Abstract
The ATLAS experiment is a multi-purpose detector built for analyzing LHC collision data. In July 2012, ATLAS announced the discovery of the Higgs boson, the last undiscovered particle in the Standard Model of Particle Physics. The ATLAS Liquid Argon (LAr) Calorimeter played a crucial role in the discovery by providing accurate measurements of Higgs final state objects such as photons, electrons and jets. The LAr detector is a sampling calorimeter consisting of four subsystems: an electromagnetic barrel, electromagnetic endcaps, hadronic endcaps, and forward calorimeters. The purity and temperature of the liquid argon remained well above the required levels throughout the data-taking period. Overall the calorimeter performed very well, with over 99% of data it collected in 2012 proton-proton collisions being suitable for physics analyses. In order to ensure good LAr detector performance at future higher luminosity LHC operation, several upgrades are being planned and implemented.
Highlights
The design for the Liquid Argon (LAr) calorimeter in ATLAS was largely motivated by requirements on searches for the Higgs boson in which the final states contain photons, electrons, jets and missing transverse energy
During the 2013-2014 shutdown, the LAr calorimeter has been kept in operation with the HV off and regular calibrations with HV at 100V performed to check the status of the system
Ongoing studies will determine whether the hadronic endcap (HEC) electronics will require replacement due to radiation damage. This replacement will require opening the cryostats since, unlike in the electromagnetic barrel (EMB), electromagnetic endcap (EMEC) and forward calorimeters (FCal), the HEC electronics are located inside the cryostats
Summary
The design for the LAr calorimeter in ATLAS was largely motivated by requirements on searches for the Higgs boson in which the final states contain photons, electrons, jets and missing transverse energy. These particles must be accurately identified, necessitating fine lateral and longitudinal segmentation in the calorimeter. To meet the mass resolution required for the Higgs searches the sampling term is required to be 10% for the electromagnetic calorimeter, 50% for the hadronic calorimeter and 100% for the forward calorimeters. The constant term, which dominates the calorimeter resolution at high energy, is required to be 0.7% for the electromagnetic calorimeter, 3% for the hadronic calorimeter, and 10% for the forward calorimeters. In order to satisfy the linearity, stability and radiation hardness requirements, liquid argon was chosen as the active material
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