On the structure of the ηπW vertex in the standard model

Inspired by the recent claim of a ≅ 5% branching ratio for τ → ηπν, we consider the form factors f± for the weak current transition η → π by means of the MIT bag model, chiral lagrangians, and a chiral quark loop model. We find contributions to f+ and f- comparable in size and of order 10-3, which is roughly one order of magnitude below the contribution to f+ from π0-η mixing, found in a recent paper. However, the non-zero values for f- turn out to be more important for the total rate: Including contributions to f-, we find a branching ratio ≅ 10-4, one order of magnitude larger than that due to π0-η mixing alone. (orig.)