Comment on "${\eta }_c$ production in photon-induced interactions at the LHC"

Abstract

In "${\eta }_c$ production in photon-induced interactions at the LHC," \cite{Goncalves:2018yxc} Goncalves and Moreira discuss inclusive and exclusive ${\eta }_c$ production at $pp$ and $pA$ collisions at LHC energies. The exclusive channels are via two-photon and photon-Odderon interactions. This comment points out that there is a large additional source of almost-exclusive ${\eta }_c$ in ultra-peripheral collisions: from the radiative decay of $J/\psi$ that are produced in photon-nucleon interactions. Although the $J/\psi \to \gamma {\eta }_c$ branching ratio is small, the $J/\psi$ production cross-section is large enough that it dominates over the exclusive channels considered in \cite{Goncalves:2018yxc}, and is comparable to the non-exclusive production. In $J/\psi \to \gamma {\eta }_c$, the photon is very soft and therefore easy to miss, and the ${\eta }_c$ will have very similar kinematics to the $J/\psi$.

Comments: 2 pages