Prompt inclusive production of $J/ψ$, $ψ'$ and $χ_{c}$ mesons at the LHC in forward directions within the NRQCD $k_t$-factorization approach - search for the onset of gluon saturation

Abstract

We discuss prompt production of $J/ψ$ mesons in proton-proton collisions at the LHC within NRQCD $k_t$-factorization approach using Kimber-Martin-Ryskin (KMR) unintegrated gluon distributions (UGDF). We include both direct color-singlet production ($g g \to J/ψg$) as well as a feed-down from $χ_c \to J/ψγ$ and $ψ' \to J/ψX$ decays. The production of the decaying mesons ($χ_c$ or $ψ'$) is also calculated within NRQCD $k_t$-factorization approach. The corresponding matrix elements for $g g \to J/ψ$ g, $g g \to ψ'g$ and $g g \to χ_c$ include parameters of the nonrelativistic space wave functions of the quarkonia at $r = 0$, which are taken from potential models in the literature. We get the ratio of the corresponding cross section ratio for $χ_c(2)$-to-$χ_c(1)$ at midrapidities much closer to experimental data than obtained in a recent analysis. Differential distributions in rapidity and transverse momentum of $J/ψ$ and $ψ'$ are calculated and compared with experimental data of the ALICE and LHCb collaborations. We discuss a possible onset of gluon saturation effects in the production of $J/ψ$ and $χ_{c}$ mesons at forward/backward rapidities. We show that it is neccessary to modify the standard KMR UGDF to describe ALICE and LHCb data. A mixed UGDF scenario was proposed. Then we can describe the experimental data for $J/ψ$ production within model uncertainties with color-singlet component only. Therefore our theoretical results leave only a relatively small room for the color-octet contributions.