Prompt charmonia production and polarization at LHC in the NRQCD with kt-factorization. Part II: $χ_c$ mesons

TL;DR

This work analyzes prompt χ_c production in pp collisions at the LHC using k_T-factorization combined with NRQCD, focusing on χ_c1 and χ_c2 and their feed-down to J/ψ. Off-shell gg* amplitudes for g* g* → χ_cJ are convoluted with unintegrated gluon densities from CCFM-based or KMR prescriptions, and NRQCD long-distance matrix elements are fitted to ATLAS 7 TeV data to separate color-singlet and color-octet contributions. The results indicate color-singlet dominance at low to moderate p_T with a small color-octet component improving high-p_T behavior, and they favor unequal color-singlet wave functions for χ_c1 and χ_c2. The fitted NMEs successfully describe differential cross sections and the σ(χ_c2)/σ(χ_c1) ratio across ATLAS, CMS, and LHCb, providing a more accurate picture of χ_c production and its impact on J/ψ polarization via feed-down.

Abstract

In the framework of kt-factorization approach,the production of prompt $χ_c$ mesons in pp collisions at the LHC energies is studied. Our consideration is based on the off-shell amplitudes for hard partonic subprocesses $g^*g^*\toχ_{cJ}$ and non-relativistic QCD formalism for bound states. The transverse momentum dependent (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. Taking into account both color singlet and color octet contributions, we deduce the corresponding non-perturbative long-distance matrix elements from the fits to the latest ATLAS data on $χ_{c1}$ and $χ_{c2}$ transverse momentum distributions at $\sqrt s = 7$ TeV. We find that these distributions at small and moderate pt are formed mainly by the color singlet components. We successfully described the data on the differential cross sections and relative production rates $σ(χ_{c2})/σ(χ_{c1})$ presented by the ATLAS, CMS and LHCb Collaborations. We find that the fit points to unequal wave functions of $χ_{c1}$ and $χ_{c2}$ states.

Figures (3)

• Figure 1: The prompt $\chi_c$ production at the LHC calculated as a function of $\chi_c$ mesons transverse momenta at $\sqrt s = 7$ TeV. Left panel: the dashed and dotted curves correspond to the color-singlet $^3P_J^{(1)}$ and color-octet $^3S_1^{(8)}$ contributions calculated with the KMR gluon density. The solid curve represent the sum of CS and CO terms. Right panel: the solid, dashed and dash-dotted curves correspond to the predictions obtained with the A0, JH and KMR gluon densities, respectively. The experimental data are from ATLAS11.
• Figure 2: The prompt $\chi_c$ production at the LHC calculated as a function of decay $J/\psi$ transverse momenta at $\sqrt s = 7$ TeV. The solid, dashed and dash-dotted curves correspond to the predictions obtained with the A0, JH and KMR gluon densities, respectively. The experimental data are from ATLAS11.
• Figure 3: The relative production rate $\sigma(\chi_{c2})/\sigma(\chi_{c1})$ calculated as a function of $J/\psi$ meson transverse momenta at $\sqrt s = 7$ TeV. Notation of all curves is the same as in Fig. 2. The experimental data are from ATLAS11, CMS12 and LHCb13.