Abstract
We consider $J/\psi$ photoproduction in ion--ion ultraperipheral collisions (UPCs) at the LHC and RHIC in the coherent and incoherent quasielastic channels with and without accompanying forward neutron emission and analyze the role of nuclear gluon shadowing at small $x$, $x=10^{-4}-10^{-2}$, in these processes. We find that despite the good agreement between large nuclear gluon shadowing and the ALICE data in the coherent channel, in the incoherent channel, the leading twist approximation predicts the amount of nuclear suppression which is by approximately a factor of $1.5$ exceeds that seen in the data. We hypothesize that part of the discrepancy can be accounted for by the incoherent inelastic process of $J/\psi$ photoproduction with nucleon dissociation. To separate the high-photon-energy and low-photon-energy contributions to the $d \sigma_{AA\to AAJ/\psi}(y)/dy$ cross section, we consider ion--ion UPCs accompanied by neutron emission due to electromagnetic excitation of one or both colliding nuclei. We describe the corresponding PHENIX data and make predictions for the LHC kinematics. In addition, in the incoherent quasielastic case, we show that the separation between the low-photon-energy and high-photon-energy contributions can be efficiently performed by measuring the correlation between the directions of $J/\psi$ and the emitted neutrons.
Highlights
A comparison of the resulting theoretical prediction for the cross section of incoherent J/ψ photoproduction in Pb–Pb ultraperipheral collisions (UPCs) at the LHC to the ALICE data, which is characterized to correspond to an incoherent quasielastic process [1], shows that the expected suppression due to nuclear shadowing is larger than that seen in the data
As we already explained in the Introduction, the nuclear suppression factor for coherent nuclear J/ψ photoproduction determined from the corresponding UPC cross section measured by the ALICE collaboration [3,4] compares favorably with the theoretical models predicting large nuclear gluon shadowing, notably, with the leading twist approximation (LTA) [6] and with the EPS09 [5] result
We considered J/ψ photoproduction in ion–ion UPCs at the LHC and RHIC in the coherent and incoherent channels with and without accompanying forward neutron emission and analyzed the role of nuclear gluon shadowing at small x, x = 10−4−10−2, in these processes
Summary
On average, J/ψ photoproduction is dominated by small transverse size dipoles and the corresponding dipole–nucleon cross section (3) is small, the resulting nuclear shadowing is small [18,19], which contradicts the ALICE data; see the discussion in [4] In this respect, the situation is similar to the VMD case considered above. The approach allows one to make predictions for the leading twist shadowing correction to nuclear parton distributions (nPDFs), structure functions and cross sections, which are given as a series in the number of simultaneous interactions with the target nucleons (the multiple scattering series). To the case of coherent J/ψ photoproduction on nuclei considered, the VMD model, the color dipole formalism and the leading twist approximation can be used to calculate the cross section of incoherent nuclear J/ψ photoproduction, σγ A→J/ψ A , where A denotes the final nuclear state containing products of the nuclear disintegration (A = A).
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