Observation of a Structure in $pp \to ppγγ$ near the $ππ$ Threshold and its Possible Interpretation by $γγ$ Radiation from Chiral Loops in the Mesonic $σ$ Channel

Abstract

The $pp \to ppγγ$ reaction has been measured at CELSIUS using the WASA $4π$-detector with hydrogen pellet target. At $T_p = 1.20$ and 1.36 GeV, where most of the statistics has been accumulated, the $γγ$ invariant mass spectrum exhibits a narrow structure around the $ππ$ threshold, which possibly may be associated with two-photon radiation of $π^+π^-$ loops in the mesonic $σ$ channel.