Exclusive J/ψ production in ultraperipheral collisions at the LHC: constrains on the gluon distributions in the proton and nuclei

TL;DR

This paper addresses constraining the gluon distributions at small $x$ via exclusive $J/\psi$ photoproduction in ultraperipheral collisions (UPCs) at the LHC. It adopts collinear factorization in perturbative QCD and a leading-order (LO) framework, expressing cross sections in terms of $xG_T(x,\mu^2)$ and including skewness $R_g$, the real-to-imaginary ratio $\eta$, normalization $F^2(\mu^2)$, and nuclear shadowing $R(x,\mu^2)$. By simultaneously describing the proton data from LHCb and Pb data from ALICE, it finds an effective hard scale $\mu^2\simeq 3$ GeV$^2$ and a normalization factor $F^2(\mu^2)\approx 0.5$, indicating sizable nuclear shadowing down to $x\sim 10^{-3}$ and constraining the proton gluon PDF down to $x\sim 6\times 10^{-6}$. It also provides predictions for coherent $J/\psi$ production in PbPb UPCs with electromagnetic excitation and neutron emission channels, and discusses consistency with leading-twist nuclear PDFs, thereby offering a robust small-$x$ benchmark for future measurements.

Abstract

Using the framework of collinear factorization in perturbative QCD (pQCD), we analyze the recent data of the LHCb and ALICE collaborations on exclusive photoproduction of J/ψ in ultraperipheral pp and AA collisions, respectively. We demonstrate that the simultaneous analysis of the proton and Pb data allows us to reduce the ambiguity of the pQCD description of the $γp \to J/ψp$ and $γA \to J/ψp$ cross sections and, hence, to place additional constraints on the gluon distributions in the nucleon and nuclei at small x. We also make predictions for the cross section of coherent exclusive photoproduction of J/ψ in nucleus-nucleus ultraperipheral collisions accompanied by the electromagnetic excitation of nuclei and the subsequent neutron emission.

Figures (8)

• Figure 1: The $\gamma p\to J/\psi p$ cross section as a function of the $\gamma p$ center-of-mass energy $W_{\gamma p}$. Upper panel: The available high-energy data vs. the fit Guzey:2013xba (the curve labeled "Fit") and LO pQCD predictions of eq. (\ref{['csprot']}) at $\mu^2=2.4$ GeV$^2$ using various gluon distributions of the proton, see text and the Appendix. Lower panel: The same as in the upper panel, but with the pQCD predictions reduced by the multiplicative factor $\zeta=F^2(\mu^2)$ (\ref{['eq:zeta']}).
• Figure 2: The same as in figure \ref{['fig:Sigma_proton_pQCD_Q2_2_4']}, but with the LO pQCD predictions evaluated at $\mu^2=3$ GeV$^2$.
• Figure 3: The suppression factor $S(W_{\gamma p})$ for Lead of eq. (\ref{['eq:S']}) as a function of $x=M_{J/\psi}^2/W_{\gamma p}^2$. The results of the model-independent analysis of the ALICE data of Guzey:2013xba (labeled "ALICE") are compared with the LO pQCD predictions at $\mu^2=2.4$ GeV$^2$ (see text for details).
• Figure 4: The same as in figure \ref{['fig:S_pb208_Q2_2_4']}, but with the LO pQCD predictions evaluated at $\mu^2=3$ GeV$^2$.
• Figure 5: The $Pb Pb \to Pb Pb J/\psi$ cross section of exclusive $J/\psi$ photoproduction in symmetric PbPb UPCs as a function of the rapidity $y$ of $J/\psi$ at $\sqrt{s_{NN}} = 2.76$ TeV. The ALICE data points at $y=-3.1$alice2 and $y=0$alice1 are compared with the LO pQCD predictions of eq. (\ref{['csupc_2']}) at $\mu^2=3$ GeV$^2$.