Neutrino Physics: What We Have Learned, and What We Would Like To Find Out

ABSTRACT: We review what has been learned about the neutrinos since the discovery of neutrino oscillation 16 years ago. We then identify open questions, and focus on several of them, including: Is the physics responsible for neutrino masses different from that responsible for the masses of all the other known particles? Are we all descended from heavy neutrinos that lived briefly in the early universe, and then decayed in a way that led to a universe containing matter but no antimatter? What can the present-day neutrinos and the universe tell us about each other? Are there “sterile” neutrinos that do not experience any of the known forces of nature except gravity? We briefly discuss plans to answer these questions.