Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultraperipheral Pb-Pb collisions at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV

TL;DR

This study measures how the production of muon pairs in ultraperipheral Pb-Pb collisions depends on neutron emission, revealing impact parameter effects on photon transverse momentum and providing constraints for photon interaction models.

Contribution

First measurement of neutron multiplicity dependence of muon pair acoplanarity in ultraperipheral heavy ion collisions, demonstrating impact parameter influence on photon transverse momentum.

Findings

Broader muon pair azimuthal correlations with increased neutron emission.

Photon transverse momentum depends on impact parameter.

Provides baseline for studying quark-gluon plasma effects.

Abstract

The first measurement of the dependence of $\gamma\gamma$ $\to$ $\mu^{+}\mu^{-}$ 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 $\sqrt{s_\mathrm{NN}} =$ 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 $\lt$ $m_{\mu\mu}$ $\lt$ 60 GeV are extracted for events including 0, 1, or at least 2 neutrons detected in the forward pseudorapidity range $|\eta|$ $\gt$ 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.