1303.2879 (S. Descotes-Genon et al.)

Dispersive evaluation of the second-class amplitude
$τ\toηπν_τ$ in the standard model    [PDF]

S. Descotes-Genon, E. Kou, B. Moussallam

We reevaluate the two form factors relevant for the $\eta\pi$ second-class $\tau$ 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 $\rho$ resonance peak which is a background-free signature of a second-class current. Its dispersive construction requires the $\eta\pi\to\pi\pi$ scattering amplitude which we derive from a family of Khuri-Treiman equations solutions constrained with accurate recent results on the $\eta\to3\pi$ Dalitz plot.

View original: http://arxiv.org/abs/1303.2879