The following are the cuts used to identify tracks as electrons.

1. Primary track satisfies quality cuts
2. Track can be projected to the EMC
3. Track is matched to an EMC point
4. Track satisfies electron id cuts

1. Track quality cuts:

• Primary track
• DCA < 2 cm
• NHitsFit / NHitsPoss > 0.52
• NHitsFit > 15
• NHitsdEdx > 10
• Chi2 < 5

1b. Track acceptance cut:

• The track eta cut varies with event VZ to maximize statistics and minimize variation in electron id efficiency, as detailed here.
• 0.3 < eta < 0.9 for -40cm < V_Z < -20cm
• 0.2 < eta < 0.8 for -20cm < V_Z < 0cm
• 0.1 < eta < 0.7 for 0cm < V_Z < 20cm

2. Track projection:

Every track that satisfies the quality cuts above is then projected to the EMC radius using

StMuEmcPosition::trackOnEmc(StThreeVectorD&, StThreeVectorD&, StMuTrack, float, float).

The EMC radii used are obtained from

StEmcGeom::Radius()

specifically,

SMDEta radius: 230.705cm

SMDPhi radius: 232.742cm

3. Point matching:

A track is considered matched to an EMC point if the track's projected coordinates in the EMC is within 0.05 radian to the center of an EMC point. The coordinates obtained from the projection to the SMDEta (SMDPhi) for eta (phi) are compared to the respective coordinates of the EMC point.

4. Electron id cuts

There are three parameters used in these cuts:

• Ratio of momentum to deposited energy, P/E
• Shower profile from number of SMD strips
• Track energy loss, dE/dx

P/E:

The idea is that an electron would deposit most of its energy in the EMC, thus having a ratio of P/E ~ 1. For the plot below, the following cuts are used to identify electrons and hadrons:

Hadrons: NSMDEta == 1 && NSMDPhi == 1, dEdx: | NSigmaElectron | > 3 && | NSigmaPion | < 3 && | NSigmaKaon | < 3 && | NSigmaProton | < 3

Electron: NSMDEta > 1 && NSMDPhi > 1, dEdx: | NSigmaElectron | < 3 && | NSigmaPion | > 3 && | NSigmaKaon | > 3 && | NSigmaProton | > 3

Cut: 0.3 < P/E < 1.5

SMD:

Electron showers are expected to have a broader shower profile than hadrons. For tracks that satisfy the P/E and the strict dE/dx cuts specified above, the shower profile is shown below. When both SMD planes are required, the majority of hadrons trigger a single SMD strip on each plane. Electrons tend to trigger more strips. So to eliminate as many hadrons with as few electrons, we will eliminate the hadron peak at 1:1 Eta:Phi strips. However, noting the much greater number of  hadrons, the small fraction of hadrons with greater than 1:1 SMD strips are still a significant contamination in the much smaller electron sample.

Cut: NSMDEta > 1 && NSMDPhi > 1

dE/dx:

The dE/dx distribution, after P/E and SMD cuts, are fitted using 3 Gaussians to approximate the hadron and electron bands in the P>2GeV region. The cuts (for different p_T bins) are made considering the resulting cut efficiency and electron purity.

More details on how the dE/dx cuts are chosen is discussed here.