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

Abstract: 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 $|\eta^\mu|<2.4$, respectively. The distributions of azimuthal angle separation, $\Delta\phi$, for muon pairs having pseudorapidity separation $|\Delta\eta|>0.8$, 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 $\Delta\phi$ distributions at $\Delta\phi\sim\pi$, 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 $\Delta\phi$ 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.