Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{NN}}$=5.02  TeV

Bruno, Giacomo [UCL] Bury, Florian [UCL] Caputo, Claudio [UCL] David, Pieter [UCL] Delaere, Christophe [UCL] Delcourt, Martin [UCL] Donertas, Izzeddin Suat [UCL] Giammanco, Andrea [UCL] Lemaitre, Vincent [UCL] Mondal, Kuntal [UCL] Prisciandaro, Jessica [UCL] Taliercio, Angela [UCL] Teklishyn, Maksym [UCL] Vischia, Pietro [UCL] Wuyckens, Sophie [UCL] Zobec, Joze [UCL] et al.

The first measurement of the dependence of $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ production on the multiplicity of neutrons emitted very close to the beam direction in ultraperipheral heavy ion collisions is reported. Data for lead-lead interactions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$, with an integrated luminosity of approximately $1.5{\mathrm{nb}}^{-1}$, are collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region $8<m_{\mu \mu }<60\mathrm{GeV}$ are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range $|\eta |>8.3$. The back-to-back correlation structure from leading-order photon-photon scattering is found to be significantly broader for events with a larger number of emitted neutrons from each nucleus, corresponding to interactions with a smaller impact parameter. This observation provides a data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results provide new constraints on models of photon-induced interactions in ultraperipheral collisions. They also provide a baseline to search for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma produced in hadronic heavy ion collisions.