Jet energy scale setting with Gamma+Jet events at

arXiv:hep-ex/0011017v527Apr2001JOINTINSTITUTEFORNUCLEARRECEARCHJINRPreprintE2-2000-255hep-ex/0011017Jetenergyscalesettingwith”γ+Jet”eventsatLHCenergies.Detailedstudyofthebackgroundsuppression.D.V.Bandourin1†,V.F.Konoplyanikov2∗,N.B.Skachkov3†E-mail:(1)[e-mail],(2)[e-mail],(3)[e-mail]†LaboratoryofNuclearProblems∗LaboratoryofParticlePhysicsAbstractThepossibilitiesofthebackgroundeventssuppression,basedontheQCDsubprocessesofqg−,gg−,qq−scatteringwithbigcrosssections,tothesignal”γ+Jet”eventsarestudied.Basingontheintroducedselectioncriteria,thebackgroundsuppressionfactorsandsignaleventsselectionefficienciesandthenumberoftheevents,thatcanbecollectedatLHCwithlowluminosityL=1033cm2s−1duringonemonthofcontinuouswork,aredetermined.1.INTRODUCTIONThispapercontinuesourpreviouspublications[1–4],wherepossibilitiesofjetenergyscalesettingandcalibrationofthehadroncalorimeteratLHCenergiesbyusing”γ+Jet”processhavebeenstudied.Thisarticleisdevotedtothestudyofthebackgroundeventssuppressionandtodemonstrationoftheefficiencyofthecutsintroducedbyusinpaper[1]forthispurpose.2.ILLUSTRATIONOFNEWCUTSEFFICIENCYToestimatethebackgroundforthesignalevents,wehavedonethesimulation1withamix-tureofallQCDandSMsubprocessesexistinginPYTHIAwithlargecrosssections,namely:ISUB=1,2,11–20,28–31,53,68,whichcanleadtoabigbackgroundforourmain”signal”subprocesses(ISUB=14and29inPYTHIA)2:qg→q+γ(1a)qq→g+γ(1b)Threegenerations(eachof50·106events)withdifferentvaluesofminimalPtofhardprocessˆpmin⊥3havebeendone:thefirstoneiswithˆpmin⊥=40GeV/c,whilethesecondandthethird–withˆpmin⊥=100and200GeV/c,respectively.Wehaveselected“γ-candidate+1Jet”eventswithPtjet30GeV/ccontainingoneγ-candidatetobeidentifiedbythedetectorasanisolatedphotonwithPtγ≥40(100and200)GeV/cforthegenerationwithˆpmin⊥≥40(100and200)GeV/c,respectively.Oneneedstostressthathereandbelow,speakingabouttheγ-candidate,weimplyinrealityasignalthatmayberegisteredinthe3by3ECALcrystalcellwindowwiththehighestPtγ/einthecenter.Allthesephoton-candidatesweresupposedtosatisfytheisolationcriteria1–4ofSection3.2frompaper[1]withPtisolCUT=2GeV/candǫγCUT=5%.NospecialcutswereimposedonΔφ,PtoutandPtclust(thevaluesofPtclustareautomaticallylimitedfromthetopsinceweselect“γ-candidate+1jet”eventswithPtJet30GeV/c).Thecorrespondingdistributionsforthephysicalobservables,introducedinSections3.1and3.2from[1],areshownseparatelyforthesignal“γ-dir”andthebackgroundeventsinFigs.1,3,5andinscatterplots2,4,6.Firstcolumnsinthesefigures,denotedby”γ-dir”,showthedistributionsinthesignalevents,i.e.intheeventscorrespondingtotheprocesses(1a)and(1b),havingdirectphotonswithPtγ−dir≥40GeV/c.Thesecondcolumns,denotedas”γ-brem”,correspondtotheeventsinwhichthephotonswereemittedfromquarks(i.e.bremsstrahlungphotons)andhavepassedundertheimposedcuts.Thedistributionsinthethirdcolumnwerebuiltbasingontheeventscontaining“γ-mes”photons,i.e.thosephotonsthatoriginatefrommultiphotondecaysofmesons(π0,η,ω,K0S)andhavealsopassedundertheimposedcuts.FirstlyweseethatinthecaseofPtγ≥200GeV/c(seeFig.5)practicallyall“signalevents”haveΔφ15◦,andinthecasePtγ≥100GeV/c(seeFig.3)mostofthemarealso1PYTHIA5.7versionwithdefaultCTEQ2Lparametrisationofstructurefunctionsisusedhere.2AcontributionofanotherpossibleNLOchannelgg→gγ(ISUB=115inPYTHIA)wasfoundtobestillnegligibleevenatLHCenergies.3CKIN(3)parameterinPYTHIA.1withinΔφ15◦.ItisseenfromFig.1thatatlowervaluesPtγ≥40GeV/cthereisstillabignumber(about70%)ofthesignaleventsbelongingtotheintervalofΔφ15◦.FromhereweconcludethattheuppercutΔφ15◦chosenin(26)of[1]isreasonableandindeeddiscardsalotofbackgroundeventsinallPtγ-intervals.Fromthesecond“γ-brem”columnsofFigs.1,3and5onecanalsoseethatPtclustspectraoftheeventswithbremsstrahlungphotonslookquitedifferentfromtheanalogousPtclustdistributionsofthesignal”γ-dir”photons.ThelatterdistributionshavemostoftheeventsintheregionofsmallPtclustvalues.Sincethebremsstrahlung(”γ-brem”)photonsgivethemostsizablebackground,thefounddifferenceofthespectrapromptsanideaofusingacutfromthetoponthevalueofPtclusttoreduce”γ-brem”backgroundwhichdominatesatlargePtclustvalues.TheanalogousdifferenceofPtoutspectraofsignal”γ-dir”events(whichalsocon-centrateatlowPtoutvalues)fromthoseofthebackground”γ-brem”and”γ-mes”events,withlongertailsathighPtoutenablesustoimposeanuppercutonthePtoutvalue.NowfromthescatterplotsinFigs.2,4and6aswellasfromFigs.1,3and5wecanconcludethattheusageofcuts(rathersofthere,buttheirfurtherrestrictionwillbediscussedbelow):onΔφ:Δφ15◦,forPtγ≥40GeV/c;Δφ10◦,forPtγ≥100GeV/c;Δφ5◦,forPtγ≥200GeV/c,onPtclust:PtclustCUT=15GeV/c,forPtγ≥40GeV/candPtγ≥100GeV/c;PtclustCUT=20GeV/c,forPtγ≥200GeV/c,onPtoutCUT:PtoutCUT=10GeV/c,forPtγ≥40GeV/c;PtoutCUT=15GeV/c,forPtγ≥100GeV/candPtγ≥200GeV/cwouldallowtokeepthemainpartofthesignal”γ-dir”eventsandtoreducenoticeablythecontributionfromthebackground”γ-brem”and”γ-mes”events.So,thesefiguresevidentlydemonstratehownewphysicalvariablesPtclustandPtout,introducedinSections3.1and3.2ofpaper[1],canbeusefulforseparationof”γ+Jet”eventswithdirectphotonsfromthebackgroundones(asthelatter,inprincipl