Calibra'onoftheElectromagne'cCalorimeteroftheCMSexperiment

StefanoArgirò

UniversitàdiTorinoandINFNfortheCMSEcalcollabora;on

Abstract

Theelectromagne.ccalorimeter(ECAL)oftheCMSexperimentisanhomogeneous,herme.cdetectorwithhighgranularity.Itspoten.alperformancesareoutstandingintermsofenergyresolu.on,dynamicrangeandnoiselevel.Thesecharacteris.csmakethecalorimeterthemostpowerfuldeviceinthesearchofthedecayintwophotonsoftheHiggspar.cle.However,theenergyresolu.ondependscruciallyonthechanneltochannelintercalibra.onprecision.Therefore,greataGen.onmustbegiventothecalibra.onprocess.Inthiscontribu.onwewilldescribethestrategythattheECALgrouphasdevisedtocalibratethedetector.Wewillreportonthepre‐calibra.onprocessesthathavealreadybeenperformed,thestrategiesforintercalibra.onatstartupandthoseforeseenwhensufficientsta.s.cswillbeaccumulatedtouseWandZevents.Forthenormaldatatakingregime,anintercalibra.onprecisionof0.5%shouldbereached,whiletheresponseofthedetectorwillbemonitoredregularly.

Precalibra'onswithelectronbeamandcosmicmuons

Intercalibra'onwithfirstdata

Mediumandlongtermstrategy

Summary

Theelectromagne.ccalorimeteroftheCMSexperimentwillstartdatatakingwithacalibra.onbetween1.5%and2.5%inthebarrelandbetween10%and15%intheendcap.ThelevelofprecisionreachedinthebarrelissuchtonotaffectthewidthoftheZ.Afewhoursofrunningwillallowtoconsolidateintercalibra.onconstantsinthebarrelandsignificantlyimproveintheendcapusingazimuthalsymmetry,soontobecomplementedbyinter‐ringintercalibra.onprovidedbyZ‐>ee.Afewdaysofrunningshouldbesufficienttolowertheintercalibra.onprecisioninthebarrelto0.5%withπ0‐>γγ.Studiesareongoingtoapplythismethodtotheendcapsaswell.Withabout5X‐1,isolatedelectronsfromW‐>eνwillintercalibratemostofthebarrelata0.5%levelandthewholecalorimeterbelow1.5%.Withmoresta.s.cs,levelswellbelow0.5%inthebarreland1%intheendcapcanbereached.TheenergyscalewillbesetbyZ‐>eeandZ‐>μμγ.Thelasermonitoringsystemwillbeusedtoproperlytakeintoaccountthevaria.onofthetransparencyofthecrystalswithaccumulateddose.

EnergyResolu'on

Beforeinstalla.onintheCMSdetector,9ofthe36supermodulesofthebarrelwerepreciselycalibratedusingtheH4electronbeamlineatCern,usingelectronsof90and120GeVwithamomentumspreadaround0.09%.Theintercalibra.onprecisionachievedwiththistechniqueis0.3%.Theplotontherightshowsthereproducibilityofthecalibra.onasthedistribu.onofthedifferencebetweentheinter‐calibra.onconstantsfromasupermoduleexposedtobeamintwooccasionsatonemonthinterval.Intheenergyreconstruc.onprocess,themaincorrec.ontobeappliedistheoneaccoun.ngfordependenceofthecrystalresponseonthepointofimpact.

Theremaining27supermoduleswillrelyonthecalibra.onperformedusingcosmicraymuons.Theinter‐calibra.onprecisionisbetween1.5%and2.5%dependingonthepseudorapidity.Theplotontheledshowstheinter‐calibra.onprecisionasafunc.onofetaindexforoneofthesupermodules.TheprecisionisdefinedastheRMSspreadofthedifferencebetweeninter‐calibra.oncoefficientsobtainedfromthetestbeamandcosmicraydata.Forthismeasurement,thecoincidenceoftwoplanesofscin.llatorwasusedtotriggerthedataacquisi.onsystem.Thefirstscin.llatorcoveredthefullsupermodulesurfaceontheunderside,thesecondwasplacedinthefocusofthequasi‐projec.vegeometryofthecrystals,thereforeselec.ngmuonsdirectedalongtheaxisofthecrystals.

Fortheendcaps,intercalibra.onsatstartupwillrelyonlaboratorymeasurementsoflightyieldandphotodetector(VPT)gain,givingaprecisionaround9%.

ImpactofCalibra'onErrors

Themostpromisingmethodtointercalibratethecalorimeterwithabout5X‐1istheoneusingtheE/ppeakforisolatedelectronsfromtheW‐>eνdecay.Thetrackmomentumiscomparedwiththecalorimetricenergyinthe5x5crystalmatrix.Electronswhichradiateasmallfrac.onoftheirenergymustbeselected.Thismethodrequiresafullyfunc.onaltracker.Theplotontherightshowstheinter‐calibra.onprecisionasafunc.onofpseudorapiditywith5X‐1inthebarreland7X‐1intheendcap.Theprecisionismostlybetween0.4%and1.5%.With30X‐1,aprecisionbelow0.5%everywhereinthebarrelandbelow1%intheendcapwillbeachievable.

Barrel:5X‐1 Endcap:7X‐1

Intercalibra'onwithisolatedelectrons

AbsoluteenergyscalewithZ‐>μμγ

CrystalstabilityInter‐calibra.onprecisionplaysacrucialroleinthedetec.onoftheH‐>γγdecay,wheretheconstanttermdominates.Thepictureontherightshowstheeffectofmis‐calibra.ononthewidthoftheHiggs.Excellentmassresolu.onisneededtodis.nguishtheresonancefromthebackground.

Z→µµγ

S/B~80

DY+jets

ThetransparencyofthecrystalswillchangeinthecourseoftheLHCcycleduetoradia.ondamage,andispartlyrecoveredoverafewhourswithoutbeam.Alasersystemwillaccuratelymonitorthetransparencyofeachcrystalbyinjec.nglightduringtheLHCabortgaps.Theplotontheledshowstheexpectedevolu.onofthetransparencybasedontestbeamdata.Theplotontherightillustratesthestabilityaderlasercorrec.onsmeasuredduringdedicatedirradia.onrunsatthetestbeam.

•  Corrected response •  Raw response

Time (hours)

ADC

χ2/ndf =73.9/68

Test beam data

Inter‐calibra'onwithπ0‐>γγ

Apowerfulcalibra.onmethodusesunconvertedphotonspairsthatarereconstructedinfixedarraysofcrystalsandformaπ0decaycandidate,basedonshowershapeandkinema.calcuts.Simula.onsshowthata0.5%calibra.onprecisioncanbeachievedinafewdaysevenatstartupluminosi.es.Thismethodrequiresaspecialsodwaretrigger,sincethetransverseenergyofthecalibra.onπ0isoftheorderofafewGeV.

Inter‐calibra'onwithazimuthalsymmetryofenergydeposi'on

Theazimuthalsymmetryofenergydepositsfromminimumbiaseventscanbeexploitedtointercalibrateringsofcrystalsatconstantpseudorapidity.Conversionsandradia.oninthetrackermaterialdefinethesystema.climittothismethod.Incontrast,themethodisveryfast,allowingtoobtaincalibra.onconstantsinafewhourswith1kHzofminimumbiastriggerrate,evenatlowluminosi.es.Theplotsontheledshowtheintercalibra.onprecisionobtainedwith18millionsofsimulatedminimumbiaseventsinblue,whilethelimitonprecision,assumingnoknowledgeoftrackermaterial,isinred.

Limitonprecision

Minimumbiasfullsimula.onIntra‐ringprecision:1.5‐3.5%

• Precisionwith18Mevents• Limitprecision

Barrel Endcap

Z‐>ee

Zdecaysarevaluableinanumberoftasks,suchastuningofcorrec.ons,selngtheabsoluteenergyscaleorintercalibrateregionsofthecalorimeter.Inpar.cular,itcanprovideringbyringinter‐calibra.ontobeusedinconjunc.onwithazimuthalsymmetry.Thepictureontherightshowstheinter‐ringcalibra.onprecisionasafunc.onofintegratedluminosity.

Fiducialregion

Preshowerregion

Fullsimula.on(preliminary)Inter‐ringcalibra.onprecision:

• 3%@10pb‐1• ≤1%@100pb‐1

References

CMSCollabora.on,“TheElectromagne.cCalorimeterTechnicalDesignReport”,CERN/LHCC97‐33(1997)CMSCollabora.on,“TheCMSexperimentattheCERNLHC”,J.Inst.3S08004(2008)M.Bonesinietal,“Intercalibra.onoftheelectromagne.ccalorimeterwithcosmicrays”,CMSNOTE–2005/023L.Agos.noetal,“Inter‐calibra.onoftheCMSelectromagne.ccalorimeterwithisolatedelectrons”,J.Phys.G33(2007)N67‐N84P.Adzicetal,“Energyresolu.onofthebarreloftheCMSelectromagne.ccalorimeter”,J.Inst.2P04004(2007)

Theenergyresolu.onoftheCMSelectromagne.ccalorimeter(ECAL)canbeparametrizedintheform:

(σ/E)2=(3.37%/√E)2+(0.108/E)2+(0.25%)2

Thefirsttermissta.s.cal,thesecondisthenoisetermandtheconstanttermistheoneaffectedbycorrec.onsandinter‐calibra.ons.

Amethodtodeterminetheabsoluteenergyscalethatisindependentofthecalorimeter,usesanearlybackground‐freesignalanddoesnotneedtorelyonbrehmrecoverycorrec.onsistheonerelyingontheZ‐>μμγdecay.Theexpectedrateis1γ/crystalat1X‐1.Theplotontherightshowsthenumberofphotonsasafunc.onofphotonenergyfortheZ‐>μμγprocessandDrell‐Yanprocessfordifferentcuts.

TheECALbarrelconsistsof18+18supermodules(oneinyellowabove)of1700crystalsofLeadTungstate(PbWO2)each

TheECALendcacapconsistsof4“D”of3662PbWO2crystals

CA‐CS

(Δη=0.0175)