Abstract
The ATLAS experiment is designed to study the proton-proton collisions produced at the Large Hadron Collider (LHC) at CERN. Liquid Argon sampling calorimeters are used for all electromagnetic calorimetry covering the pseudorapidity region up to 3.2, as well as for hadronic calorimetry in the range 1.4-4.9. The electromagnetic calorimeters use lead as passive material and are characterized by an accordion geometry that allows a fast and uniform azimuthal response. Copper and tungsten were chosen as passive material for the hadronic calorimetry; whereas a parallel plate geometry was adopted at large polar angles, an innovative one based on cylindrical electrodes with thin argon gaps was designed for the coverage at low angles, where the particles flow is higher. All detectors are housed in three cryostats kept at 88.5 K. After installation in 2004-2006, the calorimeters were extensively commissioned over the three years period prior to first collisions in 2009, using cosmic rays and single LHC beams. Since then, around 27 fb−1 of data have been collected at a unprecedented center of mass energies between 7 TeV and 8 TeV. During all these stages, the calorimeter and its electronics have been operating with performances very close to the specification ones. After 2019, the instantaneous luminosity will reach 2-3 × 1034 cm−2s−1, well above the luminosity for which the calorimeter was designed. In order to preserve its triggering capabilities, the detector will be upgraded with a new fully digital trigger system with a refined granularity. In 2023, the instantaneous luminosity will ultimately reach 5-7 × 1034 cm−2s−1, requiring a complete replacement of the readout electronics. Moreover, with an increased particle flux, several phenomena (liquid argon boiling, space charge effects…) will affect the performance of the forward calorimeter (FCal). A replacement with a new FCal with smaller LAr gaps or a new calorimeter module are considered. The performance of these new calorimeters is being studied in highest intensity particle beams. This contribution covers all aspects of the first three years of operation. The excellent performance achieved is especially detailed in the context of the discovery of the Higgs boson announced in July 2012. The future plans to preserve this performance until the end of the LHC program are also presented.
Highlights
RejectionATLAS Simulation s = 14 TeV, μ = 80 Electron efficiency = 95% Run 2 Phase I: R η, w η,2 cutsRη, w, η,2 f cuts L1 EM ET [GeV]95% Efficiency ATLAS Simulation p T [GeV]higher radiation levels and to upgrade the trigger system
The electromagnetic barrel (EMB) is enclosed in the barrel cryostat, while the endcap cryostats contain the electromagnetic endcap (EMEC), hadronic endcap (HEC) and forward calorimeter (FCal)
2.2 Operational performance The electromagnetic part of the Liquid Argon Calorimeter has a total of 173312 channels of which 76 (0.04%) cannot be read out because of technical defects either inside the cryostat or in the readout electronics
Summary
The design for the LAr calorimeter in the ATLAS detector was largely motivated by requirements on searches for the Higgs boson in which the final states contain photons, electrons, jets and missing transverse energy. The EMB is enclosed in the barrel cryostat, while the endcap cryostats contain the EMEC, HEC and FCal. The EMB and EMEC were constructed using an accordion geometry, with copper and kapton electrodes and lead as the absorbing material. The electrodes are positioned between the absorber plates by honeycomb spacers This design provides good hermeticity, azimuthal uniformity and fast response. The HEC is segmented into four layers constructed in parallel-plate geometry. It has copper absorbers and copper and. The FCal consists of three modules constructed of electrode rods parallel to the beam pipe, sitting in a metal absorber matrix (figure 3). The first module of the FCal, consisting of a copper matrix, is used for electromagnetic measurements. The two latter modules, consisting of tungsten matrices, are used for hadronic measurements
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