photon+jet process application for setting the abs

arXiv:hep-ex/0207034v115Jul2002”γ+Jet”processapplicationforsettingtheabsolutescaleofjetenergyanddeterminingthegluondistributionattheLHC.D.Bandurin,V.Konoplyanikov,N.SkachkovJointInstituteforNuclearResearch,Dubna,RussiaAbstractThepossibilityofjetenergyscalesettingattheCMScalorimeterbyusing”γ+Jet”processisstudied.Theestimationofthenumberof”γ+Jet”eventssuitablefordeterminationofgluondistributioninsideaprotoninanewkine-maticregionofx,Q2variablesbeyondtheonecoveredbyHERAdataisalsopresented.1.Introduction.Basingontheselectioncriteriaintroducedforthefirsttimein[1]–[4](seebelowSection2),thebackgroundeventssuppressionfactors,signaleventsselectionefficienciesandthenumberoftheevents,thatcanbecollectedatintegratedluminosityLint=3fb−1aredeterminedhere.Itisalsoshownthat”γ+Jet”events,beingcollectedatLHC,wouldprovideuswiththedatasufficientforanextractionofgluondistributionfunc-tioninaproton.Anewregionof2·10−4≤x≤1and1.6·103≤Q2≤8·104(GeV/c)2canbecovered.Theratesofgc→γdir+Jeteventsarealsogiven.2.DefinitionofselectioncutsforphysicalvariablesandthescalarformofthePtbalanceequation.1.Weshallselecttheeventswithonejetandone“γdir-candidate”(inwhatfol-lowswedesignateitasγandcallthe“photon”forbrevityandonlyinSection3,devotedtothebackgrounds,wedenoteγdir-candidateby˜γ)withPtγ≥40GeV/candPtJet≥30GeV/c.(1)Theelectromagneticcalorimeter(ECAL)signalcanbeconsideredasacan-didateforadirectphotonifitfitsinsidethe5×5ECALcrystalcellwindowhavingacellwiththehighestPtγ/einthecenter([6]).ThejetisdefinedhereaccordingtothePYTHIA[7]jetfindingalgorithmLUCELL.ThejetconeradiusRintheη−φspacecountedfromthejetinitiatorcell(ic)istakentobeRic=((Δη)2+(Δφ)2)1/2=0.7.12.Tosuppressthebackgroundprocesses,i.e.toselectmostlytheeventswith“isolated”photonsandtodiscardtheeventswithfake“photons”(thatmayorig-inateas“γdir-candidates”frommesondecays,forinstance),werestricta)thevalueofthescalarsumofPtofhadronsandotherparticlessur-roundinga“photon”withinaconeofRγisol=((Δη)2+(Δφ)2)1/2=0.7(“ab-soluteisolationcut”)Xi∈RPti≡Ptisol≤PtisolCUT;(2)b)thevalueofafraction(“relativeisolationcut”)Xi∈RPti/Ptγ≡ǫγ≤ǫγCUT.(3)3.TobeconsistentwiththeapplicationconditionoftheNLOformulae,oneshouldavoidaninfrareddangerousregionandtakecareofPtpopulationintheregionclosetoaγdir-candidatewealsorestrictinaccordancewith[8]and[9]thescalarsumofPtofparticlesarounda“photon”withinaconeofasmallerradiusRγsingl=0.175=1/4Rγisol:Xi∈RγsinglPti≡Ptsingl≤2GeV/c(i6=γ−dir).(4)4.Weacceptonlytheeventshavingnochargedtracks(particles)withPt1GeV/cwithintheRγsinglconearoundtheγdir-candidate.5.Tosuppressthebackgroundeventswithphotonsresultingfromhigh-energyπ0,η,ωandK0Smesondecays,werequiretheabsenceofahighPthadroninthecalorimetertowercontainingtheγdir-candidate:Pthadr≤5GeV/c.(5)AtthePYTHIAlevelofsimulationthiscutmayeffectivelytakeintoaccounttheimposingofanuppercutonthehadroniccalorimeter(HCAL)signalinthetowersbehindtheECALtowerfiredbythedirectphoton.6.Weselecttheeventswiththevector~PtJetbeing“back-to-back”tothevector~Ptγ(intheplanetransversetothebeamline)withinΔφdefinedbytheequation:φ(γ,jet)=180◦±Δφ(Δφ=15◦,10◦,5◦)(6)(5◦isoneHCALtowersizeinφ),whereφ(γ,jet)istheanglebetweenthePtγandPtJetvectors:~Ptγ~PtJet=PtγPtJet·cos(φ(γ,jet)),wherePtγ=|~Ptγ|,PtJet=|~PtJet|.27.Theinitialandfinalstateradiations(ISRandFSR)manifestthemselvesmostclearlyassomefinalstatemini-jetsorclustersactivity.Tosuppressit,weim-poseanewcutconditionthatwasnotformulatedinanevidentforminpreviousexperiments:wechoosethe”γ+Jet”eventsthatdonothaveanyotherjet-likeorclusterhighPtactivitybyselectingtheeventswiththevaluesofPtclust(theclusterconeRclust(η,φ)=0.7),beinglowerthansomethresholdPtclustCUTvalue,i.e.weselecttheeventswithPtclust≤PtclustCUT(7)(PtclustCUT=15,10,5GeV/caremostefficientaswillbeshowninSection3).Here,theclustersarefoundbyoneandthesamejetfinderLUCELL.Nowwepasstoanothernewquantity(introducedalsoforthefirsttimein[1]–[4])thatcanbemeasuredattheexperiment.8.Welimitthevalueofthemodulusofthevectorsumof~Ptofallparticles,exceptthoseofthe”γ+Jet”system,thatfitintotheregion|η|5.0coveredbytheECALandHCAL,i.e.,welimitthesignalinthecells“beyondthejetandphoton”regionbythefollowingcut:Xi6∈Jet,γ−dir~Pti≡Ptout≤PtoutCUT,|ηi|5.0.(8)TheimportanceofPtoutCUTandPtclustCUTforselectionofeventswithagoodbalanceofPtγandPtJetandforthebackgroundreductionwillbedemonstratedinSection3.Belowthesetofselectioncuts1–8willbereferredtoas“Selection1”.Thelasttwoofthem,7and8,arenewcriteria[1]–[4]notusedinpreviousexperiments.Inadditiontothemonemorenewobject,introducedin[4]andnamedan“isolatedjet”,willbediscussed.9.Wealsoinvolveanewrequirementof“jetisolation”,i.e.thepresenceofa“cleanenough”(inthesenseoflimitedPtactivity)regioninsidethering(ofΔR=0.3orofapproximatelyasizeofthreecalorimetertowers)aroundthejet.Followingthispicture,werestricttheratioofthescalarsumoftransversemomentaofparticlesbelongingtothisring,i.e.Ptring/PtJet≡ǫjet,wherePtring=Xi∈0.7R1|~Pti|.(9)(ǫjet≤3−5%).Thesetofeventsthatpasscuts1–9willbecalled“Selection2”.3TheexactvaluesofthecutparametersPtisolCUT,ǫγCUT,ǫjet,PtclustCUT,PtoutCUTwillbespecifi