13. Neutrino mixing 1 13. NEUTRINO MASS, MIXING, AND OSCILLATIONS

The experiments with solar, atmospheric, reactor and accelerator neutrinos have provided compelling evidences for oscillations of neutrinos caused by nonzero neutrino masses and neutrino mixing. The data imply the existence of 3-neutrino mixing in vacuum. We review the theory of neutrino oscillations, the phenomenology of neutrino mixing, the problem of the nature- Dirac or Majorana, of massive neutrinos, the issue of CP violation in the lepton sector, and the current data on the neutrino masses and mixing parameters. The open questions and the main goals of future research in the field of neutrino mixing and oscillations are outlined. 13.1. Introduction: Massive neutrinos and neutrino mixing It is a well-established experimental fact that the neutrinos and antineutrinos which take part in the standard charged current (CC) and neutral current (NC) weak interaction are of three varieties (types) or flavours: electron, νe and ν̄e, muon, νµ and ν̄µ, and tauon, ντ and ν̄τ. The notion of neutrino type or flavour is dynamical: νe is the neutrino which i