Viewpoint: Rethinking the Neutrino

Abstract

To some, this may be the year of the dragon, but in neutrino physics, this is the year of θ[subscript 13]. Only one year ago, this supposedly “tiny” mixing angle, which describes how neutrinos oscillate from one mass state to another, was undetected, but the last twelve months have seen a flurry of results from experiments in Asia and Europe, culminating in the result from the Daya Bay Collaboration, now being reported in Physical Review Letters, that shows that θ[subscript 13] is not small after all [1]. A not-so-tiny mixing angle forces us to rethink theory, calling for new explanations for why quarks and leptons are so different. It also opens the door to new experiments, potentially allowing the discovery of CP violation—a difference between neutrinos and antineutrinos that may be related to the matter asymmetry of the early universe.