Exclusive photoproduction of charmonia in $γp \to V p$ and $p p \to p V p$ reactions within $k_t$-factorization approach

TL;DR

This work computes exclusive $J/ψ$ and $ψ′$ photoproduction in $γp$ and hadronic collisions using a $k_t$-factorization framework with multiple unintegrated gluon distributions. It includes both Dirac and Pauli electromagnetic couplings and accounts for elastic absorption, comparing predictions to HERA and LHCb data. The results show nonlinear UGDFs can better describe recent LHCb measurements, suggesting saturation effects may be present, though conclusions on saturation onset remain inconclusive. The approach also reproduces Tevatron data, and future work aims to refine the UGDF to simultaneously fit DIS structure functions and semi-exclusive data while incorporating inelastic photon-proton contributions.

Abstract

The amplitude for $γp \to J/ψp$ ($γp \to ψ' p$) is calculated in a pQCD $k_{T}$-factorization approach. The total cross section for this process is calculated for different unintegrated gluon distributions and compared with the HERA data and the data extracted recently by the LHCb collaboration. The amplitude for $γp \to J/ψp$ ($γp \to ψ' p$) is used to predict the cross section for exclusive photoproduction of $J/ψ$ ($ψ'$) meson in proton-proton collisions. Compared to earlier calculations we include both Dirac and Pauli electromagnetic form factors. The effect of Pauli form factor is quantified. Absorption effects are taken into account and their role is discussed in detail. Different differential distributions e.g. in $J/ψ$ ($ψ'$) rapidity and transverse momentum are presented and compared with existing experimental data. The UGDF with nonlinear effects built in better describe recent experimental data of the LHCb collaboration but no definite conclusion on onset of saturation can be drawn. We present our results also for the Tevatron. A good agreement with the CDF experimental data points at the midrapidity for both $J/ψ$ and $ψ'$ is achieved.

Figures (19)

• Figure 1: Diagrams representing amplitude for the $\gamma p \to J/\psi$ p process.
• Figure 2: Diagrams representing Born amplitudes considered for the $p p \to p p J/\psi$ process.
• Figure 3: Diagrams representing absorptive corrections considered for the $p p \to p p J/\psi$ process.
• Figure 4: Total cross section for the $\gamma p \to J/\psi p$ as a function of the subsystem energy together with the HERA data and pseudodata obtained by the LHCb collaboration. Three disfferent UGDFs have been used: Ivanov-Nikolaev (solid), Kutak-Stasto linear (dashed) and Kutak-Stasto nonlinear (dash-dotted). The dotted line represents calculation with a simple power-like parametrization of the old HERA data H1_b. The HERA data points HERA_new and the LHCb data ponits LHCb_firstLHCb_second are shown for comparison.
• Figure 5: $J/\psi$ rapidity distribution calculated with the Born amplitudes for three different UGDFs from the literature for $\sqrt{s}$ = 7 TeV. The dashed lines include contributions with Dirac $F_1$ electromagnetic form factor and the solid lines include in addition Pauli $F_2$ electromagnetic form factor. The new LHCb data points LHCb_second are shown for comparison.