Azimuthal angle correlations of muons produced via heavy-flavor decays in 5.02 TeV Pb+Pb and $pp$ collisions with the ATLAS detector

Angular correlations between heavy-quarks provide a unique probe of the quark-gluon plasma created in ultra-relativistic heavy-ion collisions. Results are presented of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy-quarks produced in 5.02 TeV Pb+Pb and $pp$ collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying $p_{\mathrm{T}}^{\mu }>4$ GeV and $\mathrm{\mid }{\eta }^{\mu }\mathrm{\mid }<2.4$, respectively. The distributions of azimuthal angle separation, $\mathrm{\Delta }\phi$, for muon pairs having pseudorapidity separation $\mathrm{\mid }\mathrm{\Delta }\eta \mathrm{\mid }>0.8$, are measured in different Pb+Pb centrality intervals and compared to the same distribution measured in $pp$ collisions at the same center-of-mass energy. Results are presented separately for muon pairs with opposite-sign charges, same-sign charges, and all pairs. A clear peak is observed in all $\mathrm{\Delta }\phi$ distributions at $\mathrm{\Delta }\phi \sim \pi$, consistent with the parent heavy-quark pairs being produced via hard-scattering processes. The widths of that peak, characterized using Cauchy-Lorentz fits to the $\mathrm{\Delta }\phi$ distributions, are found to not vary significantly as a function of Pb+Pb collision centrality and are similar for $pp$ and Pb+Pb collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quark-gluon plasma.