Dispersive evaluation of the second-class amplitude τ→ηπντ in the standard model

Abstract We reevaluate the two form factors relevant for the ηπ second-class τ decay mode, making systematic use of analyticity, unitarity, combined with updated inputs to the NLO chiral constraints. We focus in particular on the shape of the ρ resonance peak which is a background-free signature of a second-class current. Its dispersive construction requires the η π → π π scattering amplitude which we derive from a family of Khuri-Treiman equations solutions constrained with accurate recent results on the η → 3 π Dalitz plot.