Exclusive photoproduction of $\chi_{c}\gamma$ pairs

Abstract

In this manuscript we study the exclusive photoproduction of $\chi_{c}\gamma$ pairs in the collinear factorization framework. We found the coefficient functions for all possible spins and helicity projections of $\chi_{c}$ mesons and final-state photons in the leading order in the strong coupling $\alpha_{s}$. In our analysis we focused on the contribution of the leading twist chiral even GPDs, and found that for all spin states of $\chi_{c}$ the suggested process is determined by the behavior of the gluon GPDs $H_{g}$ in the ERBL kinematics. Using the phenomenological parametrizations of the GPDs from the literature, we estimated numerically the photoproduction cross-sections and the expected counting rates in the kinematics of middle-energy photoproduction experiments that could be realized at the Electron-Ion Collider. We observed that the $\chi_{c1}$, $\chi_{c2}$ mesons are produced predominantly with the same polarization as the incoming photon, and the expected counting rates of $\chi_{c1}\gamma$ and $\chi_{c2}\gamma$ pairs are sufficiently large for their experimental study. We also analyzed the angular distribution of $\chi_{c}$ mesons in electroproduction experiments, and noticed that for some helicity components there are sizable angular asymmetries, which can be used as complementary observables for experimental study. Finally, we estimated the role of this process as a potential background to $\chi_{c}$ photoproduction, which has been recently suggested as a tool for studies of odderons. We found that the contribution of $\chi_{c}\gamma$ (with undetected photon) is on par with expected contributions of odderons and Primakoff mechanisms in the kinematics of small momentum transfer $|t|\lesssim1$ GeV$^{2}$, but becomes negligible at larger $|t|$.