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ing the vertex. We find that the vertex-finding efficiency ϵ$_{vertex}$ for Υ events is 96.4 ± 0 . 9%. In the 2006 run, the luminosity was high enough that 

there can be multiple primary vertices due to pileup events in the TPC. We find that about ≈ 9% of the Υ-triggered events have 2 or more vertices. For this analysis, we searched for candidates from all the vertices found in an event. We chose the vertex by requiring that the vertex also matched the high momentum TPC tracks that were already selected based on the BEMC tower extrapolation. Since the BEMC is read out after every bunch crossing, out-of-bunch pileup (interactions that happen before or after the triggered bunch cross- ing) is rejected by the TPC track–BEMC cluster match- ing requirement. We end up with a unique, unambiguous vertex in all events. Within-bunch pileup (multiple inter- actions in the same bunch crossing) is negligible for this analysis. Pileup rejection will become more important as the luminosity of RHIC is increased. 

Electrons were identified by selecting charged parti- cle tracks with specific ionization energy loss dE/dx in the TPC consistent with the ionization expected for elec- trons. In the momentum region of interest ( p ≳ 3 GeV /c ), there is overlap between electrons and charged pions, so a cut was placed that yielded an electron ef- ficiency of ϵ$_{dE/dx}$ = 84%. The cut was chosen to opti- mize the effective signal S$_{eff}$ of single electrons, S$_{eff}$ = S/ (2 B/S + 1), where S is the electron signal and B is the hadron background. To do this, we construct a normal distribution of ionization measurements. We 

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use the measured ionization of each track, compare it to the expected ionization for an electron and divide by the expected dE/dx resolution (which depends on the track length and number of measurements). This yields a normalized Gaussian distribution of ionization measure- ments, nσ$_{dE/dx}$ . We fit this distribution with one Gaus- sian function to represent the electrons signal S , and two Gaussian functions to model the background B from pi- ons and other hadrons. 

In addition, we used the combined information of the EMC energy E , as obtained by the 3-tower can- didate clusters from L2, and the TPC track momen- tum p to compute the E/p ratio, which should be unity for ultra-relativistic electrons to a very good approxi- mation. Figure 9 shows the E/p distribution compari- son from an electron sample obtained from the Trigger I and II datasets (filled circles). Cuts were placed at E/p = 1 . 0 ± 0 . 3, which is close to a 2 σ cut, given that fitting with a Gaussian gives σ = 0 . 157 ± 0 . 002. A shape that is approximately Gaussian is expected for the E/p distribution due to the Gaussian shape of the resolution in both the energy measurement done in the BEMC and in the curvature measurement done in the TPC (curva- ture is proportional to 1 /p$_{⊥}$ ). We see that the E/p distri- bution in the data shows a non-Gaussian tail. We studied the shape in several ways. To rule out distortions from misidentification of hadrons, we show only positrons to avoid any antiproton contamination and use tight dE/dx 

