Tau longitudinal and transverse polarizations from visible kinematics in (anti-)neutrino nucleus scattering

Since the $\nu_\tau(\bar\nu_\tau) A_Z \to \tau^\mp X$ reaction is notoriously difficult to be directly measured, the information on the dynamics of this nuclear process should be extracted from the analysis of the energy and angular distributions of the tau decay visible products. These distributions depend on the components of the tau-polarization vector. We give, for the first time, the general expression for the outgoing hadron (pion or rho meson) energy and angular differential cross section for the sequential $\nu_\tau A_Z \to \tau^-(\pi^- \nu_\tau, \rho^-\nu_\tau) X$ and $\bar\nu_\tau A_Z \to \tau^+(\pi^+ \bar\nu_\tau, \rho^+ \bar\nu_\tau) X$ reactions. Though all possible nuclear reaction mechanisms contribute to the distribution, it may be possible to isolate/enhance one of them by implementing appropriate selection criteria. For the case of the quasi-elastic reaction off oxygen and neutrino energies below 6 GeV, we show that the pion distributions are sensitive to the details of the tau-polarization components. We find significant differences between the full calculation, where the longitudinal and transverse components of the tau polarization vector vary with the energy and the scattering angle of the produced tau, and the simplified scheme in which the polarizations are set to one and zero (respective asymptotic values in the high energy regime). In addition to its potential impact on neutrino oscillation analyses, this result can be used to further test different nuclear models, since these observables provide complementary information to that obtained by means of the inclusive nuclear weak charged-current differential cross section. We also study the effects on the cross section of the $W_4$, $W_5$ nuclear structure functions, which contributions are proportional to the charged lepton mass, and therefore difficult to constrain in muon and electron neutrino experiments.
Comment: 11 pages, 5 figures. We have corrected a numerical error in our calculation. Figures have changed but the discussion and conclusions remain the same. An erratum will be published