This image of the Calorimeter elements was taken from the 2008 CDS category Computer generated images  [ one finds also there the detection principle "pie"  ]

 

Design facts and figures

In a sampling calorimeter the material that produces the particle shower is distinct from the material that measures a fraction of the deposited energy [ typically 15% ]. The two materials alternate, and a calibration procedure is developed to estimate the total shower energy.

Few numbers extracted from the ATLAS Fact Sheet 2011:

The Liquid Argon (LAr) Calorimeter works with Liquid Argon at -183 degrees C

• The EM barrel calorimeter is 6.4 m long, 53 cm thick, 110 000 channels. EM endcaps have thickness 0.632 m and radius 2.077 m.
• LAr endcap cryostats also include Hadronic endcaps [ two wheels of thickness 0.8 and 1.0 m with radius 2.09 m ] and Forward calorimeters  [ three modules of radius 0.455 m and thickness 0.450 m each ].

The Tile calorimeter (TileCal) measures the remaining ~30% of hadronic energy after the LAr calorimeter. It counts 500 000 scintillator tiles, integrated in steel absorbers.

• Barrel made of 64 wedges, each 5.6 m long and 20 tons.
• Each Endcap has 64 wedges, each 2.6 m long.

 

Stories, images and videos

• Few videos & images are on the ATLAS web site calorimeters presentation.
• For more photos use the "best of ATLAS" series on the calorimeters construction and installation
• Stories: ATLAS public web site updates tagged as calorimeter
• For ATLAS insiders & alumni: Daniel Fournier gave a talk on the EM design and construction for the ATLAS25 celebration
• For an advanced audience: Isabelle Wingerter's 2017 summer student 4th detector lecture [ see here the full list ]

 

Run 1 and 2 achievements

Electrons and photons energy calibration @ 13 TeV 2016 CONF note [ and Run 1 paper ]:

A precise knowledge of the energy of electrons and photons is important in many analyses, in particular precision measurements such as the  Higgs boson mass.

The reconstruction starts with the raw signal from each calorimeter cell, which is amplified, shaped and sampled. Energy deposits are then grouped into clusters, and the association with tracks reconstructed by the Inner Detector allows to discriminate “unconverted photon”, “converted photons” and electrons. Calibration is applied to convert the measured signal into an estimate of the incident particle energy.

At the start of LHC the calibration of the LAr calorimeter was primarily based on test-beam measurements. The uncertainty on the detector material upstream of the calorimeter, which is of primary importance in understanding its response to incident particles, was estimated using engineering drawings and material survey during construction. The achieved precision was 0.5 to 1%. The calibration scheme established in Run-1 was gradually refined and optimized using large data samples and multivariate analyses:

• Local and data-driven corrections are first applied (to data) to mitigate the non-uniformity in detector response – for examples in case of non nominal high-voltage. These can vary with time and are periodically updated during the data taking period.
• The inter-calibration of the calorimeter longitudinal layers is then adjusted, using simulation: the longitudinal shower depth is used to correct for the effect of the material traversed by particles before reaching the calorimeter [schema].
• Final data-driven scale factors are estimated using Z decays into 2 electrons, and verified using J/psi decays. Uncertainties are evaluated at each step and propagated to the final calibrated energy. 

For Run-1 an accuracy of 0.05 % was achieved in most of the acceptance, rising to 0.2% in regions with large amount of passive material. All parameters such as uniformity and linearity meet the design goals. One of the most demanding and important physics analyses wich required years of developments is the recent measurement of the W mass

Due to the increased data rate in Run-2, the number of digitized samples readout by the calorimeter electronics was reduced from 5 to 4, and the set of coefficients used to convert the ADC counts into energy deposits was re-optimized. The whole calibration chain is thus being revisited, and more events are being collected and studied. 

Stories: check on the ATLAS public web site updates tagged as EGAM group [ mainly photons, as electrons are in most publications combined with muons ]

 

Jet energy measurements @ 13 TeV 2017 paper:

Jets are important elements of the final state in LHC pp collisions, and used in many measurements and searches for new physics. They are reconstructed using clutering algorithms and also calibrated with a series of simulation-based corrections and in-situ techniques, which exploit the transverse momentum balance between a jet and a reference object such as photon, Z boson or multijets systems. An uncertainty of less than 1% is found in the central region, 2% in the forward regions. It grows to 4.5% for low pT jets. 

Stories: check on the ATLAS public web site updates tagged as JETM group, and in particular Making the most of the ATLAS detector  [ briefing on jets and particle flow recent improvements ]

 

Missing energy @ 8 TeV 2016 paper

Momentum conservation transverse to the beam axis implies that the transverse momenta of all particles in the final state should sum to zero. Any imbalance may indicate the presence of undetectable particles such as neutrinos or new, stable particles escaping detection. The missing transverse momentum is reconstructed as the negative vector sum of the transverse momenta of all detected particles, and its magnitude is represented by the symbol ETmiss . The measurement of Emiss strongly depends on the energy scale and resolution of the reconstructed “physics objects” electrons, photons, muons, ττ -leptons, and jets. Various techniques have been developed to reduce the impact of additional pp collisions, referred to as “pileup”, concurrent with the hard-scatter process.

Stories: check on the ATLAS public web site updates tagged as JETM group, and in particular the blog What happens when energy goes missing ?

 

Future challenges

In the 2015 ATLAS Phase-II Upgrade plans:

The Liquid Argon, Tile and Hadronic Endcap calorimeters maintain their performance and do not need interventions, but their electronics will be completely replaced, both because of radiation tolerance and to adapt to the new trigger system rate and latencies. The performance of the Forward Calorimeters (FCal) will be degraded in a region specially important for physics processes (vector boson fusion and scattering). On the other hand a replacement would require the opening of the LAr cryostat and several alternatives are studied.

 

How much of it can we see @P1 ?

Underground: the calorimeters are not visible during EYTS (end of the year technical stops). On the way from the lift to the -1 view point one sees [ left hand side, through a closed door ] the Tilecal optic fibers, electronics and high voltage rack. The orange UPS and pumps [ which are not visible but noisy ] allow to keep the LAr cryogeny and voltages on [ even during technical stops and LS1 ]

Surface: the calorimeter desks are closest to the Visitor Center. Shifters run the calibration between LHC fills and checking possible High-Voltage or noise burst issues.

 

Latest update of this page: 01.12.2017 Version 0