北京谱仪Ⅱ上利用效率比方法进行μ鉴别的研究

1、31 11 2007 11HIGHENERGYPHYSICSANDNUCLEARPHYSICSVol.31,No.11Nov.,2007MuonIdentificationUsingtheEfficienciesRatioMethodatBES *GUOYun-Jun1,2;1)HEKang-Lin2;2)1(NanjingUniversity,Nanjing210093,China)2(InstituteofHighEnergyPhysics,CAS,Beijing100049,China)AbstractThemomentumandpolarangledependedpositionres

2、olutionandhiteciencyineachlayerofBES( )muondetectorarecalibratedbyusingcosmicrayandhadronsamples.Basedonthecalibrationresults,theecienciesratiofunctionisconstructedtoseparateµ/eectivelyandappliedinphysicsanalysis.Keywordsmuonidentication,cosmicray,positionresolution,hiteciency,ecienciesratio1In

3、troductionMuonidenticationplaysacrucialroleinmanycoil,andtheironshieldisalsousedasauxreturnforthemagneticeld.Eachcountingtubehas8single-wirecells,ar-rangedintwoosetsub-layerstosolvetheleft-rightambiguityproblem.Thez-positionofthemuonhitsisdeterminedbychargedivisionbetweenthetwoendsofthemuoncounter.T

4、hepositionisdeterminedbythetubesize.Theresolutionthusattainedhasbeen5cminthez-direction,and3cminthe-direction.importantphysicstopicsat-charmenergyregion,suchas:searchingforthepureleptonicdecayofD+(s)¯srepresentthecharmandanti-strangequark.The¯µ,cµ+µ¯thedecayofµs,wh

5、erecandµ+µtomeasurefD/fDs,andthestudyofmomentumofmuonfromtheabovedecaymodesliesbelow1.2GeV/c,aneectiveidenticationforlowmomentummuonsmustberequired.Inthispaper,wepresentahitecienciesratiomethodinmuonidenticationatBES .3TheselectionofcosmicraysampleAlargeandpurecosmicraysamplewhichcoverslar

6、gemomentumrangeiscollectedbytheBESdetec-torwith1Hzeventrateduringthedatataking.Inthecosmicsample,muonsarethemaincomponents,hadronsandotherelectromagneticparticlesarel-teredbytheironyoke,onlyafewpercent(<1%)ofhadronbackgroundremains.Generally,asshowninFig.1(a),twotrackswithoppositechargewillbere-c

7、onstructedwhenasinglecosmicraypassesthroughthedetector.The“up-track”traversesfromtheoutermostdetector(muoncounter)tothebeampipewhoseazimuthangleislessthan;the“down-track”tra-32TheBES muonidentierBESisaconventionalsolenoidalmagneticdetec-torthatisdescribedindetailinRef.1.BES istheupgradedversionofthe

8、BESdetector.TheBESandBES muonidentierconsistsof189modules,eachmoduleconsistsof8proportionalmuoncountingtubes,interspersedwiththreelayersofabsorbingironshields.ItisschematicallyshowninFig.1(a).Theentiremuonstructureresidesoutsidethesolenoidal211 µ 10511+p2,wherep=(p1+p2)/2,p1andp2arethemomentumo

9、ftwotracks,thefac-torof0.1correspondstoa3cutwithbeingthemomentumsresolution.Aftertheabovecriteria,thedown-trackwillbean“ideal”muontrack.Hereandbelow,themuontrackinthecosmicraysampleisimplicitlytobethedown-track.Fig.1(b)showsthedistributionofmomentumformuonsinthesample.Hadronmaybemisidentiedasamuon.M

10、ostofkaonscanbeeasilyrejectedbyusingTOFinfor-mation.Pionhassimilarbehaviorwithmuoninin-nerdetectors,andmustbedistinguishedbymuoncounter.Tostudythepunch-throughofhadrons,largepionsamplesfromtheJ/andJ/+areselected.Themuonandpionsamplesaredividedintotwocategories:oneisfordetectorcalibration;anotheroneg

11、oesforthemuonIDperfor-mancechecks.4ThepositionresolutionofmuoncounterThedierencesinandzdirectionbetweentheprojectedandmeasuredposition=projandz=zhitprojzhitarecheckedwithmuonsam-plesindierentmomentumrange,wherethesub-script“proj”indicatesfortheprojectedpositiontothemuoncounterfromtheinnertrackingsys

12、tem,andthe“hit”indicatesforthemeasuredpositionbythemuoncounter.AsshowninFig.2,thepositionres-olutionineachlayerismomentumdepended,whichcanbeparameterizedas,z(p)=a+b1052 (HEP&NP) 3111 µ 10532ppthrinf 1+erf 2Eloss (6)forthreehitlayersorwiththedierenceoftwoerrorfunctionsforoneandtwohitlayers,w

13、hereinfrepre-sentstheeciencyatlargemomentum,theslopeoferrorfunctionElossrepresentsforthestandarddevi-ationofenergylossinironyoke,andpthrrepresentsthemomentumthresholdat50%pointofeciencycurve.Fig.5.Theinf,pthrandElossfromtwohitlayerseciencycurvesvaryasfunctionsofcosfor(a)muonsand(b)pions.Theparameter

14、sofinf,pthrandElossformuonandpionaredrawninFigs.5(a)and5(b),respec-tively.TheionizationrelatedparameterspthrandElossformuonandpionareverysimilar,bothofthemcanbeexpressedasafunctionofpthr,Eloss(1+cos2),(7)wherethepositiveparameterscanbedeterminedfromdata.Theinfformuonisalmostaconstantoverallrangeofco

15、sexceptthegeometryboundaryinzdirection.Thedropsofinfatlargevalueof|cos|canbecorrectedbyusingtheµ-pairsample.Withtheincreasingofthepathlength,moreinteractedpi-onswillbeabsorbedbytheironyoke.Theinfforpionreducedatlargevalueof|cos|,andcouldbeexpressedas2inf(1+cos),(8)wherethenegativeparameterscanb

16、edeterminedfromdata.6ApplyingtheecienciesratiomethodtomuonidenticationForachargedparticle,thesumofprobabilitiesofzero,one,twoandthreeeectivelayersinmuondetectormustequaltoone.Thuswecantreatthen-layerhiteciencyasa“Likelihood”todomuoniden-tication.Theprobabilitydensityfunction(PDF)canbeconstructedwith

17、thefollowingvariables:thenumberofeectivehitlayers,themomentumandthecosofchargedtrack.ThelikelihoodfunctionLwilltakethesimpleformL(H)L(H;n,p,cos)=Hn(p,cos),(9)whereHisthehypothesisofparticle(muonorhadron),Hnthen-layerhiteciencyinmuoncounter,nthenumberofeectivehitlayers,pthemomentum,cosiscalculatedbyE

18、q.(3)usingthepositioninformation.Generally,thefractionofthelikelihoodF(µ)=L(µ)1054 (HEP&NP) 31 References1BAIJZetal.(BESCollaboration).Nucl.Instrum.Meth-ods,1994,A344:3192BAIJZetal.(BESCollaboration).Nucl.Instrum.Meth-ods,2001,A458:6273GUJian-Hui,YUZhong-Qiang,HEP&NP,1996,20(1):2631(inChinese)( , . ,1996,20(1):2631)4YAOWMetal.JournalofPhysics,2006,G33:15BAIJZetal.(BESCollaboration).Phys.Rev.Lett.,1995,74:45996BAIJZetal.(BESCollaboration).Phys.Lett.,1998.B429:1887BES DesignReport.InteriorDocum