Phys. Rev. Lett. 107, 241801 (2011)

Neutrinos oscillate — that is, each of the three types of neutrino (electron, muon and tau) can evolve into one of the others, a behaviour that is captured in a matrix of mixing angles. This also brings the possibility of 'CP violation' in the neutrino sector, complementing that seen in the mixing of quarks, which could be spotted through a detectable difference in the oscillation probabilities for neutrinos and antineutrinos.

Discrepancies between neutrino and antineutrino data have been noticed by some experiments, notably MINOS and MiniBooNE, which both use neutrino beams generated at Fermilab, in the United States. K. Abe and colleagues have now searched data collected for atmospheric neutrinos over an 11-year period by the Super-Kamiokande apparatus, located in the Mozumi mine under Mount Kamioka in Japan.

In Super-Kamiokande's huge underground tank, lined with photomultiplier tubes and filled with 50,000 tons of water, Abe et al. sought evidence of the disappearance of muon neutrinos and muon antineutrinos as they oscillated into the other particle types. Through a challenging analysis that allows for 120 sources of systematic uncertainty, the authors conclude that, as yet, the mixing parameters for neutrinos and antineutrinos from atmospheric sources are entirely consistent with each other.