Current Performance of the SLD VXD3

arXiv:hep-ex/9909048v127Sep1999SLAC-PUB-8239September1999CurrentPerformanceoftheSLDVXD3ToshinoriAbeStanfordLinearAcceleratorCenter,StanfordUniversity,Stanford,CA94309RepresentingtheSLDCollaborationPresentedat8thInternationalWorkshoponVertexDetectors(VERTEX99),TheNetherlands,20-25June1999.AbstractDuring1996,theSLDcollaborationcompletedconstructionandbeganoperationofanewcharge-coupleddevice(CCD)vertexdetector(VXD3).Sincethen,itsperformancehasbeenstudiedindetailandanewtopologicalvertexingtechniquehasbeendeveloped.Inthispaper,wediscussthedesignofVXD3,proceduresforaligningit,andthetrackingandvertexingimprovementsthathaveledtoitsworld-recordperformance.1IntroductionAtechniqueofcharge-coupleddevice(CCD)detectorsforhighenergyex-perimentswasestablishedbytheNA32fixed-targetexperimentatCERNinmid80s[1].TheyusedaCCD-basedvertexdetectorfortheidentificationofcharmedparticles.Theirresultsshowedexcellentvertexingperformanceforshort-livedparticles.Itwasrealizedthatthesedevicesofferedthepossibilityofexcellentphysicsperformanceinthee+e−linearcolliderenvironment.TheSLDexperimentattheSLACLinearCollider(SLC)isthefirstexperimentwhichusesCCDsasavertexdetectorinacollidingbeamexperiment.AftertestswithaprototypedetectorVXD1,the120MpixeldetectorVXD2[2,3]1WorkSupportedbyDepartmentofEnergycontractDE–AC03–76SF00515(SLAC)PreprintsubmittedtoElsevierPreprint7February2008wasinstalledforphysicsrunsstartinginJanuary1992.DuringtheSLDrunswithVXD2,wedevelopedatopologicalvertexfindingalgorithm[4]totagheavy-quarkjets.Theverysmall(μmsize)andstableSLCinteractionpoint(IP)andcleanlyandpreciselymeasuredspacepoints(∼5.4μm)inVXD2per-mitefficientidentificationofsecondaryandeventertiaryvertices.WithVXD2andthistechnique,weenjoyedanadvantageinbothb-andc-jettagging[5]overothercolliderexperiments.RapidadvancesinCCDtechnologyoverthepast10yearsmadeitpossibletoreplaceVXD2withamuchmorepowerfulvertexdetector.TheupgradedvertexdetectoriscalledVXD3[6]andwasinstalledinDecember1995.VXD3providesmuchbetterimpactparameterresolution,largersolidanglecover-age,andvirtuallyerror-freetracklinking.AllthesefeaturesenhancetheSLDheavy-quarkmeasurements.OneofthemostexcitingpossibilitiesisthesearchforB0smixing,leadingtothemeasurementoftheoscillationfrequency,Δms,andanimproveddeterminationoftheCKMmatrixelement,Vtd.Theini-tialperformanceofVXD3canbefoundinRefs.[6–8].Sincethen,significantimprovementsinalignmentandtrackingalgorithmshaveledtoremarkableperformanceresults.Inthispaper,wereportthecurrentperformanceoftheSLDVXD3.Thefea-turesofCCDvertexdetectorsandVXD3aredescribedinSection2.Section3discussesthealignmentcorrectionforprecisespacepointdeterminationandcurrentachievedspatialresolution.InSection4,wepresenttheVXD3-aidedtrackfinderwhichimprovesthesolidanglecoverageoftrackreconstructionatSLD.Section5describesthetrack-fitterimprovementandanewtopologicalvertexingtechnique.Ourlatestimpactparameterresolutionandtopologicalvertexingperformancesarealsoshowninthissection.FinallyasummaryisgiveninSection6.2TheSLDVXD3CCD-basedvertexdetectorsarewellmatchedtoe+e−linearcolliders,pro-vidingnearlyidealexperimentalconditionsforheavyflavorphysics,forthefollowingreasons:(1)Verysmallbeamspots(μmsize),henceawelldefinedprimaryvertexforeveryevent.(2)Highlysegmentedpixelstructure,whichprovidesnatural3-dimensionalspacepointsandwouldcomfortablyabsorbhighbackgroundperbunchcrossing,likelytobefoundinalinearcollider.(3)Precisespace-pointresolution,resultingtodateinameasurementpreci-sionof4μminspacepoints.2(4)Verythindetectorsandsmallbeampiperadius,hencedegradationofimpactparameterresolutionduetomultiplescatteringcouldbegreatlyreduced.(5)Longintervalbetweenbunchcrossings.Whilethisisnotsufficientforcompletereadout,theaverageintegratedbackgroundduringreadoutwasonly∼10bunchcrossings.TheSLDexperimentistheonlycolliderexperimentwhichsatisfiestheaboveconditions.ThedetaileddescriptionofVXD3isfoundinRef.[6].Usingad-vancesinCCDtechnology,inparticularincreasingthedeviceactivearea,VXD3hasthefollowingfeatures:(1)Extendedpolaranglecoverage,tobenefitfromthelargepolarizedasym-metryinphysicsprocessesinthemostvaluableregionsofhigh|cosθ|.(2)Fullazimuthalcoverageineachofthreebarrels,toachieveredundancyandself-trackingcapabilityindependentoftheCentralDriftChamber(CDC),andconsequentlyimprovedoveralltrackingefficiency.(3)Optimizedgeometrywithstretchedradialleverarmandreducedmaterialineachlayer,forsignificantlyimprovedimpactparameterresolution.VXD3consistsof96CCDs[9]arrangedon3cylindricallayersofberylliumsupportingladdersaroundtheinteractionpoint(IP).Only2CCDscovertheentirelengthofthe159mmladderwithanoverlapofabout1mmintheregionnearz=0,seeFig.1a.Laddersinthesamelayerareplacedina‘shingled’layoutwithasmallcantangleof9−10◦.Thelayoutprovidesazimuthalcoverageoverlapintherangeof300μmto1mm,dependingonlayerandCCDlocation,seeFig.1b.Theoverlapsallowaninternaldetectoralignmentbyusingthetrackspassingthroughtheoverlapregions,discussedinSection3.Withabeampipeinnerradiusof23.5mmandthelayer-1radiusof28.0mm,thelayer-3radiusof48.3mmachievescompleteazimuthalcoverageoutto|cosθ|0.85