Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{NN}}$=5.02 TeV
(2021) Physical Review Letters — Vol. 127 (2021)
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- The first measurement of the dependence of <math display="inline"><mrow><mi>γ</mi><mi>γ</mi><mo stretchy="false">→</mo><msup><mrow><mi>μ</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>μ</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> 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 <math display="inline"><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>5.02</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>, with an integrated luminosity of approximately <math display="inline"><mrow><mn>1.5</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>nb</mi></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup></mrow></math>, are collected using the CMS detector at the LHC. The azimuthal correlations between the two muons in the invariant mass region <math display="inline"><mrow><mn>8</mn><mo><</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>μ</mi><mi>μ</mi></mrow></msub><mo><</mo><mn>60</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range <math display="inline"><mrow><mrow><mo stretchy="false">|</mo><mi>η</mi><mo stretchy="false">|</mo></mrow><mo>></mo><mn>8.3</mn></mrow></math>. 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.
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Bruno, G., Bury, F., Caputo, C., David, P., Delaere, C., Delcourt, M., Donertas, I. S., Giammanco, A., Lemaitre, V., Mondal, K., Prisciandaro, J., Taliercio, A., Teklishyn, M., Vischia, P., Wuyckens, S., Zobec, J., & et al. (2021). Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{NN}}$=5.02 TeV. Physical Review Letters, 127. https://doi.org/10.1103/PhysRevLett.127.122001 (Original work published 2021)