CMS-PAS-HIN-19-014

CMS-PAS-HIN-19-014
Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultra-peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV
CMS Collaboration
June 2020

Abstract: The first measurement of forward neutron multiplicity dependence of $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ production in ultra-peripheral PbPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 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 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.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

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