Abstract
The CMS ECAL is a high resolution electromagnetic calorimeter which relies upon precision calibration in order to achieve and maintain its design performance. Variations in light collected from the lead tungstate crystals, due to intrinsic differences in crystals/photodetectors, as well as variations with time due to radiation damage for example, need to be taken into account. Sophisticated and effective methods of inter-crystal and absolute calibration have been devised, using collision data from the 2011 LHC run and a dedicated light injection system. For inter-calibration, low mass particle (π0 and η) decays to two photons are exploited, as well as the azimuthal symmetry of the average energy deposition at a given pseudorapidity. The light injection system monitors the channel response in real-time and enables the re-calibration of the measured energies over time. This is cross-checked by the comparison of E/p measurements of electrons from W decays (where the momentum is measured in the CMS tracker) with/without these re-calibrations applied. Absolute calibration has been performed using Z decays into electron–positron pairs.
Highlights
The electromagnetic calorimeter (ECAL) [1] of the CMS experiment is a homogeneous, near-hermetic detector made of 75848 lead-tungstate (PbWO4) crystals
The barrel (EB) is organized into 36 supermodules and is closed at each end by endcaps (EE), each consisting of two dees
For light collection it is equipped with avalanche photodiodes (APD) in the barrel part and vacuum phototriodes (VPT) in the endcaps
Summary
The electromagnetic calorimeter (ECAL) [1] of the CMS experiment is a homogeneous, near-hermetic detector made of 75848 lead-tungstate (PbWO4) crystals. The barrel (EB) is organized into 36 supermodules and is closed at each end by endcaps (EE), each consisting of two dees. For light collection it is equipped with avalanche photodiodes (APD) in the barrel part and vacuum phototriodes (VPT) in the endcaps. A silicon/lead pre-shower detector (ES) is installed in front of the endcaps to improve the γ/π0 discrimination. ECAL is a crucial detector for several physics studies, above all the search the Higgs boson through its γγ decay. The Higgs discovery potential depends on ECAL energy and position resolution which in turn rely on stability of the channel response over extended periods of time and on the precision of their energy calibration. The calibration procedures and the results obtained in 2011 will be discussed
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