Coherent J/$ψ$ photoproduction at forward rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$ TeV

TL;DR

The study performs the first rapidity-differential measurement of coherent $J/\psi$ photoproduction in ultra-peripheral Pb–Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV, using forward rapidity data from the ALICE muon spectrometer. By fitting dimuon invariant masses and transverse momenta with STARlight-based templates, the analysis extracts the coherent cross section across six rapidity bins and the $\psi'$/ $J/\psi$ ratio, accounting for feed-down and veto effects. The results show suppression relative to the impulse approximation, indicating significant gluon shadowing with an effective $R_g \,\sim\,0.8$ around $x\sim 10^{-2}$, and favor shadowing-inclusive models over naive approaches; the $\psi'$ to $J/\psi$ ratio agrees with proton-photoproduction benchmarks. Together with comparisons to EPS09 LO, LTA, and CGC-based dipole models, the work constrains low-$x$ nuclear gluon densities and informs future studies of nuclear shadowing in heavy-ion UPCs.

Abstract

The ALICE collaboration performed the first rapidity-differential measurement of coherent J/$ψ$ photoproduction in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV. The J/$ψ$ is detected via its dimuon decay in the forward rapidity region ($-4.0 < y < -2.5$) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 $μ$b$^{-1}$. The cross section for coherent J/$ψ$ production is presented in six rapidity bins. The results are compared with theoretical models for coherent J/$ψ$ photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of $ψ'$ to J/$ψ$ coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons.

Figures (4)

• Figure 1: Left: (color online) Invariant mass distribution for muon pairs satisfying the event selection described in the text. The dashed green line corresponds to the background. The solid magenta and red lines correspond to Crystal Ball functions representing ${\mathrm J}/\psi$ and $\psi'$ signals, respectively. The solid blue line corresponds to the sum of background and signal functions. Right: transverse momentum distribution for muon pairs in the range $2.85 < m_{\mu\mu} < 3.35$ GeV/$c^{2}$(around the ${\mathrm J}/\psi$ mass).
• Figure 2: Invariant mass distributions in six rapidity bins for muon pairs satisfying the event selection described in the text.
• Figure 3: Transverse momentum distributions in six rapidity intervals for muon pairs satisfying the event selection described in the text.
• Figure 4: Measured coherent differential cross section of ${\mathrm J}/\psi$ photoproduction in ultra-peripheral Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeemV. The error bars represent the statistical uncertainties, the boxes around the points the systematic uncertainties. The theoretical calculations Klein:1999qjGuzey:2013xbaKlein:2016yzrGuzey:2016piuGoncalves:2014wnaSantos:2014znaLappi:2010ddLappi:2013amCepila:2016ukuCepila:2017nef described in the text are also shown. The green band represents the uncertainties of the EPS09 LO calculation.