Like Lazarus from the dead, a controversial idea that there may be a new, superhard-to-spot kind of particle floating around the universe has made a comeback. Using a massive particle detector, physicists in Illinois have studied the way elusive particles called antineutrinos transform from type or "flavor" to another, and their data bolster a decade-old claim that the rate of such transformation is so high that it requires the existence of an even weirder, essentially undetectable type of neutrino. Ironically, the same team threw cold water on that idea just 3 years ago, and other researchers remain skeptical.
Of the known fundamental bits of matter, neutrinos are by far the shiest. Emitted in certain types of radioactive decays and pumped into space in huge numbers by the sun, they hardly interact with other particles of matter. As a result, of the trillions of neutrinos that strike every square meter of Earth every second, all but a few pass through the planet unimpeded. Nonetheless, physicists have been able to study the little devils, which come in three different types or flavors: electron neutrinos, muon neutrinos, and tau neutrinos. The three types of neutrinos can morph or "oscillate" into one another at a rate that depends on their masses and energies, as researchers long suspected but first proved in 1998.
Even before that, however, physicists working on an experiment called the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico found evidence that muon antineutrinos morphed into electron antineutrinos. But those results, published in 1995, came with a catch: The transformation seemed to happen so fast that the antineutrinos would have to be much more massive than other experiments indicated. One way out of that paradox was to assume that an undiscovered particle—a kind of supermassive "sterile" neutrino—mixes with the usual neutrinos but interacts with other matter only through the incredibly feeble force of gravity. But other researchers considered that ad hoc scheme unlikely, and some, including members of the Los Alamos team, worried that the result was a statistical fluke or even a mistake.
Now, an international team working at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, reports results that bolster the LSND experiment and again raise the possibility of new particles. Researchers with the Mini Booster Neutrino Experiment (MiniBooNE) produced a beam of muon antineutrinos and other particles using Fermilab's Booster proton accelerator and fired them into a detector 500 meters away, hoping to see the muon antineutrinos transform into electron antineutrinos. Over a 4-year run, they found in the same energy range as LSND about 43 more electron antineutrinos than the 234 they would expect if no muon antineutrinos transformed, again suggesting that muon antineutrinos transform into electron antineutrinos at a very high rate. They calculated that excess had about a 0.5% probability of happening by chance alone. Ironically, in 2007 the MiniBooNE team reported results on neutrinos—instead of antineutrinos—that found no excess of transformations in the same energy range as LSND, but did see a few extra at lower energies. At the time, many researchers concluded that those results ruled out the LSND claim.
Team members say they are excited but remain cautious. "If it's right, it has huge consequences," says Richard Van de Water, a physicist at Los Alamos and a member of the MiniBooNE team. Reported online 26 October in Physical Review Letters, the results support the idea of a sterile neutrino.
The difference seen by MiniBooNE between neutrinos and antineutrinos also suggests an asymmetry between the way the two behave called charge parity (CP) violation in leptons, a group of particles including electrons and neutrinos. CP violation has been seen in other types of particles, but some theorists think that CP violation among neutrinos may be key in explaining why the universe developed so much matter and so little antimatter. However, "we are not claiming ... we've discovered CP violation" in neutrinos, says Bill Louis, a member from Los Alamos of both the MiniBooNE and the LSND teams. Researchers would like to run the experiment for "another 4 years to firm this up," he says.
"I'm probably less excited," says Maury Goodman of Argonne National Laboratory in Illinois who works on another neutrino experiment called the Main Injector Neutrino Oscillation Search (MINOS) Experiment. "There's probably nothing there, but like good physicists," the experimenters should follow up, he says. MINOS found a similar anomaly earlier this summer, although Goodman says he is skeptical of that result, too. The anomalous findings at MINOS and MiniBooNE may result from an undiscovered systematic problem, he says. The MiniBooNE analysis also assumed that muon antineutrinos could transform only into electron antineutrinos; if they also transform into tau antineutrinos, Goodman says, then the assumption might have made the results look more anomalous than they really are.