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 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 for f+ from πo-η mixing, found in a recent paper. However, the nonzero values for f- turn 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 πo-η mixing alone