Observation of impact parameter dependence of $\mu^{+}\mu^{-}$ acoplanarity in ultra-peripheral PbPb collisions*

Photon-photon interactions have been observed in hadronic heavy-ion collisions at very low transverse momentum ($p_T$) regions and the measured $p_T$ and azimuthal angular correlations of lepton pairs via $\gamma\gamma$ scattering in hadronic events exhibit significant broadening compared to that from vacuum production in ultra-peripheral events. There is still no consensus on the origin of the observed broadening, which is mainly from $p_T$ hardening of initial scattered photons as impact parameter decreases toward central hadronic collisions or final-state electromagnetic modifications of lepton pairs in presence of a QGP medium. In this talk, the azimuthal angular correlations and mass spectra of $\mu^{+}\mu^{-}$ pairs via $\gamma\gamma$ scattering will be presented as a function of forward neutron multiplicity and rapidity in ultra-peripheral PbPb collisions with the CMS experiment. The forward neutron multiplicity dependence of $\gamma\gamma \rightarrow \mu^{+}\mu^{-}$ production provides key insight to the origin of observed broadening for photon-photon produced lepton pairs in hadronic collisions while rapidity dependence constrains the relative contributions from leading order and high order photon-photon interactions to measured $\mu^+\mu^-$ pairs.