Probing new physics and CP violation in ντn-* Λcτ - ðπ-ντþ and ν¯τp-* Λτ plus ðπ plus ν¯τþ

Journal article, 2026

We study the processes nu tau n-* Ac tau-& eth;pi-nu tau & THORN; and nu & strns;tau p-* A tau & thorn;& eth;pi & thorn;nu & strns;tau & THORN;, 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 +/- -* pi +/-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.