Observation of Centrality-Dependent Acoplanarity for Muon Pairs Produced via Two-Photon Scattering in $\mathrm{Pb}+\mathrm{Pb}$ Collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ with the ATLAS Detector

This Letter presents a measurement of $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ production in $\mathrm{Pb}+\mathrm{Pb}$ collisions recorded by the ATLAS detector at the Large Hadron Collider at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ with an integrated luminosity of $0.49{\mathrm{nb}}^{-1}$. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from $\gamma \gamma$ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.

Figure 1

Template fits (left) to the $d_{0\mathrm{pair}}$ distributions are shown for the 0–10% interval. The $\alpha$ (center) and $A$ (right) distributions are shown before background subtraction (points). These distributions are normalized to unity over their measured range. In the central and right plots, the background contributions with normalization fixed by the template fitting are indicated by the dashed line with the uncertainties represented by the shaded band.

Figure 2

The background-subtracted distributions are shown for $\alpha$ (upper row) and $A$ (lower row). Each distribution is normalized to unity over its measured range. Moving from left to right, the data (circles) are shown for increasingly peripheral collisions (lower degree of overlap, higher percentile). The distributions obtained from the MC simulation ($\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ generated by STARlight and overlaid on data) are shown for the corresponding centrality interval as a filled histogram. The distribution measured in the most peripheral collisions, the $>80\%$ interval (diamonds) is repeated in each panel to facilitate a direct comparison. The error bars include the statistical and systematic uncertainties. Uncertainties related to the background normalization are not shown.

Figure 3

Results of fits to the muon pair $\alpha$ distributions using the sum of Gaussian and background functions. A standard Gaussian function is shown as a solid curve while the dotted curve shows a Gaussian function in $\alpha$ convolved with the measured $p_{\mathrm{T}\mathrm{avg}}$ distribution. The background distributions are indicated by the dashed lines.

Figure 4

The $k_{\mathrm{T}}^{\mathrm{rms}}$ values obtained from the fits shown in Fig. 3 as a function of $⟨N_{\mathrm{part}}⟩$. The shaded bands indicate the total uncertainty accounting for both the systematic and statistical uncertainties in the $\alpha$ distributions and background. The data points have been horizontally offset for visualization purposes, and the horizontal sizes of the error bands are arbitrary.