Energy Calibration and Resolution of the CMS Electromagnetic Calorimeter in $pp$ Collisions at $\sqrt{s} = 7$ TeV

The energy calibration and resolution of the electromagneticcalorimeter (ECAL) of the CMS detector have been determined usingproton-proton collision data from LHC operation in 2010 and 2011 at acentre-of-mass energy of √s = 7 TeV with integrated luminositiesof about 5\fbinv.Crucial aspects of detector operation, such as the environmentalstability, alignment, and synchronization, are presented. The in-situcalibration procedures are discussed in detail and include themaintenance of the calibration in the challenging radiationenvironment inside the CMS detector. The energy resolution forelectrons from Z-boson decays is better than 2% in the centralregion of the ECAL barrel (for pseudorapidity |η| < 0.8) and is2–5% elsewhere. The derived energy resolution for photons from125 GeV Higgs boson decays varies across the barrel from 1.1% to2.6% and from 2.2% to 5% in the endcaps. The calibration of theabsolute energy is determined from Z→e+e− decays to aprecision of 0.4% in the barrel and 0.8% in the endcaps.