Coherent and incoherent $J/\psi$ photonuclear production in an energy-dependent hot-spot model

In a previous publication, we have presented a model for the photoproduction of $J/\psi$ vector mesons off protons, where the proton structure in the impact-parameter plane is described by an energy-dependent hot-spot profile. Here we extend this model to study the photonuclear production of $J/\psi$ vector mesons in coherent and incoherent interactions of heavy nuclei. We study two methods to extend the model to the nuclear case: using the standard Glauber-Gribov formalism and using geometric scaling to obtain the nuclear saturation scale. We find that the incoherent cross section changes sizably with the inclusion of subnucleonic hot spots and that this change is energy dependent. We propose to search for this behavior by measuring the ratio of the incoherent to coherent cross sections at different energies. We compare the results of our model to results from the Relativistic Heavy-Ion Collider (RHIC) and from run 1 at the Large Hadron Collider (LHC), finding satisfactory agreement. We also present predictions for the LHC at the new energies reached in run 2. The predictions include $J/\psi$ production in ultraperipheral collisions, as well as the recently observed photonuclear production in peripheral collisions.

Figure 1

Diffractive photonuclear production of $J/\psi$ in $AA$ collisions. In diagram (a), the photon interacts with the full nucleus $A$, while in panel (b) it scatters off one of the nucleons in $A$. Accordingly, these processes are called coherent and incoherent $J/\psi$ production, respectively.

Figure 2

Predictions for the coherent photonuclear production of $J/\psi$ off lead targets as a function of $x$ ($W_{\gamma \mathrm{Pb}}$) compared with data extracted from ALICE measurements [28, 29, 30], as explained in Ref. [17].

Figure 3

Predictions for the incoherent photonuclear production of $J/\psi$ off lead targets as a function of $x$ ($W_{\gamma \mathrm{Pb}}$) compared with data extracted from ALICE measurements [29], as explained in Ref. [17].

Figure 4

Ratio of the incoherent to the coherent cross section for the photonuclear production of $J/\psi$ off lead targets as a function of $x$ ($W_{\gamma \mathrm{Pb}}$) compared with ALICE [29].

Figure 5

Predictions for the $t$ dependence at midrapidity for the LHC run 1 of the coherent and incoherent photonuclear cross section using the GG approach with (hs) or without ($\mathrm{n}$) subnucleonic degrees of freedom.

Figure 6

Cross section for coherent photonuclear $J/\psi$ production in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV, corresponding to the LHC run 1 energies, as a function of rapidity. Predictions of the model are compared to data measured in UPC by ALICE [28, 29] and CMS [32], as well as data measured by ALICE [30] in peripheral collisions in the 70–90% centrality class.

Figure 7

Cross section for incoherent photonuclear $J/\psi$ production in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV, corresponding to the LHC run 1 energies, as a function of rapidity. Predictions of the model are compared to data measured in UPC by ALICE [29].

Figure 8

Cross section for coherent photonuclear $J/\psi$ production in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, corresponding to the LHC run 2 energies, as a function of rapidity.

Figure 9

Cross section for coherent photonuclear $J/\psi$ production in Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, corresponding to the LHC run 2 energies, as a function of rapidity.