Study of Light Scalars in Photon-Photon Collisions, the learned Lessons

Abstract

After a short review of the nature of light scalar mesons, we dwell on the investigation of the high-statistics Belle data on two-photon productionof light scalar mesons and ascertain that these data allow us to establish their production mechanisms. We show that light scalars are produced in two-photon collisions via four-quark transitions in contrast to the classic P -wave tensor q q ¯ mesons which are produced via two-quark transitions γ γ → q q ¯ . In particular, we show that the ideal q q ¯ model prediction g f 0 γ γ 2 : g a 0 γ γ 2 = 25 : 9 is excluded by experiment. Thus we get new strong evidence of the four-quark nature of these states. A programme of further investigations is presented.