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

TL;DR

This paper investigates prompt $J/$ meson production and polarization at LHC energies using the $k_T$-factorization approach within NRQCD, fitting non-perturbative parameters to experimental data and analyzing polarization behavior.

Contribution

It provides a comprehensive analysis of $J/$ production and polarization at LHC energies using $k_T$-factorization and fits non-perturbative matrix elements to recent data, offering insights into polarization mechanisms.

Findings

Non-perturbative color-octet matrix elements fitted to CMS data.

Predicted $J/\psi$ polarization is unpolarized at high transverse momentum.

Qualitative agreement with LHC polarization measurements.

Abstract

In the framework of $k_T$-factorization approach, the production and polarization of prompt $J/\psi$ 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 $\chi_c$ and $\psi(2S)$ 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 $J/\psi$ transverse momentum distributions and then applied to describe the ATLAS and LHCb data on $J/\psi$ production and polarization at $\sqrt s = 7$, $8$ and $13$ TeV. We perform an estimation of polarization parameters $\lambda_\theta$, $\lambda_\phi$ and $\lambda_{\theta \phi}$ which determine $J/\psi$ 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 $J/\psi$ production at high transverse momenta, that is in qualitative agreement with the LHC data.