Visible energy and angular distributions of the charged particle from the $\tau-$decay in $b\to c \tau\, (\mu \bar \nu_\mu \nu_\tau,\pi\nu_\tau,\rho\nu_\tau) \bar\nu_\tau$ reactions

Abstract

We study the $d^2 \Gamma_d /(d\omega d\cos\theta_d) $, $d\Gamma_d /d\cos\theta_d$ and $ d\Gamma_d /dE_d $ distributions, which are defined in terms of the visible energy and polar angle of the charged particle from the $\tau-$decay in $b\to c \tau\, (\mu \bar \nu_\mu \nu_\tau,\pi \nu_\tau,\rho\nu_\tau) \bar\nu_\tau$ reactions. The first two contain information on the transverse tau-spin, tau-angular and tau-angular-spin asymmetries of the $H_b\to H_c\tau\bar\nu_\tau$ parent decay and, from a dynamical point of view, they are richer than the commonly used one, $d^2 \Gamma_d /(d\omega dE_d) $, since the latter only depends on the tau longitudinal polarization. We pay attention to the deviations with respect to the predictions of the standard model (SM) for these new observables, considering new physics (NP) operators constructed using both right- and left-handed neutrino fields, within an effective field-theory approach. We present results for $\Lambda_b\to\Lambda_c\tau\, (\mu \bar \nu_\mu \nu_\tau,\pi\nu_\tau,\rho\nu_\tau)\bar\nu_\tau$ and $\bar B \to D^{(*)}\tau\, (\mu \bar \nu_\mu \nu_\tau,\pi\nu_\tau,\rho\nu_\tau) \bar\nu_\tau$ sequential decays and discuss their use to disentangle between different NP models. In this respect, we show that $d\Gamma_d /d\cos\theta_d$, which should be measured with sufficiently good statistics, becomes quite useful, especially in the $\tau\to \pi \nu_\tau$ mode. The study carried out in this work could be of special relevance due to the recent LHCb measurement of the lepton flavor universality ratio ${\cal R}_{\Lambda_c}$ in agreement with the SM. The experiment identified the $\tau$ using its hadron decay into $\pi^-\pi^+\pi^-\nu_\tau$, and this result for ${\cal R}_{\Lambda_c}$, which is in conflict with the phenomenology from the $b$-meson sector, needs confirmation from other tau reconstruction channels.