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 ultrarelativistic heavy-ion collisions. Results are presented from a measurement of the azimuthal angular correlations between muons originating from heavy-quark half-decays produced in 5.02 TeV Pb þ Pb and pp collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying pμ T > 4 GeV and |ημ| < 2.4, respectively. The azimuthal angular separation distributions Δϕ for muon pairs with pseudorapidity separation |Δη| > 0.8 are measured in different Pb þ Pb centrality intervals and compared with the same distribution measured in pp collisions at the same center-of-mass energy. The results are presented separately for muon pairs with opposite charges, like charges, and all pairs. A clear peak is observed in all Δϕ distributions at Δϕ ∼ π, which is consistent with the production of the parent heavy quark pairs by hard scattering processes. The width of this peak, as characterized by Cauchy-Lorentz fits to the Δϕ distributions, has been found not to vary significantly as a function of the Pb + Pb collision centrality and to be similar for pp and Pb + Pb collisions. This observation will provide important constraints on theoretical descriptions of heavy quark interactions with the quarkgluon plasma.