b-Quark Physics with the LEP Collider

Theb-physicsresultsfromtheLEPdatasetsexceededallexpectationsinboththe range of topics covered and the precision of measurements that were eventually possible. Thesuccesswasduetoseveralfactorssuchastherapiddevelopmentof silicon-strip vertex detectors, advances in the theoretical description of b-hadrons and not least because of the inventiveness, skill and determination of the expe- mentersperformingthedataanalysis. Inordertoutilisefullytheprecisionofthe newvertexdetectors,carefulcommissioningandeventreconstructionwasneeded. Thistogetherwithnewmethodsandtechniquesofdataanalysis,particularlyinthe areaofinclusiveb-hadronreconstruction,werethekeytoawholehostofmeasu- mentsthatspannedtheentire?eldofb-physics. Theaimofthisbookisnottogiveanotherreviewofb-physicsresults. Instead the focus is on reviewing the main experimental methods that evolved (hardware andsoftware)andthelessonslearntfromsomebodywhowasinvolved?rst-handin theirdevelopmentanduse. Thehopeisthatthisworkwillbothstandasarecordof whatwasachievedwiththeLEPdata,andalsoasareferencesourceofb-physics analysisideasforexperimentersembarkingonprojectsinthefuturee. g. attheLHC oralinearcollider. ResultsfromallfouroftheLEPexperimentsarereportedbutsomebiastowards DELPHIisinevitable,especiallytoillustrategeneralpoints,sinceIamacolla- rationmemberandhaveeasieraccesstomaterial. Iwishtothankmyfriendsand colleaguesoftheDELPHIcollaborationformakingtheprojectsuchasuccess. InadditionIwishtogivespecialthanksto:Prof. Dr. MichaelFeindtwhowasthe instigatorofinclusiveanalysismethodsforb-physicswiththeLEPdata;Prof. Dr ThomasMüllerforgivingmetheopportunityandencouragementtowritethisar- cle;Dr. RichardHawkings(OPAL)andDr. ChristianWeiser(DELPHI)forallowing meaccesstosomeunpublishedresultsandforusefuldiscussions. FinallyIwould liketothankthepeopleatSpringer,especiallyUteHeuserforherprofessionalh- dlingoftheprojectandpatiencewithmyregularfailuretomeetdeadlines. UniversityofWarwick,UK GaryJ. Barker September2009 vii Contents 1 b-physics at LEP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1. 2 TheLEPMachineandExperiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. 3 b-PhysicsOverview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. 3. 1 b-QuarkProduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 b 1. 3. 2 Forward-BackwardAsymmetry A . . . . . . . . . . . . . . . . . . . . . 7 FB 1. 3. 3 R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 b 1. 3. 4 HeavyQuarkSymmetry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. 3. 5 b-QuarkFragmentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1. 3. 6 b-HadronSpectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 1. 3. 7 b-HadronProductionFractions. . . . . . . . . . . . . . . . . . . . . . . . . . 13 1. 3. 8 Weakb-QuarkDecay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1. 4 Combiningb-PhysicsResults. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1. 4. 1 AveragingofElectroweakHeavyFlavourQuantities. . . . . . . . 19 1. 4. 2 AveragingofNon-electroweakHeavyFlavourQuantities. . . . 20 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2 Silicon Vertex Detectors and Particle Identi?cation . . . . . . . . . . . . . . . . . 23 2. 1 Silicon-StripVertexDetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2. 2 ParticleIdenti?cation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2. 2. 1 Speci?cIonisationEnergyLoss,dE/dx . . . . . . . . . . . . . . . . . . 30 ? 2. 2. 2 CerenkovRingImaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2. 2. 3 OverallDetectorPerformance . . . . . . . . . . . . . . . . . . . . . . . . . . 32 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 0 ¯ 3 Experience in Reconstructing Z ? bb Events. . . . . . . . . . . . . . . . . . . . . . 37 3. 1 ImpactParameterandDecayLength . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3. 1. 1 ClusterFinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3. 1. 2 TrackFittingandPatternRecognition . . . . . . . . . . . . . . . . . . . . 41 3. 1. 3 VertexDetectorAlignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3. 1. 4 SingleHitUncertainties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3. 1. 5 ResolutionTuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .