Elastic double diffractive production of axial-vector χ_c(1^{++}) mesons and the Landau-Yang theorem

TL;DR

This study derives the exclusive double diffractive production amplitude for χ_c(1^+) in proton–antiproton collisions within a kt-factorisation framework, explicitly incorporating off-shell gluons that lift the Landau–Yang suppression. The calculated g*g*→χ_c(1^+) vertex, together with off-diagonal UGDFs, yields nonzero cross sections highly sensitive to the UGDF model, with χ_c(1^+) production being substantially smaller than χ_c(0^+) yet potentially observable in the J/ψγ decay channel due to the large BR. Spin-summed and polarized results reveal a strong dependence on gluon virtualities and t-values, and the ratio BR·σ_tot(χ_c(1^+))/BR·σ_tot(χ_c(0^+)) remains roughly model-independent while varying with kinematics. The work highlights experimental prospects to test off-shell effects and UGDFs, while noting the need to include absorption corrections for a complete phenomenology.

Abstract

We discuss exclusive elastic double diffractive axial-vector χ_c(1^{+}) meson production in proton-antiproton collisions at the Tevatron. The amplitude for the process is derived within the k_t-factorisation approach with unintegrated gluon distribution functions (UGDFs). We show that the famous Landau-Yang theorem is not applicable in the case of off-shell gluons. Differential cross sections for different UGDFs are calculated. We compare exclusive production of χ_c(1^+) and χ_c(0^+). The contribution of χ_c(1^+) to the J/Ψ+ γchannel is smaller than that of the χ_c(0^+) decay, but not negligible and can be measured. The numerical value of the ratio of the both contributions is almost independent of UGDFs modeling.

Figures (5)

• Figure 1: The sketch of the bare QCD mechanism for diffractive production of the $\chi_c(1^{+})$ meson. Some kinematical variables are shown in addition.
• Figure 2: Coordinate basis in the center-of-mass system of incoming protons $p_{1,2}$.
• Figure 3: Distributions in rapidity of $\chi_c(1^+)$ meson (left panel) and $\chi_c(0^+)$ meson (right panel) for different UGDFs. Dash-dotted line corresponds to BFKL UGDF, long-dashed line -- GBW, short-dashed line -- KL, and two solid lines -- Gaussian UGDFs for $\sigma_0=0.5$ GeV$^2$ (upper line) and $\sigma_0=1.0$ GeV$^2$ (lower line).
• Figure 4: Distribution in $t_{1,2}$ of $\chi_c(1^+)$ meson (left panel) and $\chi_c(0)$ meson (right panel) for different UGDFs.
• Figure 5: Distribution in relative azimuthal angle $\Phi$ of $\chi_c(1^+)$ (left panel) and $\chi_c(0^+)$ (right panel) meson production for different UGDFs.