Forward $\chi_c$ production in $pp$ and $pA$ collisions within the CGC+NRQCD framework

Abstract

We study the forward $\chi_{cJ}$ production in the low $p_{\perp}$ region in proton-proton $(pp)$ and proton-nucleus $(pA)$ collisions at the LHC within the framework of non-relativistic QCD (NRQCD) factorization and the Color Glass Condensate (CGC) effective field theory. Incorporating the small-$x$ gluonic evolution via the running-coupling Balitsky-Kovchegov equation with various initial conditions, we systematically analyze the cross section ratio $R_{\chi_{c}}=\sigma_{\chi_{c2}}/\sigma_{\chi_{c1}}$, and find that the theoretical predictions weakly depend on these inputs. Our predictions align well with the LHCb measurements for $p\mathrm{Pb}$ collisions, however, overshoot those for $pp$ collisions. We further investigate the feed down fractions $F_{\chi_{c1,2}\to J/\psi}=\sigma_{\chi_{c1,2}\to J/\psi}/\sigma_{J/\psi}$. We notice that with the experimental measurement of prompt $J/\psi$ cross section as inputs in the denominator the CGC+NRQCD predictions broadly match the data when choosing the standard McLerran-Venugopalan initial conditions. In contrast, implementing purely theoretical CGC+NRQCD predictions for both $J/\psi$ and $\chi_{c}$ production yield systematic discrepancies across all tested parameters. This highlights a persistent challenge in achieving a simultaneous, unified description of $\chi_{c}$ and prompt $J/\psi$ production within the current framework.