Left: raw mass distributions of same-sign and opposite-sign pairs from same events and mixed events in the 10% most central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. Right panel: corresponding ratios of same-event and mixed-event same-sign distributions, as well as estimations of the full and uncorrelated combinatorial backgrounds. Statistical uncertainties are represented by vertical bars.