Two values of isolation cone radii were tried (0.5 and 0.7 in η-Φ), first only with the EMCal, and then with both EM and HCal. 

Fig. 1. Effect of isolation cut on prompt photon signal (fraction of the events that pass the cut).

Fig. 2. Effect of isolation cut on the background (fraction of the events that pass the cut).

Discriminating power, defined as the imporvement of the signal to background ratio with respect to no isolation cut, is highest with the most stringent cut. 

Fig. 3. Discriminating power

The improvement on signal to background can be as large as a factor of 4 at ~3 GeV of p_T. The effect is much greater for π⁰ background. 

Fig. 4. NLO pQCD prediction from Werner

Fig. 5. Event count after isolation cut

Fig. 6. The current estimate of the STATISTICAL uncertainty, no simulation error included.