Azimuthal Angle Correlations of Muons Produced via Heavy-Flavor Decays in 5.02 TeV $\mathrm{Pb}+\mathrm{Pb}$ 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 of a measurement of the azimuthal angle correlations between muons originating from semileptonic decays of heavy quarks produced in 5.02 TeV $\mathrm{Pb}+\mathrm{Pb}$ and $pp$ collisions at the LHC. The muons are measured with transverse momenta and pseudorapidities satisfying $p_{\mathrm{T}}^{\mu }>4\mathrm{GeV}$ and $|{\eta }^{\mu }|<2.4$, respectively. The distributions of azimuthal angle separation $\mathrm{\Delta }\varphi$ for muon pairs having pseudorapidity separation $|\mathrm{\Delta }\eta |>0.8$, are measured in different $\mathrm{Pb}+\mathrm{Pb}$ 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 $\mathrm{\Delta }\varphi$ distributions at $\mathrm{\Delta }\varphi \sim \pi$, 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 $\mathrm{\Delta }\varphi$ distributions, are found to not vary significantly as a function of $\mathrm{Pb}+\mathrm{Pb}$ collision centrality and are similar for $pp$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions. This observation will provide important constraints on theoretical descriptions of heavy-quark interactions with the quark-gluon plasma.

Figure 1

Measured two-muon correlation functions $C(\mathrm{\Delta }\varphi )$. Top row: results for $\mathrm{Pb}+\mathrm{Pb}$ data in the 0%–10% centrality interval; bottom row: $pp$ results; left column: same-sign pairs; middle column: opposite-sign pairs; right column: all pairs. Also shown are the results of fits of the correlation functions to Eq. (1), along with different components of the fits.

Figure 2

Top panels: The measured widths $\sigma$ of the away-side peak in the two-particle correlation functions in $\mathrm{Pb}+\mathrm{Pb}$ collisions versus centrality. Also shown are the values of the $\mathrm{\Gamma }$ parameter in Eq. (1). The $\sigma$ and $\mathrm{\Gamma }$ measured in $pp$ collisions are also shown. The vertical lines (bands) on the data points represent the statistical (systematic) uncertainties. The horizontal lines indicate the nominal $pp$ values. Bottom panels: The difference between the squared-widths of the $\mathrm{Pb}+\mathrm{Pb}$ and $pp$ measurements. Also shown are the upper limit on ${\sigma }_{\mathrm{int}}^2$ and ${\sigma }_{\mathrm{int}}$ at the 90% C.L. As the 90% C.L. on ${\sigma }_{\mathrm{int}}^2$ is negative for the 0%–10% centrality interval, the corresponding 90% C.L. on ${\sigma }_{\mathrm{int}}$ cannot be evaluated, and is not shown. The left, middle, and right panels correspond to same-sign pairs, opposite-sign pairs, and all pairs.