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,theecienciesratiofunctionisconstructedtoseparate/eectivelyandappliedinphysicsanalysis.Keywordsmuonidentication,cosmicray,positionresolution,hiteciency,ecienciesratio1Introduc

3、tionMuonidenticationplaysacrucialroleinmanycoil,andtheironshieldisalsousedasauxreturnforthemagneticeld.Eachcountingtubehas8single-wirecells,ar-rangedintwoosetsub-layerstosolvetheleft-rightambiguityproblem.Thez-positionofthemuonhitsisdeterminedbychargedivisionbetweenthetwoendsofthemuoncounter.Theposi

4、tionisdeterminedbythetubesize.Theresolutionthusattainedhasbeen5cminthez-direction,and3cminthe-direction.importantphysicstopicsat-charmenergyregion,suchas:searchingforthepureleptonicdecayofD+(s)srepresentthecharmandanti-strangequark.The,c+thedecayofs,wherecand+tomeasurefD/fDs,andthestudyofmomentumofm

5、uonfromtheabovedecaymodesliesbelow1.2GeV/c,aneectiveidenticationforlowmomentummuonsmustberequired.Inthispaper,wepresentahitecienciesratiomethodinmuonidenticationatBES .3TheselectionofcosmicraysampleAlargeandpurecosmicraysamplewhichcoverslargemomentumrangeiscollectedbytheBESdetec-torwith1Hzeventrated

6、uringthedatataking.Inthecosmicsample,muonsarethemaincomponents,hadronsandotherelectromagneticparticlesarel-teredbytheironyoke,onlyafewpercent(1%)ofhadronbackgroundremains.Generally,asshowninFig.1(a),twotrackswithoppositechargewillbere-constructedwhenasinglecosmicraypassesthroughthedetector.The“up-tr

7、ack”traversesfromtheoutermostdetector(muoncounter)tothebeampipewhoseazimuthangleislessthan;the“down-track”tra-32TheBES muonidentierBESisaconventionalsolenoidalmagneticdetec-torthatisdescribedindetailinRef.1.BES istheupgradedversionoftheBESdetector.TheBESandBES muonidentierconsistsof189modules,eachmo

8、duleconsistsof8proportionalmuoncountingtubes,interspersedwiththreelayersofabsorbingironshields.ItisschematicallyshowninFig.1(a).Theentiremuonstructureresidesoutsidethesolenoidal211 10511+p2,wherep=(p1+p2)/2,p1andp2arethemomentumoftwotracks,thefac-torof0.1correspondstoa3cutwithbeingthemomentumsresolu

9、tion.Aftertheabovecriteria,thedown-trackwillbean“ideal”muontrack.Hereandbelow,themuontrackinthecosmicraysampleisimplicitlytobethedown-track.Fig.1(b)showsthedistributionofmomentumformuonsinthesample.Hadronmaybemisidentiedasamuon.MostofkaonscanbeeasilyrejectedbyusingTOFinfor-mation.Pionhassimilarbehav

10、iorwithmuoninin-nerdetectors,andmustbedistinguishedbymuoncounter.Tostudythepunch-throughofhadrons,largepionsamplesfromtheJ/andJ/+areselected.Themuonandpionsamplesaredividedintotwocategories:oneisfordetectorcalibration;anotheronegoesforthemuonIDperfor-mancechecks.4ThepositionresolutionofmuoncounterTh

11、edierencesinandzdirectionbetweentheprojectedandmeasuredposition=projandz=zhitprojzhitarecheckedwithmuonsam-plesindierentmomentumrange,wherethesub-script“proj”indicatesfortheprojectedpositiontothemuoncounterfromtheinnertrackingsystem,andthe“hit”indicatesforthemeasuredpositionbythemuoncounter.Asshowni

12、nFig.2,thepositionres-olutionineachlayerismomentumdepended,whichcanbeparameterizedas,z(p)=a+b1052 (HEP&NP) 3111 10532ppthrinf 1+erf 2Eloss (6)forthreehitlayersorwiththedierenceoftwoerrorfunctionsforoneandtwohitlayers,whereinfrepre-sentstheeciencyatlargemomentum,theslopeoferrorfunctionElossrepresents

13、forthestandarddevi-ationofenergylossinironyoke,andpthrrepresentsthemomentumthresholdat50%pointofeciencycurve.Fig.5.Theinf,pthrandElossfromtwohitlayerseciencycurvesvaryasfunctionsofcosfor(a)muonsand(b)pions.Theparametersofinf,pthrandElossformuonandpionaredrawninFigs.5(a)and5(b),respec-tively.Theioniz

14、ationrelatedparameterspthrandElossformuonandpionareverysimilar,bothofthemcanbeexpressedasafunctionofpthr,Eloss(1+cos2),(7)wherethepositiveparameterscanbedeterminedfromdata.Theinfformuonisalmostaconstantoverallrangeofcosexceptthegeometryboundaryinzdirection.Thedropsofinfatlargevalueof|cos|canbecorrec

15、tedbyusingthe-pairsample.Withtheincreasingofthepathlength,moreinteractedpi-onswillbeabsorbedbytheironyoke.Theinfforpionreducedatlargevalueof|cos|,andcouldbeexpressedas2inf(1+cos),(8)wherethenegativeparameterscanbedeterminedfromdata.6ApplyingtheecienciesratiomethodtomuonidenticationForachargedparticl

16、e,thesumofprobabilitiesofzero,one,twoandthreeeectivelayersinmuondetectormustequaltoone.Thuswecantreatthen-layerhiteciencyasa“Likelihood”todomuoniden-tication.Theprobabilitydensityfunction(PDF)canbeconstructedwiththefollowingvariables:thenumberofeectivehitlayers,themomentumandthecosofchargedtrack.The

17、likelihoodfunctionLwilltakethesimpleformL(H)L(H;n,p,cos)=Hn(p,cos),(9)whereHisthehypothesisofparticle(muonorhadron),Hnthen-layerhiteciencyinmuoncounter,nthenumberofeectivehitlayers,pthemomentum,cosiscalculatedbyEq.(3)usingthepositioninformation.Generally,thefractionofthelikelihoodF()=L()1054 (HEP&NP

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