Current Situation in the Neutrino (and Charged-Lepton) Sector

Neutrino masses are nonzero. Theoretically, we are still in the process of un- derstanding what these tiny masses and the pattern of lepton mixing mean, and how they Þt into a new and improved standard model. Nonetheless, the very successful parameterization of the new neutrino sector (three neutrino masses plus a 3 × 3 unitary leptonic mixing matrix) allows one to identify what we know we donÕt know about neutrinos and to deÞne a rich experimen- tal program in neutrino physics. This experimental program must not only answer these Òneutrino questions,Ó but also test the underlying three-active- ßavors paradigm and point the way toward a deeper understanding of neutri- nos (and particle physics). Given what we learned about neutrinos, searches for charged-lepton ßavor-violating phenomena are poised to Þnd new heavy physics at the weak scale (or well above) and provide vital clues regarding the neutrino mass mystery.