Effective-field theory analysis of the $τ^- \to η^{(\prime)} π^- ν_τ$ decays

Abstract

The rare $τ^- \to η^{(\prime)} π^- ν_τ$ decays, which are suppressed by $G$-parity in the Standard Model (SM), can be sensitive to the effects of new interactions. We study the sensitivity of different observables of these decays in the framework of an effective field theory that includes the most general interactions between SM fields up to dimension six, assuming massless neutrinos. Owing to the strong suppression of the SM isospin breaking amplitudes, we find that the different observables would allow to set constraints on scalar interactions that are stronger than those coming from other low-energy observables.

23 pages, 8 multiple figures and one table. Extended discussion of the hadronization of the tensor current and improved presentation of the plots. Accepted for publication in JHEP