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Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultraperipheral Pb-Pb collisions at √sNN=5.02 TeV

The first measurement of the dependence of γγ→μ+μ− 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 √sNN=5.02 TeV, with an integrated luminosity of approximately 1.5 nb-1, were collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region 8<mμμ<60 GeV are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range |η|>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.

high energy physics ; experimental particle physics ; LHC ; CMS ; Hadron-Hadron scattering (experiments) ; Higgs physics ; p p: colliding beams ; Higgs particle: pair production ; bottom: 2 ; p p: scattering ; Higgs particle: coupling ; Higgs particle: hadroproduction ; Higgs particle: decay modes ; bottom: pair production ; photon: pair production ; top: pair production ; coupling: Yukawa ; coupling constant: upper limit ; vector boson: fusion ; mass spectrum: two-photon ; standard model

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