Photon identification efficiency measurement in ATLAS experiment and its application to the $H\to\gamma\gamma$ analysis

Photon candidates in ATLAS experiment on LHC after their reconstruction are highly contaminated by misidentified QCD jets. Further photon identification (PID) is based on differencies in calorimetric variables distributions between photons and jets. The PID efficiency has been measured from simulated MC samples and compared with results obtained using three different data-driven techniques. This comparison gives the PID uncertainty for physics measurements. The uncertainty from photon identification decreased from 11% to around 5% for diphoton events, but still gives one of the biggest impacts in systematics for $H\to\gamma\gamma$ strength measurement.