Abstract
This paper presents the electron and photon energy calibration achieved with the ATLAS detector using about 25 fb$^{-1}$ of LHC proton--proton collision data taken at centre-of-mass energies of $\sqrt{s}$ = 7 and 8 TeV. The reconstruction of electron and photon energies is optimised using multivariate algorithms. The response of the calorimeter layers is equalised in data and simulation, and the longitudinal profile of the electromagnetic showers is exploited to estimate the passive material in front of the calorimeter and reoptimise the detector simulation. After all corrections, the $Z$ resonance is used to set the absolute energy scale. For electrons from $Z$ decays, the achieved calibration is typically accurate to 0.05% in most of the detector acceptance, rising to 0.2% in regions with large amounts of passive material. The remaining inaccuracy is less than 0.2-1% for electrons with a transverse energy of 10 GeV, and is on average 0.3% for photons. The detector resolution is determined with a relative inaccuracy of less than 10% for electrons and photons up to 60 GeV transverse energy, rising to 40% for transverse energies above 500 GeV.
Highlights
Precise calibration of the energy measurement of electrons and photons is a fundamental input to many physics measurements
The η categories are defined as bins of size η = 0.2 in the barrel; a more complicated structure is defined in the endcap, according to the presampler layer (PS) acceptance boundaries and the η-dependent high voltage (HV) settings
The calibration procedure for electron and photon energy measurement with the ATLAS detector is presented using LHC Run 1 proton–proton collision data corresponding to a total integrated luminosit√y of about 25 √fb−1 taken at centre-of-mass energies of s = 7 and s = 8 TeV
Summary
Precise calibration of the energy measurement of electrons and photons is a fundamental input to many physics measurements. After the discovery of the Higgs boson by the ATLAS and CMS experiments [1,2], an accurate determination of its properties is of primary importance. A precise measurement of the W boson mass is a long-term goal of the LHC experiments, and requires an excellent accuracy of the electron energy calibration. The achieved calibration was accurate to 0.5–1 % for electrons, depending on pseudorapidity and energy. This paper presents the calibration scheme developed for precision measurements involving electrons and photons with |η| < 2.471 and mostly derived from collision data. It comprises local corrections to the calorimeter energy measurement, and the intercalibration of its longitudinal layers;
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