Flavor oscillation may be different in neutrinos and antineutrinos

In 1995 William Louis and colleagues at Los Alamos National Laboratory reported the first evidence of neutrino flavor oscillation in an accelerator experiment. The report was disquieting, and became more so over the next three years as its statistical significance was bolstered by more data from the LANL experiment. Why the disquiet? After all, neutrino oscillation, the metamorphosis of neutrino flavors with a probability that oscillates with travel distance L like sin2(L/λ), was already well attested for neutrinos from the Sun and from cosmic-ray showers in the atmosphere. The characteristic oscillation length λ is given by 4ħE/c3Δm2, where E is the neutrino’s energy and Δm2 is the difference between the squared masses of the two neutrino mass eigenstates involved. The prevailing model of neutrino oscillation assumes that there are three different neutrino mass eigenstates in nature and that they are different linear superpositions of the three flavor eigenstates νe, νμ , and ντ , associated respectively with the three charged leptons: the electron, the muon, and the much heavier tau. The standard oscillation phenomenology presumes that, to adequate approximation, only two of the three neutrino mass states are involved in any one observational oscillation regime. The LANL data, seeming to reveal the metamorphosis νμ→ νe over distances of less than 100 meters in a lowenergy accelerator beam, were well fitted by Δm2 of order 1 eV2. That’s several hundred times bigger than the Δm2 measured for atmospheric neutrino oscillation and ten thousand times bigger than what’s found for solar neutrinos. But if only three mass eigenstates exist, no one Δm2 can exceed the sum of the other two. By 1995 electron–positron collider experiments had already excluded the existence of more than three neutrino flavors participating in the weak interactions. So the LANL data seemed to require an additional “sterile” neutrino flavor, impervious to the weak interactions. That prospect was unappealing; it would have cluttered the elegant prevailing theory. So experimenters at Fermilab responded by building the MiniBoone neutrino beam and detector, a facility explicitly designed to confirm the LANL result or lay it to rest. And indeed in 2007, with three years of data in hand, the MiniBoone collaboration announced that its results were incompatible with LANL’s claim (see PHYSICS TODAY, June 2007, page 18). The neutrino-physics community breathed a sigh of relief.