A team of physicists, including some from MIT, has found surprising differences between the flavor-switching behavior of neutrinos and antineutrinos. If confirmed, the finding could help explain why matter, and not antimatter, dominates our universe. Neutrinos, elementary particles generated by nuclear reactions in the sun, "suffer from an identity crisis as they cross the universe, morphing between three different flavors.” Their antimatter counterparts do the same thing.
“People are very excited about it because it suggests that there are differences between neutrinos and antineutrinos,” says Georgia Karagiorgi, an MIT graduate student and one of the leaders of the analysis of experimental data produced by the Booster Neutrino Experiment (MiniBooNE) at the Fermi National Accelerator Laboratory.
The new results appear to be one of the first observed violations of CP symmetry: the theory that matter and antimatter should behave in the same way. CP symmetry violation has been seen before in quarks, but never in neutrinos or electrons. CP symmetry means in the words of Roger Penrose (The Road to Reality), "CP sends a particle's zig into its antiparticle's zag, and vice versa."
The finding could also force physicists to revise their Standard Model, which catalogs all of the known particles that make up matter. The model now posits only three flavors of neutrino, but a fourth (or fifth or sixth) may be necessary to explain the new results.
“If this should be proven to be correct, it would have major implications for particle physics,” says John Learned, professor of physics at the University of Hawaii, who is not part of the MiniBooNE team.
So far, the researchers have enough data to present their results with a confidence level of just below 99.7% (also called 3 sigma), which is not high enough to claim a new discovery. To reach that level, 5-sigma confidence (99.99994%) is required. “People are going to rightfully demand a really clean, 5-sigma result,” says Learned.
Since the 1960s, physicists have been gathering evidence that neutrinos can switch, or oscillate, between three different flavors—muon, electron, and tau, each of which has a different mass. However, they have not yet been able to rule out the possibility that more types of neutrino might exist.
In an effort to determine the number of neutrinos, MiniBooNE physicists send beams of neutrinos or antineutrinos down a 500-meter tunnel, at the end of which sits a 250,000-gallon tank of mineral oil. When neutrinos or antineutrinos collide with a carbon atom in the mineral oil, the energy traces left behind allow physicists to identify what flavor of neutrino took part in the collision. Neutrinos, which have no charge, rarely interact with other matter, so such collisions are rare.
The MiniBooNE team plans to collect antineutrino data for another 18 months. Conrad also hopes to launch a new experiment that would use a cyclotron, a type of particle accelerator in which particles travel in a circle instead of a straight line, to help confirm or refute the MiniBooNE results.Casey Kazan