北京谱仪Ⅱ上利用效率比方法进行μ鉴别的研究

131ò111Ï2007c11pUÔn†ØÔnHIGHENERGYPHYSICSANDNUCLEARPHYSICSVol.31,No.11Nov.,2007MuonIdentificationUsingtheEfficienciesRatioMethodatBES///*GUOYun-Jun1;2;1)HEKang-Lin2;2)1(NanjingUniversity,Nanjing210093,China)2(InstituteofHighEnergyPhysics,CAS,Beijing100049,China)AbstractThemomentumandpolarangledependedpositionresolutionandhiteciencyineachlayerofBES(/)muondetectorarecalibratedbyusingcosmicrayandhadronsamples.Basedonthecalibrationresults,theecienciesratiofunctionisconstructedtoseparate=eectivelyandappliedinphysicsanalysis.Keywordsmuonidentication,cosmicray,positionresolution,hiteciency,ecienciesratio1IntroductionMuonidenticationplaysacrucialroleinmanyimportantphysicstopicsat-charmenergyregion,suchas:searchingforthepureleptonicdecayofD+(s)!+tomeasurefD=fDs,andthestudyofthedecayof!,c!+s,wherecandsrepresentthecharmandanti-strangequark.Themomentumofmuonfromtheabovedecaymodesliesbelow1.2GeV/c,aneectiveidenticationforlowmomentummuonsmustberequired.Inthispaper,wepresentahitecienciesratiomethodinmuonidenticationatBES/.2TheBES///muonidentierBESisaconventionalsolenoidalmagneticdetec-torthatisdescribedindetailinRef.[1].BES/istheupgradedversionoftheBESdetector[2].TheBESandBES/muonidentierconsistsof189modules,eachmoduleconsistsof8proportionalmuoncountingtubes,interspersedwiththreelayersofabsorbingironshields.ItisschematicallyshowninFig.1(a).Theentiremuonstructureresidesoutsidethesolenoidalcoil,andtheironshieldisalsousedasauxreturnforthemagneticeld.Eachcountingtubehas8single-wirecells,ar-rangedintwoosetsub-layerstosolvetheleft-rightambiguityproblem.Thez-positionofthemuonhitsisdeterminedbychargedivisionbetweenthetwoendsofthemuoncounter.Thepositionisdeterminedbythetubesize.Theresolutionthusattainedhasbeen5cminthez-direction,and3cminthe-direction.3TheselectionofcosmicraysampleAlargeandpurecosmicraysamplewhichcoverslargemomentumrangeiscollectedbytheBESdetec-torwith1Hzeventrateduringthedatataking.Inthecosmicsample,muonsarethemaincomponents,hadronsandotherelectromagneticparticlesarel-teredbytheironyoke,onlyafewpercent(1%)ofhadronbackgroundremains[3].Generally,asshowninFig.1(a),twotrackswithoppositechargewillbere-constructedwhenasinglecosmicraypassesthroughthedetector.The\up-tracktraversesfromtheoutermostdetector(muoncounter)tothebeampipewhoseazimuthangleislessthan;the\down-tracktra-Received12February2007*SupportedbyNationalNaturalScienceFoundationofChina(10491300,10605030)1)E-mail:[e-mail]2)E-mail:[e-mail]1050|1055111ÏHþµ®Ì¤/þ|^Ç'{?1OïÄ1051versesfromthebeampipetotheouterdetectorwhoseazimuthangleisgreaterthan.Theselectionofcosmicrayproceedsasfollows[3].Therearetwochargedtrackswithgoodhelixmea-surementsinthemaindriftchamber.Thenumberofhitsinthemuoncounterisrequiredtobegreaterthan3.Thetotalenergydepositsinthebarrelshowercounterisrequiredtobelessthan1.5GeVtorejecttheBhabhaevents.EachchargedtrackmusthaveagoodTOFmeasurement.Comparedwiththecol-lisionevent,theeventstarttime(T0)ofcosmicrayhasashiftT0=(T1+T2)=2;(1)whereT1andT2arethemeasuredtime-of-ightintheupsideanddownsidebarrelTOFcounters,whichareproducedbyasinglecosmicraytrack.TheT0calculatedinEq.(1)willbeappliedtocorrectthedrifttimeinthetrackreconstructionprocess.Fig.1.(a)Schematicviewofthemuonidenti-erinxy-planeandaneventdisplayforcos-micray;(b)Themomentumdistributionforcosmicmuonsample.ThetrackparametersareimprovedaftertheT0correctioninthereconstruction.Furtherselectionsareemployedtotheabovesample:1)Ifonlyonedown-trackisfound,itmustbeneartheinteractionpoint(IP)bytherequirementofjRxyj5cmandjzj30cm,whereRxyandzaretheclosestapproachofthetracktotheIPinxy-planeandzdirection;2)ThetimedierenceofthetwoTOFcountersisrequiredtobegreaterthan5ns;3)Thedierencesofthetwotrackvertexarerequiredtobelessthan3mmatxy-planeandlessthan25mminzdirection;4)Thedierenceofthetwotrackmomentumssat-ises
p0:1pp1+p2,wherep=(p1+p2)=2,p1andp2arethemomentumoftwotracks,thefac-torof0.1correspondstoa3cutwithbeingthemomentumsresolution.Aftertheabovecriteria,thedown-trackwillbean\idealmuontrack.Hereandbelow,themuontrackinthecosmicraysampleisimplicitlytobethedown-track.Fig.1(b)showsthedistributionofmomentumformuonsinthesample.Hadronmaybemisidentiedasamuon.MostofkaonscanbeeasilyrejectedbyusingTOFinfor-mation.Pionhassimilarbehaviorwithmuoninin-nerdetectors,andmustbedistinguishedbymuoncounter.Tostudythepunch-throughofhadrons,largepionsamplesfromtheJ=!andJ=!!+areselected.Themuonandpionsamplesaredividedintotwocategories:oneisfordetectorcalibration;anotheronegoesforthemuonIDperfor-mancechecks.4ThepositionresolutionofmuoncounterThedierencesinandzdirectionbetweentheprojectedandmeasuredposition
=projhitand
z=zprojzhitarecheckedwithmuonsam-plesindierentmomentumrange,wherethesub-script\projindicatesfortheprojectedpositiontothemuoncounterfromtheinnertrackingsystem,andthe\hitindicatesforthemeasuredpositionbythemuoncounter.AsshowninFig.2,thepositionres-olutionineachlayerismomentumdepended,whichcanbeparameterizedas;z(p)=a+bpc;(2)whereandzarethe