The Large Hadron Collider (LHC) at CERN in Geneva (Switzerland) will go in
operation in the coming months and will soon enable us to analyze the highest
energy collisions ever produced at an accelerator. Beyond Standard Model
searches at LHC require a detailed understanding of the detector performance,
reconstruction algorithms and triggering. Precision measurements of Standard
Model processes are also mandatory to acquire the necessary knowledge of
Standard Model background. Both ATLAS and CMS efforts are hence addressed to
determine the best calibration candles and to design a realistic plan for the
initial period of data taking.Comment: Invited talk at the Hadron Collider Physics Symposium (HCP2008),
  Galena, Illinois, USA, May 27-31, 2008; 8 pages, LaTeX, 12 eps figure

Beauceron, S.

English

arXiv

The Large Hadron Collider (LHC) at CERN in Geneva (Switzerland) will go in operation in the coming months and will soon enable us to analyze the highest energy collisions ever produced at an accelerator. Beyond Standard Model searches at LHC require a detailed understanding of the detector performance, reconstruction algorithms and triggering. Precision measurements of Standard Model processes are also mandatory to acquire the necessary knowledge of Standard Model background. Both ATLAS and CMS efforts are hence addressed to determine the best calibration candles and to design a realistic plan for the initial period of data taking

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arXiv:0808.3795v1  [hep-ex]  27 Aug 2008Hadron Collider Physics Symposium (HCP2008), Galena, Illinois, USAEarly Standard Model Measurement and Determination of Standard ModelBackground for SearchesS. Beauceronon behalf of ATLAS and CMS collaborationCERN, CH-1211 Gene`ve 23, SwitzerlandThe Large H adron Collider (LH C) at CER N in G eneva (Switzerland) willgo in operation in the com ing m onthsand willsoon enable us to analyze the highest energy collisions ever produced at an accelerator. Beyond StandardM odel searches at LH C require a detailed understanding of the detector perform ance, reconstruction algorithm sand triggering. Precision m easurem ents ofStandard M odelprocesses are also m andatory to acquire the necessaryknowledge ofStandard M odelbackground. Both ATLA S and CM S eortsare hence addressed to determ ine the bestcalibration candles and to design a realistic plan forthe initialperiod ofdata taking.1. INTRODUCTIONATLAS [1]and CM S [2]detectoraretwo generaldetectorswhich havebeen designed in orderto scrutinizeproton-proton collisions ofLHC.The m ajor goalofthese experim ent is to search for beyond Standard M odelprocessesand/orto push lim itsofStandard M odeltheory.The rststepsofthese searcheswillbe to establish with precisionStandard M odelprocessesfora centerofm assof14 TeV.W ithin thestatisticsdelivered by LHC overtherstyearsofrunning,precised Standard M odelm easurem ents willconstraint beyond Standard M odeltheories and allow usto understand these background forsearches.In thispaper,the Standard M odelm easurem entwillbe presented asa function ofincrease lum inosity. At each stage,these m easurem entswillbe exposed asa background fora givensearch.Before collisions,the com m issioning ofthe detectorsis crucialto already understand their response. Then with10pb  1 ofrecorded collisions,detector synchronization,alignm ent ofdetectors and com m issioning ofrst physicsobjectswillbedone.Therstphysicswillbethen dom inated by jetphysics.W ith lessthan 100pb  1,m easurem entofStandard M odelprocessesusing leptonscan beaddressed with a high precision and allow thestartofthesearches.Studiesofcom plex nalstatessuch asttproduction willhelp the nalization ofthe com m issioning period.Beyond1000pb  1,the area ofsearcheswillbegin.2. FIRST PHYSICS USING JETSTherstphysicseventsthatwillberecorded by thedetectorswillbem ainly m inim um biasevents.Them inim umbiaseventswillbeused attherststageto calibrateand align detectors.In them eantim ea rstlook atthechargedhadron spectrum atcenterofenergy of14 TeV willbe possible. In the Fig.1,onescan see the dierent dEdXfromproton,kaonsand pionsfortwo ofthe CM S trackerdetector.In parallel,the studiesofunderlying eventsare m andatory atstartup.The currentsim ulation ofthese eventsisbased on an extrapolation oftheTevatron energy at2TeV up to LHC energy at14TeV.Theseextrapolationslead todierentvaluesasshown in Fig.2.Itisthem im portantto be ableto discrim inatebetween dierentextrapolationsin orderto ne tune the sim ulation as such eventswillplay an crucialrole in isolation criteria to dened isolatedleptons.Jetswillbetherstreconstructedobjectsatstartup butthesearealsothem ostdicultobjectstofullyunderstand.It has to give a good description ofjet/parton properties from an interpretation ofcalorim eters response. TheFigure1:D istribution ofthetruncated m ean estim atordE /dx asa function ofm om entum p forthepixelshits(left)and striphits(right)ofthe CM S detector.Figure 2: D ensities dN/dd (left) and dpsumT/dd (right) for tracks with pT > 0:5 G eV=c,as a function ofthe leadingcharged jetpT ,in the transverse region,foran integrated lum inosity of100pb  1collected by CM S detector.response ofthe calorim eterswillbe inuenced by experim entalfactorsand physicsfactors.The m ain experim entalfactorsthatoneshasto take care when building a jetare the am ountofdead m aterialin frontofthe calorim eters,longitudinalleakageand lateralshowersize,non linearitiesin theread outaswellasthenon-com pensated behaviorofthecalorim eter.Them ain physicsfactorsaretheunderstanding ofinitial/nalstateradiation,fragm entation,theam ountofunderlying events/m inim um biasevents.Nevertheless,the physics program with jets is large and already som e searches/com plem entary m easurem ent ofthe Standard M odelcan be established. O ne ofthe rst m easurem entcan be the dierentialcross section ofjetsproduction which willallow a testofQ CD theories:high m om entum objectscan be inuenced by processesbeyondStandard M odel.Thecontactinteraction willtend to enlargethe rateofproduction ofjetsathigherm om entum asshown in Fig.3. CM S analysis[3]describeswith 10 pb  1 ofdata collected a sensitivity to + > 2.7 TeV which isthe currentlim itofTevatron experim ent.Therstadvanceeventtopology studied willbe the studiesofdijeteventsand m ainly the invariantm assofdijetevents.Thedijetresponsewillbeused atstartup in orderto providea rstjetenergy scale.Thisscaleism andatoryto perform studiesofinvariantm assbut,already,with a few ofinversepicobarnsofintegrated lum inosity,som etestsbeyond Standard M odelcan be perform ed.Figure 3: Rate ofJets within the centralpart ofCM S detector as a function ofenergy for 10 pb  1 ofexpected integratedlum inosity.Contactinteraction willincrease the rate atlarge energy.3. CHALLENGING MISSING ETEven thoughtthe m issing ET (6E T )variablecan be calculated assoon asthe detectorsarecloseand ready,itwilltake som e tim e before to fully understand it. In the m eantim e,6E T isone ofthe variable the m ostsensitive to newphysics.Sourcesoffake6E T arem ainly beam gasinteractions,dead/hot/noisy cells/area in thecalorim etersystem s,non linearity/non-com pensated detectors,nite energy resolution or from m uons escaping detection as shown inFig.4 [5].ATLAS and CM S collaboration develop techniquesto handle the bias,forexam ple,com ing from jetenergy scalecorrection [4]. Techniquesare also developed in orderto estim ate the 6E T com ing from physicseventsasZ0! from \visibleprocesses" asZ 0 ! ll.4. PHYSICS WITH LEPTONSJetsand 6E T are m ainly calorim etric object. The com bination ofthe trackerwith otherdetectorswillbring theleptonsinto the gam e. Butthe leptonswillm ainly rely on the quality ofthe alignm entofthe trackerand also ontheknowledgeofm agneticelds.Thesequantitiescan bequickly estim ated by looking atresonancesasJ=	 and within 1 pb  1 ofintegrated lum inosity asshown in Fig.5.The LHC willbe a W  /Z 0 factories so with a few am ount ofdata collected,cross section production can beestablished fora energy in the centerofm assof14 TeV.The m ain studiesthatwillbe done atthe beginning withthese events willbe the com m issioning ones. Indeed,Z 0 resonancesare really im portantto tune the EM -scale ofthe calorim eterorto controlelectrom agneticcalorim etercalibration in case ofa decay in the electronic channel,to (GeV)missTE100 200 300 400 500 600 700 800 900 1000Arbitrary units110210310410missTTrue EmissTFake E (with true muons)missT - Fake EmissTFake E (GeV)missTE100 200 300 400 500 600 700 800 900 1000Arbitrary units110210310missTTrue EmissTFake EFigure 4: Left: D istribution of6E T variable for generator inform ation (triangle) and for fake 6E T (round). The fake 6E T fromm uon escaping the acceptance ofATLAS detector is com pared to the overallfake 6E T (black point). Right: D istribution of6E T variableforgeneratorinform ation (triangle)and forfake6E T oncethe6E T iscorrected from m uonsescaping theacceptanceofATLAS detector(round).Figure 5:D ilepton invariantm assofthe expected rstinverse picobarnsofdata collected by ATLAS detector.J=	 peak aswellas are clearly visible.im prove/validate the alignm entoftrackerand m uonscham bersin case ofa decay m uonically. The resonanceswillbealso used to determ inetheecienciesoftheleptonsby using theTag-and-Probem ethod [6].TheTag-and-Probem ethod rely on wellknown resonances. O ne ofthe leg ofthe resonancesis asked to have a perfectly wellidentifylepton.Theinvariantm assofthetwo leptonsshould bewithin theresonancem asswindow.In thatcasethesecondlepton hasa high probability to be a true isolated lepton and itisaected only by the biasfrom the kinem aticsofthe resonanceproduction.Thissecond lepton sam plecan then be used to study lepton identication criteria.Figure 6:Invariantm assoftwo opposite charge m uonswithin CM S detectorforan expected lum inosity of100 pb  1.A clearsignalofZ willbe seen on top ofthe dierentbackground.In addition to crosssection m easurem ent,itwillbe possible to im prove the constraintson PDF asatLHC,theW  allow to accesslow-x range.W ith a few events,a m easurem entwith a precision lowerthan 5% ,a gain aslargeas40% on system aticscan be expected on low-x gluon shape[7].At this stage,Standard M odelphysicsusing leptons as m ain nalstate willbe established. W ith a few inversepicobarn ofdata,m ain propertiesofsuch eventsata centerofm assof14 TeV willbestudied.Som eearly discoveriescan happen in these topologiesby sim ply looking atthe tailofthe invariantm assdistribution. Forexam ple,CM Sanalysiswith 100pb  1 ofdata can see an excess ofeventsin the tailofthe invariantm ass oftwo opposite signedm uons asshown in Fig.6. Forsuch analysisan optim aldetector is notm andatory but the interpretation ofsuchsignalasa possibleZ 0 candidateorasa graviton willrequirem orestudies.5. TOP PHYSICSThe ttcross section production is a factor 100 greater than the one at Tevatron. The statistics ofsuch eventswillbewithoutcom parison and so itwillbepossibleto used thesevery com plex eventsin orderto validate/performcalibration. From the sem i-leptonic decay ofa ttevent,it willbe possible to constraint the two non b-jet to theinvariantm assoftheW and in thatcaseto im provethejetenergy scale.Thepresenceofb-jetin dileptonicdecay ofsuch eventswillgive usa high quantity ofeventswith a high purity ofb-jetsin orderto study b-tagging eciency.The 6E T can also be controlled by constraining the sem i-leptonic decay to the W m ass. Allthese studies can beFigure 7: Left: Invariant m ass of3 jets in sem ileptonic tt events for 100pb  1ofexpected lum inosity recorded by ATLASdetector.SU4 signalwilldouble the levelofbackground.Right:Signicance asa function ofintegrated lum inosity with andwithouta SU4 signalwhich willdouble the background.perform ed once only top eventsare seen in ATLAS and CM S detector. CM S analysis[8]shown thatwith 10pb  1ofcollected data,the rst m easurem ent ofthe tt cross section production can be done. The analysis have beenperform ed forthree channelsofdileptonic decay ofttevents(di-electron,dim uon and one electron plusone m uon)and in the caseofW ! m u forsem ileptonic decay.Physicsbeyond Standard M odelcan alsoplay an im portantrolein thestudiesofttsignalatLHC.ATLAS analysisstudied a squark signalwithin m Sugra fram ework which willappearasdoubling thebackground forttsem i-leptonicanalysis. The invariantm ass of3 jets as wellas signicance as function ofintegrated lum inosity are presented inFig.7.O ncettproduction isestablished ata centerofm assof14 TeV studiesofthe invariantm assofthettsystem canbe scrutinized.An excessin such distribution can givea hinttowardsbeyond Standard M odelprocesses.Contrary to tt system ,the production ofsingle top willrequire a integrated lum inosity ofaround 1fb  1 to beestablished.Nevertheless,thisprocesseswillallow usto testquite som e theories.The crosssection production canbe enlarged in caseofb0;t0 production ifM b0 > M t0,W0 production,avorchanging neutralcurrentaswellasSusycorrection etc.Thetop physicsisa really challenging onedueto thecom plex nalstatebutthisalso concludethecom m issioningphaseofthe detectorsand allow usto push forward discoveries.6. HIGGS BOSON SEARCHThe lastrem aining piece ofthe Standard M odelwhich isstillm issing isthe search fora Higgssignal.Dependingon them assoftheHiggsboson,theHiggsboson can bediscovered in a really clean and controlled m ultilepton nalstate with a few hundreds ofpicobarns ofdata ifthe Higgs boson m ass is larger than 130 G eV.In that case theHiggsboson decay willbe essentially via a pair ofW boson. These bosonswillbe studied in their leptonic decayand m ainly electron and m uon. Due to the large 6E T expected from the W boson decay,the m assofHiggsbosoncannotbe reconstructed.TheHiggsboson isa spin 0 particleso the two leptonscom ing from W decay willtend tobecollinear.Thisproperty givesa handleto discrim inateHiggsboson decay from W W Standard M odelproduction.Fig.8 presentsforCM S analysis,the invariantm assofthe dilepton system and the azim uthalangularseparationbetween the two leptons for the   channelafter the selection applied and for a Standard M odelHiggs bosonm asshypothesisofm H = 160 G eV [9]TheHiggsboson discovery oritsexclusion willplay a crucialrolefortheseacrhesbeyond Standard M odelphysicsifby thattim e nothing would havebeen already observed.Figure 8:Left:Invariantm assofthedilepton system reconstructed in theCM S detector,right:azim uthalangularseparationbetween the two leptonsforthe channelafterthe selection and fora Standard M odelHiggsboson m asshypothesisofm H = 160 G eV.Figure9:Integrated Lum inosity asa function oftim ewith expected pointwherediscovery ofprocessbeyond Standard M odelcan be perform ed.LHC willallow usto achieve large searches.7. CONCLUSIONO nce the LHC willstart and once ATLAS and CM S experim ent willhave com m issioned their detectors usingStandard M odelprocesses,the vastarea ofsearchesforprocessesbeyond Standard M odelwillreally begin. Som eprocessescan alreadyappearwithin thecom m issioningphaseoratleastindicatethatbeyond Standard M odelphysicsis at the corner. Fig.9 presents as a function oftim e and integrated lum inosity the discovery potentialowing toLHC m achineand detectors.AcknowledgmentsIwould like to thank the conference organisersforthe invitation to give thistalk. Thanks,also,to ATLAS andCM S colleaguesfortheirinputm aterial.References[1]ATLAS Collaboration,\ATLAS detectorand physicsperform ance",TechnicalDesign Report,CERN-LHCC-99-15/CERN-LHCC-99-14 (1999)[2]CM S Collaboration,\TheCM S experim entattheCERN LHC",Subm itted to JournalofInstrum entation (2008)[3]CM S Collaboration,\SearchesforNew Physicsusing high ET dijetevents",CM S PAS SBM -07-001 (2007)[4]CM S Collaboration,\M issing ET perform ancein CM S",CM S PAS JM E-07-001 (2007)[5]ATLAS Collaboration,\TheATLAS Experim entatthe CERN LargeHadron Collider",2008 JINST 3 S08003[6]CM S Collaboration,\M easuring Electron EcienciesatCM S with Early Data",CM S PAS EG M -07-001 (2007)[7]A.Tricoli,A.Cooper-Sarkarand C.G wenlan,\Uncertaintieson W and Z production attheLHC",hep-ex/0509002[8]CM S Collaboration,\O bservability ofTop Q uark PairProduction in the Sem ileptonic M uon Channelwith therst10 pb  1 ofCM S Data",CM S PAS TO P-08-005 (2008)[9]CM S Collaboration,\Search fortheHiggsboson in theW W ( )decay channelwith theCM S experim ent",CM SPAS Hig-07-001 (2007)

Early Standard Model Measurement and Determination of Standard Model Background for Searches

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