Virtual radiative pion capture: Current algebra description, information on nucleon axial form factors and chiral symmetry breaking

Abstract

We analyse the reaction π−p → l+l−n at low energies, in the «quasi-threshold» configuration, using equal-time current commutators. We derive a simple expression of the amplitude, in terms of nucleon axial vector form factors, and a consistency relation for the nonstrange quark massmp, indicating a small valuemp ≈ 15 MeV.