Coherent J/psi photoproduction in ultra-peripheral PbPb collisions at sqrt(s[NN]) = 2.76 TeV with the CMS experiment

TL;DR

This study reports the first CMS measurement of coherent J/ψ photoproduction in ultra-peripheral PbPb collisions with a neutron break-up pattern Xn0n, extracting a differential cross section dsigma^coh/dy for a restricted rapidity range and extrapolating to the total coherent cross section using neutron-breakup fractions. The analysis uses a dedicated UPC trigger, precise vertexing, and a simultaneous fit to invariant mass and pT to separate coherent and incoherent J/ψ and γγ backgrounds, with detailed acceptance and efficiency corrections. The results strongly disfavor the impulse approximation and align with leading-twist calculations that include nuclear gluon shadowing, highlighting the role of nuclear effects at low x in γA interactions. Break-up mode ratios measured in the data corroborate model predictions and demonstrate the utility of neutron tagging in constraining nuclear gluon distributions via photonuclear processes.

Abstract

The cross section for coherent J/psi photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, X[n]0[n], is measured with the CMS experiment in ultra-peripheral PbPb collisions at sqrt(s[NN]) = 2.76 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159 inverse microbarns, collected during the 2011 PbPb run. The J/psi mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is dsigma[coh,X[n]0[n]] / dy(J/psi) = 0.36 +/- 0.04 (stat) +/- 0.04 (syst) mb in the rapidity interval 1.8 < abs(y) < 2.3. Using a model for the relative rate of coherent photoproduction processes, this X[z,n,z] measurement gives a total coherent photoproduction cross section of dsigma[coh] / dy(J/psi) = 1.82 +/- 0.22 (stat) +/- 0.20 (syst) +/- 0.19 (theo) mb. The data strongly disfavour the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent J/psi photoproduction in gamma + Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing.

Figures (2)

• Figure 1: Results from the simultaneous fit to dimuon invariant mass (top) and $p_{\mathrm{T}}$ (bottom) distributions from opposite-sign muon pairs with $p_{\mathrm{T}}\xspace<1.0\,\text{Ge\spaceV}\xspace$, $1.8<\lvert y \rvert<2.3$ and $2.6<m(\mu^+\xspace\mu^-\xspace)<3.5$$\,\text{Ge\spaceV}$ for the $X_{\mathrm{n}}0_{\mathrm{n}}\xspace$ break-up mode, after all selections are applied. In the left panel the green curve represents the $\gamma + \gamma$ component (second-order polynomial) and the black curve the sum of the $\gamma + \gamma$, incoherent ${J}\space/\space\psi$, and coherent ${J}\space/\space\psi$ components (see text for details). In the right panel the green, red, and blue curves represent $\gamma + \gamma$, coherent ${J}\space/\space\psi$, and incoherent ${J}\space/\space\psi$ components, respectively. The black curve represents the sum of the $\gamma + \gamma$, coherent ${J}\space/\space\psi$, and incoherent ${J}\space/\space\psi$ components. Only statistical uncertainties are shown. The data are not corrected by acceptance and efficiencies, and the MC templates are folded with the detector response simulation.
• Figure 2: Differential cross section versus rapidity for coherent ${J}\space/\space\psi$ production in ultra-peripheral PbPb collisions at $\sqrt{s_{NN}}=2.76$$\,\text{Te\spaceV}$, measured by ALICE Abelev:2012baAbbas:2013oua and CMS (see text for details). The vertical error bars include the statistical and systematic uncertainties added in quadrature, and the horizontal bars represent the range of the measurements in $y$. Also the impulse approximation and the leading twist approximation calculations are shown (see text for details).