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

TL;DR

This study reports a resonance structure at 360 MeV in the gamma-gamma invariant mass spectrum observed in deuteron-carbon interactions, with detailed measurements of its properties, but not seen in proton-carbon collisions, suggesting a unique nuclear effect.

Contribution

The paper provides the first observation of a resonance at 360 MeV in gamma-gamma spectra in $d$C interactions, with detailed measurements and analysis of its properties.

Findings

Resonance at 360 MeV observed in $d$C interactions.

No such resonance observed in $p$C collisions.

Estimated width and production cross section of the resonance.

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.