Photon energy scale determination and commissioning with radiative Z decays

The CMS electromagnetic calorimeter (ECAL) is composed of 75848 lead-tungstate scintillating crystals. It has been designed to be fast, compact, and radiation-hard, with fine granularity and excellent energy resolution. Obtaining the design resolution is a crucial challenge for the SM Higgs search in the two photon channel at the LHC, and more generally good photon calibration and knowledge of the photon energy scale is required for analyses with photons in the final state. The behavior of photons and electrons in the calorimeter is not identical, making the use of a dedicated standard candle for photons, complementary to the canonical high-yield $Z^0$ decay to electrons, highly desirable. The use of $Z^0$ decays to a pair of muons, where one of the muons emits a Bremstrahlung photon, can be such a standard candle. These events, which can be cleanly selected, are a source of high-purity, relatively high-pt photons. Their kinematics are well-constrained by the $Z^0$ boson mass and the precision on the muon momenta, and can be used for numerous calibration and measurement purposes. This poster presents the event selection method and the results of the photon energy scale measurement via $Z^0 \rightarrow \mu \mu \gamma$ events as well as their use in evaluating the efficiency of photon identification requirements, based on data recorded by the CMS experiment in 2010.