Measurement of $\gamma\gamma\rightarrow\mu^+\mu^-$ pairs in non-ultra peripheral Pb+Pb collisions with the ATLAS detector*

ATLAS measurements of dimuons produced via $\gamma\gamma$ scattering processes in inelastic, non-ultra-peripheral Pb+Pb collisions at 5.02 TeV are presented using an integrated luminosity of 1.9 nb$^{-1}$. The $\gamma\gamma\rightarrow\mu^+\mu^-$ pairs are identified via selections on pair momentum asymmetry and acoplanarity, and the contribution from the heavy flavor decay background is estimated using a template fit method. The pair yields are measured differentially as functions of the centrality, average transverse-momentum ($p_T$) and rapidity of the pair. The measurement shows a depletion in the number of muon pairs near zero acoplanarity in central events, resulting in the distributions peaking at non-zero values of acoplanarity. Fits to the perpendicular transverse momentum ($k_\perp$) distributions are used to estimate the centrality dependence of this peak position. The most probable is shown to increase from the most peripheral to the most central collisions, reaching a value of $k_\perp=36\pm1$ MeV in the 0-5\% most-central collisions. The capacity of these measurements to distinguish between recent theoretical predictions and underlying physical mechanisms for the observed behavior are discussed.