Coherent $\mathrm{J}/\psi$ and ${\uppsi '}$ photoproduction at midrapidity in ultra-peripheral Pb–Pb  collisions at $\sqrt{s_{\mathrm {NN}}}~=~5.02$ TeV

Abstract

The coherent photoproduction of $\mathrm{J}/\psi$ and ${\uppsi '}$ mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy $\sqrt{s_{\mathrm {NN}}}~=~5.02$ TeV  with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $\mathrm{J}/\psi$ is reconstructed using the dilepton ($l^{+} l^{-}$) and proton–antiproton decay channels, while for the ${\uppsi '}$  the dilepton and the $l^{+} l^{-} \pi ^{+} \pi ^{-}$ decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 $\mu {\mathrm{b}}^{-1}$. The results are compared with theoretical models for coherent $\mathrm{J}/\psi$ and ${\uppsi '}$ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around $6\times 10^{-4}$, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent $\mathrm{J}/\psi$ cross section including ALICE measurements at forward rapidity. The ratio of ${\uppsi '}$ to $\mathrm{J}/\psi$ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.

1 Introduction

Photonuclear reactions can be studied in ultra-peripheral collisions (UPCs) of heavy ions where the two nuclei pass by with an impact parameter larger than the sum of their radii. Hadronic interactions are suppressed and the dominant electromagnetic interactions are mediated by photons of small virtualities. The intensity of the photon flux is growing with the squared nuclear charge of the colliding ion resulting in large cross sections for the photoproduction of vector mesons in heavy-ion collisions. The photoproduction process has a clear experimental signature: the decay products of vector mesons are the only signal in an otherwise empty detector.

The physics of vector meson photoproduction is described in [1,2,3,4]. Photoproduction of vector mesons in ion collisions can either be coherent, i.e. the photon interacts consistently with all nucleons in a nucleus, or incoherent, i.e. the photon interacts with a single nucleon. Experimentally, one can distinguish between these two production types through the typical transverse momentum of the produced vector mesons, which is inversely proportional to the transverse size of the target. While the coherent photoproduction is characterized by the production of mesons with low transverse momentum ($\langle p_{\mathrm{T}} \rangle \sim$ 60 MeV/c), the incoherent is dominated by mesons with higher values ($\langle p_{\mathrm{T}} \rangle \sim$ 500 MeV/c). In the first case, the nuclei usually do not dissociate, but the electromagnetic fields of ultrarelativistic heavy nuclei are strong enough to develop other independent soft electromagnetic interactions accompanying the coherent photoproduction process and resulting in the excitation of one or both of the nuclei. In the second case, the nucleus breaks up and usually emits neutrons close to the beam rapidities which can be measured in zero-degree calorimeters (ZDC) placed at long distances on both sides of the detector [5]. The incoherent photoproduction can also be accompanied by the excitation and dissociation of the target nucleon resulting in even higher transverse momenta of the produced vector mesons [6].

Coherent heavy vector meson photoproduction is of particular interest because of its connection with the gluon distribution functions (PDFs) in protons and nuclei [7]. At low Bjorken-x values, nuclear parton distribution functions are significantly suppressed in the nucleus with respect to free proton PDFs, a phenomenon known as parton shadowing [8]. Shadowing effects are usually attributed to multiple scattering and addressed in various phenomenological approaches based on elastic Glauber-like rescatterings of hadronic components of the photon, Glauber–Gribov inelastic rescatterings, and high-density QCD [9,10,