Figure 1

Top panels: invariant mass distributions for opposite-sign (OS) and mixed-events (ME) electron pairs. Bottom panels: OS invariant mass spectra, after the subtraction of the ME distributions, with a comparison to the Monte Carlo signal (solid lines) superimposed. The MC signal is scaled to match the integral of the OS distribution within the mass counting window. From left to right, distributions correspond to the centrality ranges 0-10%,10-40% and 40-90%, respectively. The panels for the 0-10% and 10-40% centrality ranges are obtained from the 2011 data, while the ones for the 40-90% centrality range are obtained from the 2010 data. The two vertical dashed lines shown in each panel indicate the mass interval used for signal counting.