Measurement of muon pairs produced via <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>γ</mml:mi><mml:mi>γ</mml:mi></mml:mrow></mml:math> scattering in nonultraperipheral <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Pb</mml:mi><mml:mo>+</mml:mo><mml:mi>Pb</mml:mi></mml:mrow></mml:math> collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:msub><mml:mi>s</mml:mi><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:msub></mml:msqrt><mml:mo>=</mml:mo><mml:mn>5.02</mml:mn></mml:mrow></mml:math> TeV with the ATLAS detector

In ultrarelativistic heavy-ion collisions the Lorentz-contracted electromagnetic fields of the nuclei are a source of high-energy quasireal photons which can produce muon pairs via the process $\ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}+{\ensuremath{\mu}}^{\ensuremath{-}}$. In hadronic Pb+Pb collisions the two leptons in the final state provide a sensitive probe of the quark-gluon plasma created in the Pb+Pb collision or the strong magnetic fields that the plasma generates. This work presents new measurements by the ATLAS collaboration at the LHC of the Pb+Pb collision centrality dependence of the angular decorrelation of muon pairs created via the $\ensuremath{\gamma}\phantom{\rule{0}{0ex}}\ensuremath{\gamma}$ process. Comparisons of the results to theoretical calculations suggest that the observed decorrelation in more central collisions results from an initial-state broadening of the photon transverse momenta and not from the interaction of the muons with the plasma or its magnetic field.

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