Meson pair production in γγ scattering and implications for scalar mesons

Abstract

We describe a simple analytic unitary model which provides with a parametrization of low energy (from threshold up to 1.4GeV) two photon production of ππ andK\(\bar K\) pairs in term of a few parameters related to the γγ width of the scalar and tensor resonances. It can be useful for analysing data because of the large continuum found in γγπ+π− production besides thef0 (1,270) peak in recent experiments. We show that the present data already provides a strong evidence for the existence of a broad low mass scalar state, the old σ or e (600) with a γγ width of 5 to 9KeV, and that it will be difficult to get more information on the controversialS* parameters from ππ pair production only. In thef0 range we explain the downwards shift of the π+π− peak but we expect the π0π0 peak around 1.27–1.29 GeV which could raise the question either of systematic errors in mass calibration of crystal Ball experiment or of the precise value of thef0 mass.