Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultra-peripheral PbPb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$

The first measurement of forward neutron multiplicity dependence of $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ production in ultra-peripheral PbPb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$ is reported by the CMS experiment at the LHC. The azimuthal correlations between two muons in the invariant mass region $8 < M_{\mu\mu}< 60~\mathrm{GeV}$ are extracted for events associated with zero, one or at least two neutrons produced in the very forward region. The strong back-to-back correlation structure from leading-order photon-photon scattering is significantly broadened for events producing a larger number of neutrons from each nucleus, corresponding to interactions having a smaller impact parameter. This observation demonstrates the transverse momentum and energy of photons emitted from relativistic ions have impact parameter dependence. These results constrain precision modeling of initial photon-induced interactions in ultra-peripheral collisions. They also provide a controllable baseline to search for possible final-state effects on lepton pairs resulting from the production of quark-gluon plasma in hadronic heavy ion collisions.