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

Along with $pi^0$ and $eta$ mesons, a resonance structure in the invariant mass spectrum of two photons at $M_{gammagamma}= 360 pm 7 pm 9$ MeV is observed in the reaction $d Ctogamma + gamma +X$ 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 $sigma_{gammagamma}=98pm24^{+93}_{-67} {rm mu b}$, respectively. The collected statistics amount to $2339 pm 340$ events of $1.5cdot 10^6$ 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.