The handbag contribution to gamma gamma to pi pi and K K

Abstract

We investigate the soft handbag contribution to two-photon annihilation into pion or kaon pairs at large energy and momentum transfer. The amplitude is expressed as a hard gamma gamma -> q qbar subprocess times a form factor describing the soft transition from q qbar to the meson pair. We find the calculated angular dependence of the cross section in good agreement with data, and extract annihilation form factors of plausible size. A key prediction of the handbag mechanism is that the differential cross section is the same for charged and neutral pion pairs, in striking contrast with what is found in the hard scattering approach.