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JETFRAGMENTATIONANDPREDICTIONSOFTHERESUMMED PERTURBATIVEQCD By ALEXEIN.SAFONOV ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 2001 Tomyparents,LudmilaDmitriyevnaandNikolaySergeyevichSafonov. ACKNOWLEDGMENTS Most ofall, I am very grateful to Prof. Audrey Korytov, the chair ofthe dissertationcommittee,forhisinvaluablecontributiontothesuccessfulcompletionof thisdissertation.Dr.Korytov’sdesiretosharehisexperienceandgenuinededicationto physicswerethekeyfactorsthatdefinedtheoutcomeofthiswork. IamverythankfultoProf.GuenakhMitselmakherforhisadvice,helpandmany hoursofveryusefuldiscussions. IwouldliketothankDr.JoeyHuston(MichiganStateUniversity),Dr.Valery Khoze (Durham University, UK), Dr. Steve Kuhlmaim (ArgonNational Accelerator Laboratory),Dr.AviYagil(FermiNationalAcceleratorLaboratory)fortheirhelpand usefulsuggestionsthatwereveryimportantatdifferentstagesofthiswork I appreciate the opportunity 1 had to work and collaboratewith Dr. Serguei Klimenko (University ofFlorida), Dr. Jaco Konigsberg (University ofFlorida), Dr. AndreiNomerotski(Fermilab)andmanyotherpeopleworkingattheCDFExperiment, whocontributedintothesuccessfulcompletionofthisdissertation. IamthankfultpBenKilminster(UniversityofRochester)andSlavaKrutelyov (Texas A&M University) for carefully reading the manuscript and their helpful suggestions. DuringmystudyattheUniversityofFlorida,Iverymuchenjoyedthelecturesof ZonganQiu,CharlesHooperandCharlesThom,andIbelievethatthosewerealarge factorinthesuccessfulcompletionofthiswork. iii 01 TABLEOFCONTENTS page ACKNOWLEDGMENTS iii ABSTRACT vii CHAPTERS 1 QUANTUMCHROMODYNAMICSANDJETFRAGMENTATION 1 11..12 PReerstuurmbmateidveQQCCDDCalculationsandApplicabilityforJetFragmentation 42 1.2.1 ModifiedLeadingLogApproximation(MLLA) 6 1.2.2 LocalParton-HadronDualityHypothesis(LPHD) 9 1.3 MLLAPredictions 10 1.3.1 MomentumDistributionofPartonsinJets 1 1.3.2 MeanChargedParticleMultiplicity 1 1.3.3 OnDifferencesBetweenQuarkandGluonJets 12 1.3.4 Next-to-MLLACorrections 13 2 EARLYEXPERIMENTALRESULTSANDMOTIVATION 15 3 CDFATFERMILAB:EXPERIMENTALFACILITYOVERVIEW 17 3.1 ColliderDetectorsinHighEnergyPhysics 18 3.2 ColliderDetectoratFermilab:DesignandOverview 19 3.2.1 VertexDetectors(VTPCandSVX) 20 3.2.2 CentralTrackingChamber(CTC) 21 3.2.3 ElectromagneticandHadronicCalorimeters 22 3.2.4 OtherSub-Detectors 23 3.3 CDFDataReadoutandProcessing 24 4 DATAANALYSISTOOLS 26 4.1 Monte-CarloEventGenerators 26 4.2 DetectorSimulation 28 5 FEASIBILITYOFTHEJETFRAGMENTATIONSTUDIESATCDF 30 IV 6 DATASELECTION,CORRECTIONSANDERRORESTIMATES 33 6.1 DataSelectionandQualityCuts 33 6.2 CorrectionstotheData 36 6.2.1 JetCorrections 36 6.2.2 TrackingCorrections 39 6.3 SystematicUncertaintiesandMethodsofEvaluation 42 6.3.1 SystematicsAssociatedwiththeCalorimetry 43 6.3.2 SystematicsAssociatedwiththeTracking 47 7 RESULTSANDCOMPARISONSTOTHEPREDICTIONSOFTHEMODIFIED LEADINGLOGAPPROXIMATION(MLLA) 48 7.1 MeasurementoftheMomentumDistributionofChargedParticlesinJets 49 7.1.1 FitsoftheDistributionswiththeMLLALimitingSpectrum 49 7.1.2 gg^fromtheMLLAFit 54 7.1.3 FitforthePeakPositionoftheDistributions.MLLAScaling. ExtractionofQg^ 55 7.1.4 DependenceoftheFittedParameterKontheSizeoftheCone 57 7.1.5 ExtractionoftheLPHDParameter andtheRatioof MultiplicitiesinGluonandQuarkJetsr 59 7.1.6 Summary 61 7.2 MeasurementoftheInclusiveChargedParticleMultiplicity.Comparisons totheMLLAPredictions.ExtractionoftheRatiorandLPHDParameter KfSir' 62 7.3 ComparisonofthePhoton+JetandtheDijetData.Model-independent MeasurementoftheRatioofMultiplicitiesinGluonandQuarkJetsr 66 7.4 ParticlePropertiesinJetsandComparisonstoHerwigMonte-Carlo 69 7.4.1 MeanChargedMultiplicity 70 7.4.2 Distribution 72 7.4.3 dN/d(logp)MomentumDistribution 74 7.4.4 MultiplicityFlowdN/d% 76 7.5 Conclusions 77 8 SUMMARYOFTHEEXPERIMENTALRESULTSANDTHEIRIMPACTON UNDERSTANDINGOFJETFRAGMENTATION.FUTUREPERSPECTIVES 79 APPENDIX ANALYSISOFTHECENTRALTRACKINGCHAMBER(CTC) RECONSTRUCTIONEFFICIENCIES 83 TrackEmbeddingMethodandMonte-CarloTesting 84 Track-FindingAlgorithm 88 ResultsandCross-Checks 93 ConclusionAndLimitsofApplicability 98 V LISTOFREFERENCES 100 BIOGRAPHICALSKETCH 105 VI AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy JETFRAGMENTATIONANDPREDICTIONSOFTHERESUMMED PERTURBATIVEQCD By AlexeiN.Safonov May2001 Chairman: AndreyKorytov MajorDepartment: DepartmentofPhysics Thisdissertationisdedicatedtotheexperimentalanalysisofjetfragmentation,the processofformationofjetsofparticlesproducedinhigh-energycollisions,andtothe comparisonoftheresultstothepredictionsofresummedperturbativecalculationswithin QuantumChromodynamics. DatausedinthisanalysiswereobtainedbytheCollider DetectoratFermilab(CDF)Experimentinproton-antiprotoncollisionswiththecenter- of-massenergy1.8TeVproducedbytheTevatroncollideratFermiNationalAccelerator Laboratory. Jetfragmentation,becauseofitssoftness(typicaltransversemomentaofparticles injetswithrespecttothejetaxisis200-300MeV),waslongconsideredasanessentially non-perturbative,andthusincalculable,QCDprocess.Recentcalculationsperformedin theframeworkofModifiedLeadingLogApproximation(MLLA)havemadeanattempt vii toexpandtheperturbativedomaintoincludethefragmentationphenomenon.Validityof theMLLAapproachcanonlybeverifiedexperimentally. Early experimental measurements have shown a good qualitative agreement betweenthe lowerenergydataandthe MLLApredictions. However, someofthese resultsappearedtocontradicteachother. Studiespresentedinthisworksignificantlyexpandtheareaofthefragmentation studiesbyanalyzingamuchwiderrangeofjetenergies,reachingfarbeyondwhatcould beachievedate+e-colliders.Theyalsoprovideagoodtestinggroundfortheuniversality ofthejetpropertiesate+e-andhadroncollisions.ImportantMLLApredictionsofscaling werestudiedforthefirsttimeandfoundtobesupportedbythedata. Theanalysispresentedinthisworkshowsahighlevelofconsistencybetweenthe data and the MLLA predictions. This proves that thejet fragmentation is largely a perturbativeprocess inagreementwiththeassumptionsthatarethebasisofMLLA. Essentialmodelparametersareextractedandextensivecrosschecksofthemodelself- consistencywereperformed. viii CHAPTER1 QUANTUMCHROMODYNAMICSANDJETFRAGMENTATION Forthe last30years, therehasbeenanagreementthatallthematterofthe Universeisbuiltoffermions(leptonsandquarks)thatinteractwitheachotherviathe vector boson exchange. These interactions are described by gauge theories, and to properly describe a particular interaction, one needs to find an appropriate gauge symmetryandaparticularrepresentationofthetheory. Allleptonsaredividedintothreegenerations:thefirstgenerationconsistsofthe electronandelectronneutrino(e‘andVg),thesecondofthemuonandmuonneutrino(|ir andv^),andthethirdofthetauandtauneutrino(Tandv-^).Thesameistrueforquarks: thefirstgenerationofquarksconsistsofuandd-quarks,thesecondofc-ands-quarks, andthethirdoft-andb-quarks. Threeknownkindsofinteractions,apartfromgravitation,areelectromagnetic, weakandstronginteractions. Electromagneticinteractioninvolveselectricallycharged particles(allfermionsexceptneutrinos)andthecorrespondingexchangebosonisphoton (Y). The weak interaction is responsible for the well-known radioactive beta decay processesandinvolvesallfermionsmentionedabove;theweakinteractionismediated bythreebosons:W+,W-andZ.Thestronginteractionistheinteractionbetweenthe nucleiconstituents(quarksandgluons)andisresponsibleforkeepingnucleiconstituents together.Thestronginteractioninvolvesallquarksandismediatedbygluons(thereare8 differentspeciesofgluons). 1 2 Theelectromagneticandweakinteractionsaretheoldestknowninteractionsand arethebeststudiedandunderstood.TheStandardModelofElectroweakInteractions[1] definesthedynamicsofboththeparticipatingparticlesandtheexchangevectorbosons. Itallowsthecalculationofcross-sectionsofdifferentprocesseswithahighprecision (sometimesitrequiressophisticatedcalculations,butthereisawell-definedsystematic procedureofperforming suchcalculations).The StandardModelistestedinvarious experimental studies and shows averyhighlevelofconsistencywiththedata, even thoughthelastparticlecomprisingtheStandardModel,theHiggsbosonresponsiblefor generationofparticlemasses,hasnotyetbeenformd.Theexperimentalfindingofthe Higgsbosonisoneofthemostimportantdiscoveriesyettobemadebythehigh-energy physics,andalotofeffortandresourcesareappliedinthisdirection. Thestronginteractionisstrikinglydifferentfi'omtheelectromagneticandweak forcesbecauseits strengthbecomeslargerandgrowsveryquicklywiththedistance betweentheinteractingquarks,whileitalmostdisappearswhenthedistancebecomes small. This effect is called asymptotic fi’eedom. Both electromagnetic and weak interactionsshowdifferentbehavior:theirstrengthrapidlyfallswithdistance. 1.1 PerturbativeQCD Earlyexperimentsofproton-protonscattering [2] showingalowrateatlarge scatteringangleswereinterpretedtosuggestthattheprotonconsistsofanensembleof looselyboundedcloudofnon-interactingconstituents incapableofabsorbing alarge momentumtransfer.However,laterdeep-inelasticscattering(scatteringofanelectronon a proton) studies [3] have shown a substantial rate of high momentum-transfer