The cross-section requires the following components:

N_raw: number of gamma candidates for a given pt bin passing the cut on the discriminator.  Awaiting scripts from Mike.
N_pi: expected number of pions for a given pt bin.  Determined from PHENIX data.
e_gamma: efficiency of gamma maker at finding photons.  Need to write some code for this, inverting previous code for determining if a candidate is a gamma, pi0, or eta.
e_pion: efficiency of gamma maker at finding pions.  Ditto.
efficiency of the cut: Easy to find once a cut is decided on.
background rejection of the cut: Ditto.

Note: using |eta|<.8, p_T<15 for now to avoid the necessity of using theoretical calculations to determine N_pi.

Things to do:

Process gamma_jet MC background sample.  Mike may do this.
Calculate value of discriminant for data samples: see N_raw above.
Determine the gamma maker efficiencies (though for pions this depends on the gamma_jet MC background being turned into gammaTrees).
Determine a cut value.