC. But don't count out the workhorse stateside, which has a head start of many years—and reams of well-understood data—on its more powerful European counterpart. Brooijmans says his "gut feeling" is that such a particle should be observable at the LHC. "And who knows?" he adds. "It might be accessible at the Tevatron."