Speaker
Description
New measurements of impact parameter dependence of photon-photon interactions in ultraperipheral PbPb collisions (UPC) are presented, using data collected by the CMS detector during the LHC Run 2. The “UPC centrality” is classified based on the number of neutrons (including 0, 1, or at least two) detected in the very forward pseudorapidity to control the impact parameter of UPCs. The back-to-back azimuthal correlation structure of $\mu^+\mu^-$ pairs from leading-order photon-photon scattering is measured and 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 the first data-driven demonstration that the average transverse momentum of photons emitted from relativistic heavy ions has an impact parameter dependence. These results are compared with quantum electrodynamics calculations and provide crucial new constraints on models of photon-induced interactions in ultraperipheral collisions. Searches for possible final-state effects on lepton pairs caused by traversing a quark-gluon plasma in hadronic PbPb collisions and new physics beyond the standard model are also discussed with the baseline provided by the new results.
| Submitted on behalf of a Collaboration? | Yes |
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