Observation of a Structure in $pp \to pp\gamma\gamma$ near the $\pi\pi$ Threshold and its Possible Interpretation by $\gamma\gamma$ Radiation from Chiral Loops in the Mesonic $\sigma$ Channel

Abstract

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