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Submitted on 13 Apr 2004
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F. Hubaut
To cite this version:
Performance of the ATLAS electromagnetic calorimeter under beam tests FabriceHubaut
a a
CPPM,CNRS/IN2P3,Univ. Mediterranee,Marseille,France
ThephysicsprogramatLHCishighlydemandingintermsofdetectorperformance. Inparticular,theATLAS electromagnetic calorimeter has to match challenging requirements for energy, position and time resolutions. CalorimeterprototypeandproductionmoduleshavebeentestedunderelectronbeamsatCERNduringthelast threeyears. ResultsarepresentedandcomparedtoATLASrequirements.
1. Introduction
TheATLAS(AToroidalLHC ApparatuS) ex-periment [1], presently under construction, will start operation in 2007 at the LHC [2] proton-protoncollideratCERN.Thismulti-purpose de-tectorhasawidephysicsprogram,spanningfrom precision measurements of W
bosons, top and bottomquarksproperties, to Higgsboson or su-persymmetric particle searches. In most cases, theelectromagnetic(EM)calorimeterwillplaya keyrole in measuringenergy, position and time ofelectronsandphotons.
2. General layout of the ATLAS electro-magneticcalorimeter
TheLHCextremeoperatingconditionsimpose severe constraints on detectors, in terms of ra-diation tolerance, background rejection capabil-ity, noise handling, response speed, spatial cov-erage and time stability. The EM calorimeter is a lead-liquid argon (LAr) sampling calorime-ter with an accordion geometry [3], that guar-antees a full azimuthal coverage. It is divided in one barrel (jj < 1:475) and two end-caps (1:375 < jj < 3:2) and is segmented in depth in three compartments(see gure 1). The sam-pling 1(front)ismadeof narrowstripsand per-forms precise position measurements and =
0 separation. Thesampling2(middle)hasadepth of16to18X
0
andcollectsmostofthee/ shower energy. Thesampling3(back)recovershigh en-ergytailsandhelps toseparatehadronicto elec-
OnbehalfoftheATLASliquidargongroup.
tromagnetic particles. In addition, a thin pre-samplerdetectorcorrectsenergylossesinthe up-stream material for jj < 1:8. In total, almost 200,000read-outchannelsgivethedetectorahigh granularity. Liquidargonhasbeenchosenforits intrinsic linear behavior, response stability and radiationtolerance. Foreaseofconstruction,the barrel part is divided in 32 modules and each end-cap wheel is made of 8 modules. The con-struction, test and integration of these modules arepresentlywelladvanced,andaredetailedina separate contribution[4]within thispublication.
∆ϕ = 0.0245
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Figure 1. Sketch of theaccordionstructure and sampling segmentationoftheEMcalorimeter.
energeticone. Inordertocompensateforenergy lossesin frontofthecalorimeterand leakage be-yondit,weightsareappliedtothepresamplerand theback compartmentresponses. Moreover,the nite size of the cluster causesa lateral leakage a ecting the energy measurement at a level be-low0.4%, andtheaccordiongeometry inducesa modulation along with similar magnitude. In addition, the speci c setting of thehigh voltage by nite sectors in the end-cap induces alinear variationof theenergyresponse as afunction of ineachsector. Finally,unlikeinATLAS,beam test particlearrivaltimes areasynchronouswith respecttothe40MHzclock,andtheenergy mea-surementis sensitiveto this phase. All these ef-fects are found in good agreement with simula-tionsandhavebeencorrectedfor.
6. Beamtest results
The performance of the EM calorimeter has been extensively tested under electron beams using two full-size prototype modules (one for the barrel and one for the end-cap) and seven production modules (four barrel and three end-cap). An ATLAS-like electronics was used. Re-sults from prototype modules, including noise, cross-talk, time stability, temperaturee ect, re-sponse to muons, =
0
separation, are exten-sivelydescribedin[7,8]. Theyallowedtoimprove calorimeter performance. The following results havebeenobtainedwithproductionmodulesand aresimilarforalltestedmodules.
6.1. Energy resolutionand uniformity Energy scans from 10 to 245 GeV have been performed at several positions. After unfolding noiseandbeamenergyuncertainty,thesampling termaisfoundbelow10%
p
GeV (resp. 12.5)for every barrel (resp. end-cap) positions, whereas the local constant term c is everywhere smaller than 0.4%. This is in good agreementwith AT-LAS speci cations (section 3). The linearity is foundtobebetterthan1%.
Toestimatetheglobal constanttermof equa-tion(1),thelocalconstanttermhastobeadded quadratically with the response non-uniformity.
The latter has been determined by performing position scans througheverymodule cells. As a result,theresponse non-uniformityislowerthan 0.6%onthewholemoduleforthebarrelandthe end-cap(see gure3). Itis smallerthan0.5%in regionsofsize=0:20:4. Asrequired, theresultingglobalconstanttermisbelow0.7%.
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Num of Cells : 515
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(a)Entries
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(b)Energy-weightedbarycenters arecalculatedin thefrontand/or in themiddlecompartmentsto perform position measurements. Inthe direc-tion, corrections for nite cellsize are done and the beam chambers resolution is unfolded. The positionresolutioninthefront(resp.middle) sec-tionisstablealong and amountsto0:1510
3 -units (resp. 0:3510
3
). These two mea-surementscanbecombinedwiththelongitudinal showerbarycentersto estimatetheshower direc-tion. Anaveragepreliminary50mrad=
p
E(GeV) resolutionis achievedoverthewhole calorimeter whichisinagreementwithsimulationsandwithin ATLASspeci cations.
6.3. Time measurementresolution
Theoptimal lteringtechniqueprovides infor-mationontheparticlearrivaltime(equation(2)). Celltocelltimedi erencesarestudied. Figure4 showstheresultsobtainedforonebarrelcelland its neighbors,as a function of the energy. They are in agreement with the expected electronics contribution. The time resolution amounts to 70psat70GeV,whichiswithinATLAS spec-i cations.
7. Conclusions and outlook
Theseveralbeamtestsperformedonprototype and production modules show that the ATLAS EMcalorimeter meets thephysicsspeci cations. Theconstruction,testandintegrationofthe nal modules are presentlywell proceeding[4]. Com-binedruns of afull barrelwedge and ofall end-capliquidargoncalorimeterswilltakeplacenext year, and will provide a rstglimpse of the AT-LASdetector.
Acknowledgment
I thank my ATLAS-Larg-EM colleagues for providing me with gures and results. I am in-debtedtoE.Monnier,P.PralavorioandL.Serin fordiscussionsinthepreparationofthetalkand themanuscript.
Figure4. Timeresolution(symbols)asafunction of theenergy,andexpectedelectronics contribu-tion(line).
REFERENCES
1. The ATLAS Technical Proposal, CERN/LHCC/94-43(1994).
2. The Large Hadron Collider, CERN/AC/95-05(1995).
3. ATLASLiquidArgonCalorimeter,Technical DesignReport, CERN/LHCC/96-41(1996). 4. A. Jeremie, The ATLAS liquid argon
elec-tromagnetic calorimeter construction status, these proceedings.
5. ATLAS CalorimeterPerformance, Technical DesignReport, CERN/LHCC/96-40(1996). 6. W.E. Cleland, E.G. Stern, Nucl. Inst.
Meth.A338(1994)467.
