Probing New Physics and CP Violation in $\nu_\tau n \to \Lambda_c \tau^- (\pi^- \nu_\tau)$ and $\bar\nu_\tau p \to \Lambda \tau^+ (\pi^+ \bar\nu_\tau)$

TL;DR

This paper investigates tau neutrino interactions with nucleons to identify signals of new physics and CP violation, focusing on pion distributions and tau polarization, within an effective field theory framework.

Contribution

It introduces a CP-odd observable based on pion azimuthal-angle asymmetry as a novel probe for physics beyond the Standard Model in tau neutrino processes.

Findings

The proposed asymmetry is sensitive to CP-violating Wilson coefficients.

Lattice QCD form factors are used to estimate the effect's magnitude.

The observable can distinguish new physics scenarios from the Standard Model.

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.