Jet energy scale setting with photon+Jet events at

arXiv:hep-ex/0011084v327Apr2001JOINTINSTITUTEFORNUCLEARRECEARCHJINRPreprintE2-2000-253hep-ex/0011084Jetenergyscalesettingwith”γ+Jet”eventsatLHCenergies.MinijetsandclustersuppressionandPtγ−PtJetdisbalance.D.V.Bandourin1†,V.F.Konoplyanikov2∗,N.B.Skachkov3†E-mail:(1)[e-mail],(2)[e-mail],(3)[e-mail]†LaboratoryofNuclearProblems∗LaboratoryofParticlePhysicsAbstractInthispaperthestudyof”γ+Jet”eventsiscontinuedandtheeventnumberdeterminationusefulforjetenergyscalesettingwiththepossibleCMSdetectorhadroncalorimetercalibrationatLHCisfulfiled.ThesourcesofPtγandPtJetareexposed.ItisshownthattheclusterPtlimitation,whichisfoundinaneventoutsideofthejet,canbeagoodcriterionforthisdisbalancedecreasing.1.INTRODUCTIONInthisarticlewecontinueourstudyofthepp→γ+Jet+Xprocesscausedbytwopartonicsubprocesses:qg→q+γ(1a)qq→g+γ(1b)Themaingoalofthispaperistoestimatewhethertherewillbeasufficientnumberof”γ+Jet”eventsforsettingthemassscaleofajetwithagoodaccuracyandforperforminghadroncalorimeter(HCAL)calibrationatLHCenergies.WeusethePYTHIAgeneratorasamodelforthissortofestimationsupposingthattheresultswithotherphysicaleventgeneratorslikeHERWIG,andwithGEANT–basedsimulationpackageswillbediscussedinourfurtherpublications.HerewestudyindetailthesourcesofPtγandPtJetdisbalanceandtheimpactofthePtclustCUTparameter[1,2],imposedasthecutonpossibleminijetsorclustersPtforthecalibrationaccuracyimprovement1.2.DETAILED”γ+Jet”SYSTEMPtDISBALANCEDEPENDENCEONPtclustCUTPARAMETER.Inthepreviouspapers([1,2])weintroducedphysicalobservables(variables)forstudying”γ+Jet”events[1]anddiscussedwhatcutsforthemcouldleadtoadecreaseinthePtγandPtJetdisbalance[2].Thecalibrationproceduredependsonthegainedstatistics.Onecanmakethesecutstobetighterifmoreeventswouldbecollectedduringdatataking.InthetablesofAppendices1–4forfourdifferentPtγintervalsthemeanvaluesofthemostimportantvariablesthatreflectthemainfeaturesof”γ+Jet”eventsandtheothervaluesthatcharacterizePtγandPtJetbalance(aspredictedbythePYTHIAmodel),arepresented.Appendix1containstablesforPtγvaryingfrom40to50GeV/c.InthesetableswepresentthevaluesofinterestforthreedifferentSelections,mentionedinSection3.2[1].EachpagecorrespondstoadefinitevalueofΔφ(whichenterstheformula(23)of[1])asameasureofdeviationfromtheabsoluteback-to-backorientationoftwo~Ptγand~PtJetvectors.So,Tables1and2onthefirstpageandTables3and4onthesecondoneineachofAppendices1–4correspondtoΔφ=180◦(i.e.tothecaseofnorestrictionontheback-to-backnessanglechoice)andtoΔφ=15◦,respectively.OnthethirdpageofeachofAppendices1–4wepresenttheTables5and6(forthesamecutΔφ=15◦)thatcorrespondtotheSelection2casedescribedin[1].ThisSelectiondiffersfromSelection1,presentedinTables1–4,byadditionofthecut(26)of[1].Itallowsonetoselecteventswiththe”isolatedjet”,i.e.eventshavingthetotalPtactivityinΔR=0.3ringaroundajetnotexceeding2%ofjetPt.Tables7,8,shownonthefourthpageineachofAppendices,presentresultsfoundinthecaseofSelection3forthesamecutΔφ=15◦.ThelastsampleincludesonlyeventshavingthesamejetfoundsimultaneouslybybothjetfindersUA1andLUCELL.1ThedetailedinformationonthedependenceofthePtγandPtJetdisbalanceonPtclustCUTandPtoutCUTinthecaseofbackgroundeventscanbefoundin[3,4].1ThecolumnsinTables1–6correspondtofivedifferentvaluesofPtclustCUT=30,20,15,10and5GeV/c.TheupperlinesofTables1–6fromalltheAppendicescontainthenum-bersofeventsNeventexpectedfor“HBevents”,i.e.for”γ+Jet”eventsinwhichjetiscompletelyfittedintothebarrelregionofHCAL(see[2])fortheintegratedluminosityLint=3fb−1fordifferentvaluesofPtclustCUTandafixedvalueofΔφ.ThelastlinesoftheTablespresentthenumberofgeneratedevents,leftaftercuts,i.e.entries.InthenextfourlinesoftheTablesweputthevaluesofPt56,Δφ,Ptout,Ptη5definedbyformulae(3),(23),(25)and(5)of[1],respectively,andaveragedovertheeventsselectedwithachosenPtclustCUTvalue.FromtheTablesweseethatthevaluesofPt56,Δφ,Ptoutdecreasefastwithdecreas-ingPtclustCUT,whiletheaveragedvaluesofPtη5showveryweakdependenceonit(practicallyconstant).InthesixthlinetheaveragevaluesoftheinitialstatevectordisbalancePt5+6(definedby(3)of[1])arepresentedinadditiontotheroughscalarPt56estimatorvalue.ItisseenthattheyaresmallerthanPt56value.ThenextlinesrepresenttheaveragevaluesofthevariablesPtγ+part,Ptγ+jet,Ptγ+Jet,thataredefinedasaveragedvaluesof(2),(12)and(4)from[1]andserveasameasureofthePtdisbalanceinthe”γ+Jet”system.Theselinescorrespondinthefollowingorderto:thebalanceatthepartonlevel,thebalanceofthatpartofthe”γ+Jet”systemwhichcanbemeasuredbycalorimeters,i.e.definedbyPtjet(see(10)of[1]),andthebalanceofthe“γ+completejet”system.PracticallyallthevaluesofthesethreevariablesdropapproximatelybyafactoroftwowhenwemovefromPtclust=30GeV/ctoPtclust=5GeV/cforallPtγintervalsandforboththeUA1andLUCELLalgorithms.TotakeintoaccountthepartofjetPtcarriedoffbyneutrinos,wehaveintroducedin[1]thecorrectionΔνthatshouldbeaddedtoPtjettorestorePtofthecompletejetineachevent:Δν=PtJet−Ptjet(2)LetusdefineanewquantityPtJ,whichwouldserveasanestimatorforthetotalPtJetvalue,bythesumofPtjet(ameasurablepartofPtJet)andanaveragedΔνcorrection:PtJ=Ptjet+Δν(3)Thecomparis