Resonance Structure in the $\gamma\gamma$ Invariant Mass Spectrum in $p$C- and $d$C-Interactions

Abstract

Along with $\pi^0$ and $\eta$ mesons, a resonance structure in the invariant mass spectrum of two photons at $M_{\gamma\gamma}= 360 \pm 7 \pm 9$ MeV is observed in the reaction $d C\to\gamma + \gamma +X$ at momentum 2.75 GeV/c per nucleon. Estimates of its width and production cross section are $\Gamma = 63.7 \pm 17.8$ MeV and $\sigma_{\gamma\gamma}=98\pm24^{+93}_{-67} {\rm \mu b}$, respectively. The collected statistics amount to $2339 \pm 340$ events of $1.5\cdot 10^6$ triggered interactions of a total number $\sim 10^{12}$ of $d$C-interactions. This resonance structure is not observed in $p$C collisions at the beam momentum 5.5 GeV/c. Possible mechanisms of this ABC-like effect are discussed.