Nonperturbative and spin effects in the central exclusive production of tensor $\chi_c(2^+)$ meson

Abstract

We discuss central exclusive production (CEP) of the tensor $\chi_c(2^{+})$ meson in proton-(anti)proton collisions at Tevatron, RHIC and LHC energies. The amplitude for the process is derived within the $k_t$-factorisation approach. Differential and total cross sections are calculated for several unintegrated gluon distributions (UGDFs). We compare exclusive production of all charmonium states $\chi_c(0^+),\,\chi_c(1^+)$ and $\chi_c(2^+)$. Equally good description of the recent Tevatron data is achieved both with Martin-Ryskin phenomenological UGDF and UGDF based on unified BFKL-DGLAP approach. Unlike for Higgs production, the main contribution to the diffractive amplitude of heavy quarkonia comes from nonperturbative region of gluon transverse momenta $Q_{\perp}<1\,\GeV$. At $y \approx$ 0, depending on UGDF we predict the contribution of $\chi_c(1^+,2^+)$ to the $J/\Psi + \gamma$ channel to be comparable or larger than that of the $\chi_c(0^+)$ one. This is partially due to a significant contribution from lower polarization states $\lambda=0$ for $\chi_c(1^+)$ and $\lambda=0,\,\pm 1$ for $\chi_c(2^+)$ meson. Corresponding theoretical uncertainties are discussed.