Probing New Physics and CP Violation in ${\nu }_{\tau }n\to {\Lambda }_c{\tau }^{-}({\pi }^{-}{\nu }_{\tau })$ and ${\bar{\nu }}_{\tau }p\to \Lambda {\tau }^{+}({\pi }^{+}{\bar{\nu }}_{\tau })$

Abstract

We study the processes ${\nu }_{\tau }n\to {\Lambda }_c{\tau }^{-}({\pi }^{-}{\nu }_{\tau })$ and ${\bar{\nu }}_{\tau }p\to \Lambda {\tau }^{+}({\pi }^{+}{\bar{\nu }}_{\tau })$, with particular emphasis on the pion energy and angular distributions, as a possible signal for lepton flavor universality violation, and in general of physics beyond the Standard Model (SM), as well as a sensitive probe of the $\tau$ polarization vector. We work within an effective low-energy extension of the SM with all dimension-six four-fermion operators. In this framework, complex Wilson coefficients which encode new physics can generate CP-violating contributions. We propose an observable that provides a genuine CP-odd signal due to its sensitivity to particular transverse components of the $\tau$ polarization vector. Namely, we show that the asymmetry in the azimuthal-angle distribution of the pion in the decay ${\tau }^{\pm }\to {\pi }^{\pm }{\nu }_{\tau }$ constitutes a smoking-gun prediction of such a beyond the SM scenario. We estimate the strength of this effect extrapolating nucleon-hyperon form factors recently obtained from lattice QCD calculations.

Comments: 18 latex pages, 5 figures and 2 tables