Tau Polarization in Tau-Neutrino Nucleon Scattering

Abstract

We investigate the spin polarization of τ^{\pm} leptons produced in ν_τ and \barν_τ nucleon scattering via charged currents. Quasi-elastic scattering, Δresonance production and deep inelastic scattering processes are studied. The polarization information is essential for measuring the τ^{\pm} appearance rate in long baseline neutrino oscillation experiments, because the decay particle distributions depend crucially on the τ^{\pm} spin. In this article, we calculate the spin density matrix of each process and estimate the spin polarization vector in medium and high neutrino energy interactions. We find that the produced τ^{\pm}'s have high degree of polarization, and their spin direction depends non-trivially on the energy and the scattering angle of τ^{\pm} in the laboratory frame.