Prompt charmonia production and polarization at LHC in the NRQCD with $k_T$-factorization. Part III: $J/ψ$ meson

Abstract

In the framework of $k_T$-factorization approach, the production and polarization of prompt $J/ψ$ mesons at the LHC energies is studied. Our consideration is based on the non-relativistic QCD formalism for bound states and off-shell amplitudes for hard partonic subprocesses. Both the direct production mechanism and feed-down contributions from $χ_c$ and $ψ(2S)$ decays are taken into account. The transverse momentum dependent (or unintegrated) gluon densities in a proton were derived from Ciafaloni-Catani-Fiorani-Marchesini evolution equation or, alternatively, were chosen in accordance with Kimber-Martin-Ryskin prescription. The non-perturbative color-octet matrix elements were first deduced from the fits to the latest CMS data on $J/ψ$ transverse momentum distributions and then applied to describe the ATLAS and LHCb data on $J/ψ$ production and polarization at $\sqrt s = 7$, $8$ and $13$ TeV. We perform an estimation of polarization parameters $λ_θ$, $λ_ϕ$ and $λ_{θϕ}$ which determine $J/ψ$ spin density matrix and demonstrate that treating the soft gluon emission as a series of explicit color-electric dipole transitions within NRQCD leads to unpolarized $J/ψ$ production at high transverse momenta, that is in qualitative agreement with the LHC data.