Gluon shadowing at small $x$ from coherent $J/\psi$ photoproduction data at energies available at the CERN Large Hadron Collider

The cross section for coherent $J/\psi$ photoproduction in Pb-Pb collisions is the sum of two contributions: one from low-energy and the other from high-energy photon-nucleus interactions. A novel method to disentangle the two contributions, allowing one to extract the coherent photo-nuclear cross section for coherent $J/\psi$ production, ${\sigma }_{\gamma \mathrm{Pb}}$, is presented. The utility of the method is demonstrated using measurements from peripheral and ultraperipheral Pb-Pb collisions at the CERN Large Hadron Collider. Applying the proposed method to the available data, it is possible to obtain ${\sigma }_{\gamma \mathrm{Pb}}$ up to a photon-lead center-of-mass energy of 470 GeV, which corresponds to $x$ of $4.4\times {10}^{-5}$. To illustrate a possible use of the extracted photonuclear cross sections, the corresponding nuclear suppression factors are computed and they are compared to predictions of gluon shadowing calculated in the leading-twist approximation approach.

Figure 1

Contributions to coherent production of $J/\psi$ in Pb-Pb collisions. In diagram (a) the target lead nucleus is traveling away from the detector at the moment of the interaction, while in diagram (b) it is traveling towards it.

Figure 2

Form factor for lead computed using Eq. (5) for the dash-dot red line and Eq. (7) for the solid blue line.

Figure 3

Photon flux according to Eq. (3) for a photon energy of $k=39.94$ GeV (see Table 1) using the form factor of Eq. (5) for the solid blue line or the point charge form factor for the dash-dot red line.

Figure 4

Probability of no hadronic nuclear interaction according to Eq. (10).

Figure 5

Cross section for $J/\psi$ coherent photonuclear production off a lead nucleus as a function of $W_{\gamma \mathrm{Pb}}$ (upper axis) and $x$ (lower axis). Statistical uncertainties are represented by a line and systematic uncertainties by the height of the empty box, except for the middle cross section, where the bars represent both types of uncertainties. Blue symbols are evaluated at the center, while red symbols are evaluated at the mean rapidity of the rapidity ranges where the corresponding measurements were performed.

Figure 6

Nuclear suppression factor for $J/\psi$ coherent photoproduction off a lead nucleus as a function of $W_{\gamma \mathrm{Pb}}$ (upper axis) and $x$ (lower axis). Statistical uncertainties are represented by a line and systematic uncertainties by the height of the empty box, except for the middle cross section, where the bars represent both types of uncertainties. Blue symbols are evaluated at the center, while red symbols are evaluated at the mean rapidity of the rapidity ranges where the corresponding measurements were performed. The red (green) band correspond to the prediction of the LTA approach when using CTEQ6L (MNRT07) parton parametrizations. For details see [13].