Energy calibration and resolution of the CMS electromagnetic calorimeter in pp collisions at root s=7 TeV

The energy calibration and resolution of the electromagnetic calorimeter (ECAL) of the CMS detector have been determined using proton-proton collision data from LHC operation in 2010 and 2011 at a centre-of-mass energy of root s = 7 TeV with integrated luminosities of about 5 fb(-1). Crucial aspects of detector operation, such as the environmental stability, alignment, and synchronozation, are presented. The in-situ calibration procedures are discussed in detail and include the maintenance of the calibration in the challenging radiation environment inside the CMS detector. The energy resolution for electrons from Z-boson decays is better than 2 in the central region of the ECAL barrel (for pseudorapidity vertical bar eta vertical bar < 0.8) and is 2-5 elsewhere. The derived energy resolution for photons from 125GeV Higgs boson decays varies across the barrel from 1.1 to 2.6 and from 2.2 to 5 in the endcaps. The calibration of the absolute energy is determined from Z -> e(+)e(-) decays to a precision of 0.4 in the barrel and 0.8 in the endcaps.