rpp2010-rev-neutrino-mixing - 13. Neutrino mixing 1 13....

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13. Neutrino mixing 1 13. NEUTRINO MASS, MIXING, AND OSCILLATIONS Written May 2010 by K. Nakamura (IPMU, U. Tokyo, KEK) and S.T. Petcov (SISSA/INFN Trieste, IPMU, U. Tokyo, Bulgarian Academy of Sciences). 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 ±eld 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, ν e and ¯ ν e , muon, ν μ and ¯ ν μ ,and tauon, ν τ and ¯ ν τ . The notion of neutrino type or flavour is dynamical: ν e is the neutrino which is produced with e + , or produces an e in CC weak interaction processes; ν μ is the neutrino which is produced with μ + , or produces μ , etc. The flavour of a given neutrino is Lorentz invariant. Among the three di²erent flavour neutrinos and antineutrinos, no two are identical. Correspondingly, the states which describe di²erent flavour neutrinos must be orthogonal (within the precision of the corresponding data): ± ν l ± | ν l ² = δ l ± l , ± ¯ ν l ± | ¯ ν l ² = δ l ± l , ± ¯ ν l ± | ν l ² =0. It is also well-known from the existing data (all neutrino experiments were done so far with relativistic neutrinos or antineutrinos), that the flavour neutrinos ν l (antineutrinos ¯ ν l ), are always produced in weak interaction processes in a state that is predominantly left-handed (LH) (right-handed (RH)). To account for this fact, ν l and ¯ ν l are described in the Standard Model (SM) by a chiral LH flavour neutrino ±eld ν lL ( x ), l = e, μ, τ .F o r massless ν l ,thestateo f ν l ν l )wh ichthe±e ld ν lL ( x ) annihilates (creates) is with helicity (-1/2) (helicity +1/2). If ν l has a non-zero mass m ( ν l ), the state of ν l ν l ) is a linear superposition of the helicity (-1/2) and (+1/2) states, but the helicity +1/2 state (helicity (-1/2) state) enters into the superposition with a coefficient m ( ν l ) /E ,Ebe ingth e neutrino energy, and thus is strongly suppressed. Together with the LH charged lepton ±eld l L ( x ), ν lL ( x ) forms an SU (2) L doublet. In the absence of neutrino mixing and zero neutrino masses, ν lL ( x )and l L ( x ) can be assigned one unit of the additive lepton charge L l and the three charges L l , l = e, μ, τ , are conserved by the weak interaction.
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rpp2010-rev-neutrino-mixing - 13. Neutrino mixing 1 13....

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