Neutrino Masses and Mixing at the Millennium∗

Recent evidence for neutrino oscillations has revolutionized the study of neutrino masses and mixing. This report gives an overview of what we are learning from the neutrino oscillation experiments, the prospects for the near term, and the bright future of neutrino mass studies. I NEUTRINO COUNTING How many neutrinos are there? Neutrino counting at LEP of Z → νν̄ decays obtains Nν = 3 active flavors — the expected νe, νμ, ντ . However, light isosinglet, right-handed “sterile” neutrinos with no gauge boson interactions could also exist. Big Bang Nucleosynthesis determines the equivalent number of massless neutrinos at the time of nucleosynthesis. The bounds inferred (Nν < 3.2 [1], Nν < 4 [2], Nν < 5.3 [3]) depend on which measurements of the He and D/H abundances are used in the analysis. Neutrino oscillation phenomenology for Nν = 3 and Nν = 4 is very different and both options need to be considered at present.