Abstract
Liquid argon (LAr) sampling calorimeters are employed by ATLAS for all electromagnetic calorimetry in the pseudorapidity region |η| < 3.2, and for hadronic and forward calorimetry in the region from |η| = 1.5 to |η| = 4.9. In the first LHC run a total luminosity of 27 fb−1 has been collected at center-of-mass energies of 7–8 TeV. After detector consolidation during a long shutdown, Run 2 started in 2015 and 147 fb−1 of data at a center-of-mass energy of 13 TeV have been recorded. In order to realize the level-1 trigger acceptance rate of 100 kHz in Run 2 data taking, the number of read-out samples recorded and used for the energy and the time measurement has been modified from five to four while keeping the expected performance. The well calibrated and highly granular LAr calorimeter reached its design values both in energy measurement as well as in direction resolution. This contribution gives an overview of the detector operation, hardware improvements, changes in the monitoring and data quality procedures, to cope with increased pileup, as well as the achieved performance, including the calibration and stability of the electromagnetic scale, noise level, response uniformity and time resolution.
Highlights
The EM barrel (EMB) and end-cap (EMEC) calorimeters have lead plates interspaced as the passive material with electrodes arranged in accordion-like structures
1.3 Signal Measurement and Readout When an incoming electromagnetic particle hits the lead absorber, an electromagnetic shower is produced in the liquid argon, the resulting current from which is collected by electrodes
The resulting triangular pulse is sent to the on-detector Front-End Boards (FEBs) where it is amplified, split into three overlapping linear gain scales, and shaped
Summary
The LAr calorimeters operated extremely efficiently throughout Run 2, achieving a 99.6% overall data-taking efficiency. This high efficiency can be attributed to very stable system performance as well as a dedicated operations team. The main sources of this type of data loss are noisy channels, noise bursts, coverage, and high voltage (HV) trips [5]. Noise bursts are a well-studied coherent noise phenomenon that affects a large fraction of the detector for a short time (∼1 μs). They are suspected to be induced by unshielded HV cables. New power supplies were installed before 2016 data-taking, significantly reducing losses from this issue
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