An accelerator experiment at Los Alamos National Laboratory in the late 1990s reported the observation of neutrino flavor oscillation on a laboratory length scale—tens of meters. All prior oscillation observations had been over much larger distances. The disquieting implication of the LANL claim was that so small an oscillation length required the existence of a “sterile” neutrino flavor impervious to the weak interactions, which would clutter the attractively neat prevailing theory. So experimenters designed the MiniBoone facility at Fermilab to confirm the LANL result or lay it to rest. In 2007, the MiniBoone collaboration reported that its results were incompatible with the LANL claim. But now a new MiniBoone result suggests that the sterile neutrino’s obituary was premature. The new result looks a lot like the original LANL data. Like the LANL experiment, it was based on a beam of muonic antineutrinos. The 2007 refutation, however, was based on a muonic neutrino beam. The standard theory assumes that neutrinos and antineutrinos oscillate identically. But the new result, though not yet statistically robust, appears to show that antineutrinos, unlike neutrinos, do indeed oscillate on a distance scale that implies one or more sterile neutrino states. In fact, theorists are already considering how interference between two sterile states of different mass might explain such a neutrino–antineutrino asymmetry. The photo shows the photomultiplier tubes in the oil-filled MiniBoone detector that discern the flavors of neutrinos by recording the Cherenkov light of the charged leptons they create in collisions with nucleons. (A. Aguilar-Arevalo et al., http://arxiv.org/abs/1007.1150.)—Bertram Schwarzschild.
