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Using Drell-Yan to Probe the Underlying Event in Run II at Collider Detector at Fermilab (CDF)

Permanent Link: http://ufdc.ufl.edu/UFE0022563/00001

Material Information

Title: Using Drell-Yan to Probe the Underlying Event in Run II at Collider Detector at Fermilab (CDF)
Physical Description: 1 online resource (128 p.)
Language: english
Creator: Kar, Deepak
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: cdf, drell, hadron, tevatron, underlying
Physics -- Dissertations, Academic -- UF
Genre: Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: We study the behavior of charged particles produced in association with Drell-Yan lepton-pairs in the region of the Z-boson in proton-antiproton collisions at 1.96 TeV. We use the direction of the Z-boson in each event to define 'toward,' 'away,' and 'transverse' regions. For Drell-Yan production (excluding the leptons) both the 'toward' and 'transverse' regions are very sensitive to the `underlying event', which is defined as everything except the two hard scattered components. The data are corrected to the particle level and are then compared with several PYTHIA models (with multiple parton interactions) and HERWIG (without multiple parton interactions) at the particle level (i.e., generator level). The data are also compared with a previous analysis on the behavior of the 'underlying event' in high transverse momentum jet production. The goal is to produce data that can be used by the theorists to tune and improve the QCD Monte-Carlo models of the 'underlying event' that are used to simulate hadron-hadron collisions.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Deepak Kar.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Field, Richard D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022563:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022563/00001

Material Information

Title: Using Drell-Yan to Probe the Underlying Event in Run II at Collider Detector at Fermilab (CDF)
Physical Description: 1 online resource (128 p.)
Language: english
Creator: Kar, Deepak
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: cdf, drell, hadron, tevatron, underlying
Physics -- Dissertations, Academic -- UF
Genre: Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: We study the behavior of charged particles produced in association with Drell-Yan lepton-pairs in the region of the Z-boson in proton-antiproton collisions at 1.96 TeV. We use the direction of the Z-boson in each event to define 'toward,' 'away,' and 'transverse' regions. For Drell-Yan production (excluding the leptons) both the 'toward' and 'transverse' regions are very sensitive to the `underlying event', which is defined as everything except the two hard scattered components. The data are corrected to the particle level and are then compared with several PYTHIA models (with multiple parton interactions) and HERWIG (without multiple parton interactions) at the particle level (i.e., generator level). The data are also compared with a previous analysis on the behavior of the 'underlying event' in high transverse momentum jet production. The goal is to produce data that can be used by the theorists to tune and improve the QCD Monte-Carlo models of the 'underlying event' that are used to simulate hadron-hadron collisions.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Deepak Kar.
Thesis: Thesis (Ph.D.)--University of Florida, 2008.
Local: Adviser: Field, Richard D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2009-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022563:00001


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ThisdissertationistheresultofmyveyearsofworkinthedepartmentofphysicsattheUniversityofFlorida.Iwouldliketotakethisopportunitytoacknowledgethedirectandindirectcontributionsofmanypeopleonthiswork.ItisdiculttooverstatemygratitudetomyPh.D.supervisor,Prof.RichardD.Field.WhenIrstcamehere,Ihadlittleideaofhowexcitingexperimentalparticlephysicscanbe,andIleavedeterminedtohaveacareerintheeld.Hehasbeenawonderfulteacher,agreatmotivator,andaboveallagreatsourceofsupportineverythingIdid,whileallowingmeenoughfreedom.AlthoughasaPh.D.thesis,thisworkisnecessarilyauthoredbymealone,mostofithasbenetedfromhisremarkableinsightsandtirelesseorttounderstandthephysicsandexplainwhatis`goofy'.IhavebeenincrediblyfortunatetohavehadhimasmysupervisorandIdonotknowwhereIwouldbewithouthim.Hehadprovidednancialsupportfromhisresearchgrants,sothatIcouldconcentratefulltimeonresearchordotherequiredserviceworkfortheCDFcollaboration.Healsofundedmytravelstodierentconferencesandschoolsovertheseyears,andthosehelpedmeimmenselyandgavevaluableexperience.IwouldliketothankProf.KonstantinMatchevwhowasavailablewheneverIneededhisadvice.HisinsistencethatIgiveasmanytalksaspossibledenitelyhelpedmetoconquermyfearofpublicspeaking.HisQFTcoursewasagreatfoundationforstartingmyworkinthisarea.IwouldalsoliketothanktheothermembersofmyPhDcommitteewhomonitoredmyworkandtookeortinreadingandprovidingmewithvaluablecommentsonearlierversionsofthisthesis:ProfDarinAcosta,ProfJamesFryandProfSanjayRanka.IamgreatlyindebtedtoDr.RobertCraigGroup,formerlyaPh.DstudentofProf.Field,nowaresearchassociateatFermilab.Henotonlyhelpedmetogetstarted,but 4

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page ACKNOWLEDGMENTS ................................. 4 LISTOFTABLES ..................................... 9 LISTOFFIGURES .................................... 10 ABSTRACT ........................................ 14 CHAPTER 1INTRODUCTION:THESTANDARDMODELANDTHEQCD ........ 15 1.1Overview .................................... 15 1.2StandardModelofParticlePhysics ...................... 15 2THEEXPERIMENT:TEVATRON ......................... 19 2.1Introduction ................................... 19 2.2TheTevatron .................................. 20 2.3ColliderCoordinates .............................. 22 2.4TheCDFDetector ............................... 24 2.4.1Overview ................................. 24 2.4.2TrackingSystems ............................ 25 2.4.3Calorimeters ............................... 26 2.4.4MuonChambers ............................. 28 2.4.5LuminosityCounter ........................... 29 2.4.6TriggerSystem ............................. 31 2.4.7RunIIUpgrade ............................. 32 3COLLIDERPHENOMENOLOGY ......................... 34 3.1OverviewofHadronicCollisions ........................ 34 3.2TypicalColliderEvent ............................. 35 3.3TheUnderlyingEvent ............................. 37 3.4MinimumBiasCollisionsandtheUnderlyingEvent ............. 39 3.5DividingIntoRegions .............................. 40 4THEDRELL-YANPROCESS ............................ 43 4.1HistoricalPerspective .............................. 43 4.2CrossSectionCalculationsfortheDrell-YanProcess ............ 44 4.3ExperimentalStudieswiththeDrell-Yan ................... 48 4.4Drell-YanProcessandtheUnderlyingEvent ................. 50 7

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.................... 52 5.1Overview .................................... 52 5.2EventGeneration ................................ 53 5.3ExamplesofEventGenerators:PYTHIAandHERWIG .......... 54 5.4TuningPYTHIA ................................ 57 6ANALYSISSTRATEGY ............................... 62 6.1Introduction:Goal ............................... 62 6.2DataSelection .................................. 62 6.3EventSelection ................................. 63 6.4ElectronSelection ................................ 63 6.5MuonSelection ................................. 65 6.6LeptonPairFormation ............................. 68 6.7ChargedTrackSelection ............................ 70 6.8Observables ................................... 71 7RESULTS ....................................... 73 7.1EarlierWork .................................. 73 7.2CorrectingDataBacktoParticleLevel .................... 73 7.3SystematicErrors ................................ 74 7.4Drell-YanResults ................................ 76 7.4.1UnderlyingEventObservables ..................... 76 7.4.2ComparingDierentRegions ...................... 79 7.4.3ComparisonwiththeLeadingJetUnderlyingEventResults ..... 80 7.4.4CorrelationBetweenMeanTransverseMomentumandMultiplicity 81 8SUMMARYANDCONCLUSION .......................... 104 8.1Conclusions ................................... 104 8.2LookingAheadtotheLHC .......................... 106 8.3FinalWords ................................... 108 APPENDIX AALLTHENUMBERS ................................ 109 REFERENCES ....................................... 121 BIOGRAPHICALSKETCH ................................ 127 8

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Table page 2-1CDF3leveltrigger .................................. 32 4-1Drell-Yancrosssections ............................... 49 5-1PYTHIAparameters ................................. 58 5-2ParametersforseveralPYTHIA6.2tunes ..................... 60 5-3Multiplepartonscatteringcrosssection ....................... 60 6-1DataandMonte-Carlosamplesusedinthisanalysis ................ 63 6-2Electronselection ................................... 65 6-3Muonselection .................................... 68 6-4Massranges ...................................... 69 6-5Chargedtrackselection ................................ 71 6-6Observables ...................................... 72 7-1Systematicuncertainties ............................... 76 A-1Chargedmultiplicitydensity,PYTHIAtuneAW .................. 109 A-2Chargedmultiplicitydensity,data .......................... 110 A-3Chargedtransversemomentumsumdensity,PYTHIAtuneAW ......... 111 A-4Chargedtransversemomentumsumdensity,data ................. 112 A-5TransMAXandMIN,PYTHIAtuneAW ...................... 113 A-6TransMaxandMIN,data .............................. 114 A-7Chargedtransversemomentumaverage,PYTHIAtuneAW ........... 115 A-8Chargedtransversemomentumaverage,data ................... 116 A-9Chargedtransversemomentummaximum,PYTHIAtuneAW .......... 117 A-10Chargedtransversemomentummaximum,data .................. 118 A-11Correlations,PYTHIAtuneAW ........................... 119 A-12Correlations,data ................................... 120 9

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Figure page 1-1StandardModelofparticlephysics ......................... 17 2-1Collidermodes .................................... 20 2-2Tevatron ........................................ 21 2-3Collidercoordinates,z-axisandpolarangle ..................... 23 2-4Collidercoordinates,transversemomentumandpsuedorapidity ......... 23 2-5Collidercoordinates,psuedorapidity ......................... 24 2-6ColliderDetectoratFermilab ............................ 25 2-7Electromagneticshower ............................... 27 2-8ParticledetectionatCDF .............................. 29 2-9LuminosityobtainedatCDF ............................ 30 2-10TriggersystematCDF ................................ 32 3-1Hadroniccollisioncrosssection ........................... 35 3-2Hadroniccollision ................................... 35 3-3Feynmandiagramsforhadroniccollision ...................... 36 3-4Partonshower ..................................... 38 3-5Theunderlyingevent ................................. 39 3-6Dividingthecentralregion .............................. 41 3-7RegionsforaZ-bosonevent ............................. 41 3-8Transverseregions .................................. 42 4-1Drell-Yanprocess ................................... 43 4-2Drell-Yancrosssection ................................ 44 4-3Drell-YanFeynmandiagram ............................. 45 4-4Drell-YanQCDeects ................................ 47 4-5Z-bosonrecoilwithDrell-Yanproduction ...................... 48 4-6UnderlyingeventinDrell-Yanprocess ....................... 50 10

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................... 50 4-8Drell-Yanevent .................................... 51 5-1Eventgeneration ................................... 54 5-2Stringfragmentation ................................. 56 5-3Run1tunes ...................................... 59 6-1Pairproduction .................................... 66 6-2BackgroundforZ-bosonproduction ......................... 70 7-1Detectorandparticlelevel .............................. 74 7-2CorrectingdataBacktoparticleLevel ....................... 75 7-3Systematicerror,datauncorrected ......................... 76 7-4Systematicerror,datacorrected ........................... 77 7-5Drell-Yantransverseregionchargedmultiplicitydensity .............. 83 7-6Drell-Yantowardregionchargedmultiplicitydensity ............... 83 7-7Drell-Yanawayregionchargedmultiplicitydensity ................ 84 7-8Drell-YantransMAXandtransMINregionschargedmultiplicitydensity .... 84 7-9Drell-YantransDIFregionchargedmultiplicitydensity .............. 85 7-10Drell-Yantransverseregionchargedtransversemomentumsumdensity ..... 85 7-11Drell-Yantowardregionchargedtransversemomentumsumdensity ....... 86 7-12Drell-Yanawayregionchargedtransversemomentumsumdensity ........ 86 7-13Drell-YantransMAXandtransMINregionchargedtransversemomentumsumdensity ......................................... 87 7-14Drell-YantransDIFregionchargedtransversemomentumsumdensity ...... 87 7-15Drell-Yantransverseregionchargedaveragetransversemomentum ........ 88 7-16Drell-Yantowardregionchargedaveragetransversemomentum ......... 88 7-17Drell-Yanawayregionchargedaveragetransversemomentum .......... 89 7-18Drell-Yantransverseregionchargedmaximumtransversemomentum ...... 89 7-19Drell-Yantowardregionchargedmaximumtransversemomentum ........ 90 11

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......... 90 7-21Drell-Yantransverseandtowardregionschargedmultiplicitydensity ...... 91 7-22Drell-YantransMAX,transMINandtowardregionschargedmultiplicitydensity 91 7-23Drell-Yanallregionschargedmultiplicitydensity ................. 92 7-24Drell-Yantransverseandtowardregionschargedtransversemomentumsumdensity ......................................... 92 7-25Drell-YantransMAX,transMINandtowardregionschargedtransversemomentumsumdensity ...................................... 93 7-26Drell-Yanallregionschargedtransversemomentumsumdensity ......... 93 7-27Drell-Yantransverseandtowardregionchargedaveragetransversemomentum 94 7-28Drell-Yanallregionschargedaveragetransversemomentum ........... 94 7-29Drell-Yantransverseandtowardchargedmaximumtransversemomentum ... 95 7-30Drell-Yanallregionschargedmaximumtransversemomentum .......... 95 7-31Drell-Yanallregionschargedmultiplicitydensitycombined ............ 96 7-32Drell-Yanallregionschargedtransversemomentumsumdensitycombined ... 96 7-33Drell-Yanandleadingjettransverseregionchargedmultiplicitydensity ..... 97 7-34Drell-Yanandleadingjettransverseregionchargedtransversemomentumsumdensity ......................................... 97 7-35Drell-Yanandleadingjettransverseregionchargedaveragetransversemomentum 98 7-36Drell-Yanandleadingjettransverseregionchargedmaximumtransversemomentum 98 7-37Drell-YanandleadingjettransMAXandtransMINregionschargedmultiplicitydensity ......................................... 99 7-38Drell-YanandleadingjettransMAXandtransMINregionschargedtransversemomentumsumdensity ............................... 99 7-39Drell-YanandleadingjettransDIFregionchargedmultiplicitydensity ..... 100 7-40Drell-YanandleadingjettransDIFregionchargedtransversemomentumsumdensity ......................................... 100 7-41Drell-Yanandleadingjetawayregionchargedmultiplicitydensity ........ 101 7-42Drell-Yanandleadingjetawaychargedtransversemomentumsumdensity ... 101 12

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... 102 7-44Drell-Yanpairtransversemomentumvschargedmultiplicity ........... 102 7-45Drell-Yanmeanchargedtransversemomentumagainstchargedmultiplicity,(pairpT<10GeV/c) .................................... 103 8-1ChargedmultiplicitydensityatLHC ........................ 107 13

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1 ]puttogetherelectromagneticinteractionsandtheweakforceintoanuniedframework.Thetheoryisinitiallyformulatedwithfourmasslessparticlesthatcarrytheforces.Aprocessofsymmetrybreakinggivesmasstothreeofthesefourparticles-theW+,theWandtheZ0,whichparticlesarethecarriersoftheweakforce.Theparticlethatremainsmasslessisthephoton,whichisthecarrieroftheelectromagneticforce.ThistheoryistermedasQuantumElectrodynamics(QED),asthisaquantumversionoftheclassicalelectrodynamics.WeextendthetheorytodescribethestrongcolorforceandthatistermedQuantumChromodynamics(QCD)[ 2 ].ItisanalogoustoQuantumElectrodynamics,butcontainscolorchargesexceptelectricalcharges.Asymptoticfreedomguaranteesthatperturbativeexpansionsmostlyfail-soderivinganalyticresultsfromthetheoryisextremelydicult.Thecarriersarethecolorforceareeightmasslesscoloredgluons,andjustlikethequarks,theycannotbeobservedinisolation.Theelectroweaktheory,togetherwithQCDformthestandardmodel[ 3 ]ofparticlephysics.Sofar,wehavementionedthetwelveforcecarriers,whichareallspinzeroorspinonebosons.Thematterparticlesarespinhalffermions,andareoftwotypes,leptonsandquarks.Theleptonsincludetheelectron,themuonandthetauandtheassociatedneutrinosforeachofthose.Sincewemustincludetheirantiparticles,theyadduptoatotaloftwelveleptons.Therearesixdierenttypesofquarks,calledavorsforhistoricalreasonsandtheavorsareup,down,charm,strange,topandbottom.Eachofthesesixquarkavorscomesinthreedierentcolors,soincludingthecolorsandassociatedantiquarkforeachofthemweendupwithatotalofthirty-sixquarks-theyindierentcombinationsconstitutesthehadrons.Figure1-1depictsthecomponentsofthestandardmodelasdescribedabove.Thequarksandgluonsconstitutingthehadronsareoftentermedaspartons. 16

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ComponentsofTheStandardModelofparticlephysics(Figurecredit:DOE/FermiNationalAcceleratorLaboratory) StandardModelsummarizescurrentknowledgeandisverymuchconsistentwiththeavailabledata.Howeverithassignicantshortcomings.Itdoesnotincludegravityandthatpreventsanyuniedtheoryofalltheforces.StandardModeladmitsonlymasslessparticlesandweneedspontaneouselectroweaksymmetrybreakingtoproducemasses.ThisisthesocalledHiggsMechanism[ 4 ],implementingwhichresultsinyetundetectedHiggsBoson(s).TheHiggsmechanismnotonlyprovidessymmetrybreakingandparticlemassesbutalsocontrolsthehighenergybehaviorofweakinteractions.Apartfromthis,wedonotknowwhythetypicalenergyscaleassociatedwiththeelectroweaksymmetrybreaking(roughly,thetypicalsizeofallmassesofelementaryparticles)issomuch(1015times)smallerthanthePlanckenergy-whichisknownasthehierarchyproblem.Infactthereareabouttwentyparametersinstandardmodel,suchasmassesandthecouplingsandmixingangles,thatneedtobeputinbyhand. 17

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5 ]-whereeveryfundamentalfermionhasasuperpartnerwhichisabosonandviceversa.Sinceallmatterparticlesarefermionsandallforcecarriersarebosons,thissymmetryuniesmatterandforce.AnotheralternativeisStringtheory[ 6 ],whichtriestounifygravitywiththeStandardModel.However,neitherofthesehavebeenexperimentallyveried. 18

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Fixedtargetandcollidermode(Figurecredit:Fermilabpublicwebpage) insmallerbackgroundsandlessermultiplicities.However,wewouldbediscussinghadroncollisionsinthisdissertation,sincethedatawewouldbelookingatcamefromprotonantiprotoncollisionatTevatroninFermiNationalAcceleratorLaboratorynearChicagoinIllinois. 7 ]toseparatelyacceleratetheprotonsandantiprotonsto980GeV.Thepathstakenbypand 8 ],whichiseectivelyagiantcapacitor.Next,theHionsenteralinearaccelerator(Linac)[ 9 ],

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SchematiclayoutoftheFermilabacceleratorcomplex.Protons(solidarrow)areacceleratedattheCockcroft-Walton,Linac,Booster,MainInjectorandnallyattheTevatron.Theantiprotons(dashedarrow)fromtheantiprotonsourcearerstacceleratedattheMainInjectorandthenattheTevatron(FigureCredit:Fermilabpublicwebpage). approximately150mlong,wheretheyareacceleratedto400MeV.AnoscillatingelectriceldintheLinac'sRadioFrequency(RF)cavitiesacceleratestheionsandgroupsthemintobunches.Theforceoftheeldactingontheionsacceleratesthemwhiletheyareinthecavities.TheforceoftheelddeceleratestheionstheymovethroughtheRF-shieldeddrifttubes.Beforeenteringthenextstage,acarbonfoilisusedtoremovetheelectronsfromtheHions,leavingonlythebareprotons.The400MeVprotonsaretheninjectedintotheBooster,acircularrapid-cyclingsynchrotronof74.5mindiameter[ 9 ],withconventionalmagnetstofocusandsteerthebeam.TheprotonstravelaroundtheBoostertobeacceleratedtoanalenergyof8GeVbyanotherseriesofofRFcavities.Toproduceantiprotons,protonsfromtheBoosterareacceleratedto120GeVbytheMainInjector[ 10 ]andcollidedwithanickeltarget[ 7 ].Thisproducesawidespectrumofsecondaryparticles,includingantiprotons.About20antiprotonsareproducedperonemillionprotons,withameankineticenergyof8GeV.Theantiprotonsarefocusedbyalithiumlensandseparatedfromotherparticlespeciesbyapulsedmagnet.The 21

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11 ]inthe8GeVAccumulatorring.Ittakesbetween10and20hourstobuildupa`stack'ofantiprotonswhichisthenusedforcollisionsintheTevatron.Antiprotonavailabilityismostoftenthelimitingfactorforattaininghighluminosities.Onceasucientnumberof 10 ]foraccelerationto150GeVandinjectionintotheTevatron[ 7 ].TheMainRingisa1kminradius,rapid-cyclingsynchrotronringwith3.5kGaussconventionaldipolemagnetsforsteeringthebeam,quadrupolemagnetsforfocusing,andanRFcavitythatacceleratesthepto150GeVbeforetheyareinjectedintotheTevatron.Thestackscontain36bunches,withaprotonbunchcontainingaround31011protonsandantiprotonbunchcontainingaround31010antiprotons.ThesamemagneticeldintheMainRingandTevatronbendsthepand 22

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Figure2-3. Deningcollidercoordinates:z-axisandpolarangle Deningcollidercoordinates:transversemomentumandspace Therapidityisdenedasy=1 2ln(E+pz 23

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Figure2-5. Advantageofusing-coordinatesandmappingofpolarangletopsuedorapidity 2.4.1OverviewTheCDFRunIIdetector[ 12 ],inoperationsince2001,isanazimuthallyandforward-backwardsymmetricsolenoidalparticledetectordesignedtostudyp pcollisionsattheTevatron.Itisamultipurposedetector,meaningthedesignisnotaimedatoneparticularphysicsmeasurement,butratheratextractinggenerallyusefulinformationaboutthecreatedparticles.Itcombinesprecisionchargedparticletrackingwithfastprojectivecalorimetryandnegrainedmuondetection.TheCDFdetectorisshowninFigure2-5withaquadrantcutouttorevealthedierentsub-detectors,arrangedcoaxiallyaroundthebeam-pipe,whicharedescribednext.Themomentumcomponentofallhadrons,electrons,muonsandphotonstransversetothebeamaxisaremeasured(particlesthatescapealongthebeampipehavenegligibletransversemomentum)andanysignicanceimbalanceintransversemomentum,termedas 24

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SchematicdiagramoftheCDFRunIIdetector(Figurecredit:CDFpublicwebpage) missingenergycanbeattributedtopenetratingneutralparticles(mostlyneutrinos)whichhavepassedundetected.Wewilldescribehowelectrons,muonsandchargedtracks,whichareusedinthisanalysisareidentiedandreconstructedintherelevantdetectorcomponentsingreaterdetailinChapter6. 13 ]andofanopen-cellwiredriftchamber[ 14 ]thatsurroundsthesilicon.Thesiliconmicrostripdetectorconsistsofsevenlayers(eightlayersfor1:0
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14 ],a3.1-m-longlongcylindricaldriftchambercoveringtheradialrangefrom40to137cm.Itislledwithwithfastgas(50%argon,50%ethane)tomakedrifttimessmallenoughsothatthehitscanbereadoutbetweeneachTevatronbunchcrossing.Itisorganizedinto8alternatingsuperlayersof4stereo(2)and4axialwireplanes,providing96mesaurementslayers.ACOTcellhas12sensewiresorientedinaplane,at35withrespecttoradialdirectionforLorentzdrift,agroupofsuchcellsatgivenradiusformsasuperlayer(SL).TheCOTprovidescoverageforjj1.ATime-of-Flight(TOF)detector[ 15 ],basedonplasticscintillatorsandne-meshphotomultipliersisinstalledinafewcentimetersclearancejustoutsidetheCOT.TheTOFresolutionis100psandthetiminginformationfromtheTOFcanbecombinedwiththemomentummeasurementfromtheCOTtodeduceaparticle'smass.ThedefaultCOTtrackingalgorithmrstreconstructstracksegmentsineachofeightsuperlayers.Itchecksforhitslooselyconsistentwithastraightline,usingatoleranceof20ns.Theidentiedhitsineachsegmentarethenttoacirculartrajectory. 16 ]surroundthetrackingsystemandmeasuretheenergyofinteractingparticles.Particlesmakeshowers 26

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17 18 ]isshowninFigure2.6.Theshowerdevelopsuntiltheenergyreachesacriticalenergy(approx600MeV)andionizationlossesequalthoseofbremsstrahlung.Asimilarphenomenaoccurswhenhadroninteractwithmatter,whichisreferredtoasahadronicshower.Anincidenthadronundergoesaninelasticcollisionwithnuclearmatterinthedetectorresultinginsecondaryhadrons.Thesehadronsalsoundergoinelasticcollisions.Asmanydierentprocessescontributetothedevelopmentofahadronicshower,themodelingoftheshowerismuchmorecomplexthananEMshower. Figure2-7. Developmentofanelectromagneticshower Thecalorimeterhasaprojectivetowergeometry;itissegmentedinandtowersthatpointtotheinteractionregion.Thecoverageofthecalorimetrysystemis2inandjj<4:2inpseudo-rapidity.Thecalorimetersystemisdividedintothreeregions 27

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19 ]residesbeyondthecalorimeters.Wheninteractingwithmatter,muonsactasminimallyionizingparticles(lowbremsstrahlungradiationduetotheirrelativelylargemass);theyonlydepositsmallamountsofionizationenergyinthematerial.Theyaretheonlyparticleslikelytopenetrateboththetrackingandveabsorptionlengthsofcalorimetersteel,andleavetracksinthemuondetectionsystem.TheCDFdetectorhasfourmuonsystems:theCentralMuonDetector(CMU),CentralMuonUpgradeDetector(CMP),CentralMuonExtensionDetector(CMX),andtheIntermediateMuonDetector(IMU).CMUconsistsoffourlayersofplanardriftchambersanddetectsmuonswithpT>1:4GeV=c.TheCMP'sareadditionalfourlayersofplanardriftchambersinstrumentanddetectsmuonswithpT>2:0GeV=c.CMXdetectorismadeofdriftcellsandscintillationcounters,whichareusedtorejectbackgroundbasedontiminginformation.Usingthetiminginformationfromthedriftcellsofthemuonsystems,shorttracks(called`stubs')arereconstructed.TracksreconstructedintheCOTareextrapolatedtothemuonsystems.Forgoodqualitymuons,anupperlimitisplacedonthe2tvalueoftrack-stubmatch.TheCMU,CMPandCMXchamberseachprovide 28

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DierentparticlesarebeingdetectedattheCDFdetector(Figurecredit:CDFpublicwebpage) coverageinthepseudo-rapidityrangejj<0:6.TheIMUcoverstheregion1:0
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Figure2-9. AgraphofCDFluminositywithtime,showingtheremarkableimprovementovertheyears(Figurecredit:CDFpublicwebpage) 30

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pcollisionshappenatarateof2.5MHz,andthereadoutofthefulldetectorproduces250kBofdata.Thereisnomediumavailablewhichiscapableofrecordingdatathisquickly,norwoulditbepracticaltoanalyzeallofthisdatalateron.TheCDFtriggersystem[ 22 ]hasathreelevelarchitecturewitheachlevelprovidingaratereductionsucienttoallowforprocessinginthenextlevelwithminimaldeadtime

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RatereductionsatCDF3leveltriggersystem TriggerlevelRatereductionRatio Level11:7MHz!25kHz1:70Level225kHz!600Hz1:40Level3600Hz!100Hz1:6Net1:17000 preselectedcriteriaareselectedandrecorded.Oftenfurtherselectioncutsindierentvariablesaremadetorenetheevent.Sometimestheratesofsometriggersaretoohightosustainathighluminosity,thetriggerisprescaledbyaconstantfactor-thatfractionoftheeventssatisfyingthetriggercriteriaarerandomlyrejected. Figure2-10. AowchartshowingtheCDFtriggersystem(Figurecredit:CDFpublicwebpage) 32

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21 ]include,thereplacementofthecentraltrackingsystem;thereplacementofagassamplingcalorimeterintheplug-forwardregionwithascintillatingtilecalorimeter;preshowerdetectors;extensionofthemuoncoverage,aTOFdetectorandupgradesoftrigger,readoutelectronics,anddataacquisitionsystems.TheupgradedCDFIIDetectorprovidesusbettersolidanglecoverageandparticleidentication. 33

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23 ],asshowninFigure3-1.25%ofthetime,afterthecollision,theprotonandtheantiprotonscatterelasticallythroughasmallangle,exchangingmomentumbuttherearenonewparticlesorenergyloss,whichisnotveryinterestingfromourperspective.Therestofthetimewehaveinelasticcollision,whereoneorbothhadronshaveachangeinenergyanddirection.Theinelasticcrosssectionconsistsofthreeterms;singlediraction(SD),double-diraction(DD),andeverythingelse(referredtoasthe`hardcore'),IN=SD+DD++DD.InSD(12%),oneoftheincidentparticlesplitsupintootherparticlesandtheotherparticleleaveatasmallangleontheotherside.InDD(8%)boththeprotonandtheantiprotondissociatesintoabundleofhadronsandandtravelatrelativelysmallanglesoneitherside.Wearemostlyinterestedinthe(nondiractive)hardcorepart-partofitisthesoftcollision,wherethebeamhadrons`ooze'througheachotherproducinglotsofsoftparticleswithauniformdistributioninrapidityandmanyparticlesyingdownthebeampipe.Occasionallythereisahardscatteringamongconstituentpartons,producingoutgoingparticleswithlargepTinthetransverseregion.BysoftwemeanlowtransversemomentatransferfrominitialtonalstateandveryfewornoparticlesproducedwithsignicantpT.Incontrast,interactionsinvolvingthecreationofatleastoneoneparticlewithappreciablepTistermedhardscattering.HardinteractionscanbecalculatedreliablyusingperturbativeQCDwhilesoftinteractionsarenoteasilycalculablewithinQCDandrelyonad-hocmodelswhicharetakenfromdata(withsometheory).ExperimentallyitisdiculttoseparateHCfromDD.At1.8TeV(CDFRun1)thetotalproton-antiprotoncrosssectionisabout78mbandtheelasticcrosssectionisabout 34

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Showsthevariouscomponentsoftheproton-antiprotontotalcrosssection 18mb[ 24 ].Singlediractionmakesupabout9mbofthe60mbinelasticcrosssectionandHC+DD=51mb,withdoublediractionintherange4
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25 ].Figure3-3showsthetherstorderhardscatteringdiagramsforprotonantiprotonscattering.Bylookingattheatthetypicalpartondistributionfunctions[ 26 ],wecanseethatgluonsarethemostprobablepartons,exceptatthehighestmomentumfractions.ThecrosssectionatverylowpTisdominatedbygluon-gluonandquark-gluonscatteringviathetchannel. Figure3-3. Firstorderdiagramsforproton-anti-protonscattering.Ifoneassumesthattimerunsbottomtotop(thetheoristsconvention),therstcolumnindicateexchangeinthetchannel,thesecondschannelexchange,thethirdtheuchannelandthe4thisaspecialQCDdiagram. Wecannotseefreequarksandgluonsappearingatnalstagebecauseofcolorconnementandstronginteraction.Forexamplewhenaquarkisknockedoutofthe 36

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27 ],althoughtechnicallyweneedanalgorithmtoproperlydeneajet.Therecanbecorrectionstothissimplepicturefromvariouseects.Firstly,thereareQEDandQCDbremsstrahlung-typemodications,andbecauseofthelargenessofthestrongcouplingconstantsandthepresenceofthetriplegluonvertex,QCDemissionoquarksandgluonsisespeciallyprolic.Wethereforespeakaboutpartonshowers,showninFigure3-4[ 28 ]whereinasingleinitialpartonmaygiverisetoawholebunchofpartonsinthenalstate.AlsophotonemissionmaygivesizableeectsinQEDprocesses.Thebulkofthebremsstrahlungcorrectionsareuniversal,i.e.donotdependonthedetailsoftheprocessstudied,butonlyononeorafewkeynumbers,suchasthemomentumtransferscaleoftheprocess.Secondly,wehavetruehigher-ordercorrections,whichinvolveacombinationofloopgraphsandthesoftpartsofthebremsstrahlunggraphsabove,acombinationneededtocancelsomedivergences. 29 ]aseverythingexceptthehardscatteredcomponentsanditincludes 37

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Apartonshowerinwhichaquarkinitiallyproduced,radiatesgluonswhichinturnradiateadditionalgluonsandquark-antiquarkpairs.Astimeincreasestheshowerprogressestolargerdistancesfromthepointwhereininitialquarkwasproducedandhadronsareformed. the`beam-beamremnants'plusthemultiplepartoninteraction.The`hardscattering'componentconsistsoftheoutgoingtwojetsplustheinitialandnal-stateradiation.The`beam-beamremnants'arewhatisleftoverafterapartonisknockedoutofeachoftheinitialtwobeamhadronsasinFig3-5.Itisthereasonhadron-hadroncollisionsaremore`messy'thanelectron-positronannihilationsandnoonereallyknowshowitshouldbecalculated.Also,multiplepartonscatteringcontributestothe`underlyingevent'.Inadditiontothehard2-to-2parton-partonscatteringandthe`beam-beamremnants',sometimesthereisasecond`semi-hard'2-to-2parton-partonscatteringthatcontributesparticlestotheunderlyingeventasinFigure3-5.However,fromanexperimentalpointofview,itisimpossibletouniquelyseparatethehardscatterfromtheunderlyingeventcleanlyonaneventbyeventbasis.Forexample,softgluons(QCDradiation)emittedfromthehardscatterquarkswouldtypicallybepartoftheunderlyingeventbutwheresoftgluonsbecomehardgluonsandsincenotpartoftheunderlyingeventisnotanexactdenition.IfitisofhighenoughpTtohadronizeintoajetthenitisgenerallyconsideredhard,butthatalsodependsonthechosenjetalgorithm. 38

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Theunderlyingeventconsistsofeverythingexceptthehardscatteredcomponents,beam-beamremnantsandmultiplepartoninteractions. Theenvironmentathadroncollidersaredominatedbyhardscatteringeventsandthesehardscatteringeventsarecontaminatedbyunderlyingevents.Theyareunavoidablebackgroundtoallcolliderobservables.Forexample,attheTevatronboththeinclusivejetcrosssectionandtheb-jetcrosssection,aswellasisolationcuts,measurementofmissingenergydependsensitivelyontheunderlyingevent.Inallprecisionmeasurementsofhardinteractionswheresofteectsneedtobesubtracted,highertheprecisionoftheunderlyingeventmodeling,highertheaccuracyofphysicsmeasurements.Infact,aswediscussedbeforeitisnotpossibleonanevent-by-eventbasistobecertainwhatparticlescamefromtheunderlyingeventand,whichparticlesoriginatedfromthehardscattering.Increasingluminosityimpliesmorehadroniccollisionsresultinginmoreunderlyingevents(whichistechnicallyknowasthepileup 23 ],althoughdierentfromtheunderlyingevent,isanotherexcellentplacetolookatthetheoreticallypoorlyunderstoodsofter

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76 ]thatthedensityofparticlesintheunderlyingeventinjeteventsisaboutafactoroftwolargerthanthedensityofparticlesinatypicalmin-biascollision.AttheLHCthedierencemightbeevengreater. 30 ].Thedirectionoftheleadingcalorimeterjetisusedtoisolateregionsofspacethataresensitivetotheunderlyingevent.Theangle=leadingjetistherelativeazimuthalanglebetweencharged 40

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Dierentregionsinspace,withrelativetotheleadingjet particlescomingfromtheunderlyingeventandthedirectionofhardscatteredleadingjet,asinFigure3-6.LaterwewouldbelookingatleptonpairproductionfromthedecayofaZboson,thenwouldbedeterminedrelativetothedirectionoftheZboson,asinFigure3-7.Wesplitthecentralregiondenedbetweenjj<1asfollows,jj<60asthetowardregion.60120astheawayregion. Figure3-7. Dierentregionsinspace,withrelativetotheZ-boson Forhardscatteredjetsthetransverseregionsaremostsensitivetounderlyingevents,sincetheyareperpendiculartotheplaneof2-to-2hardscattering.Forthemwe 41

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Figure3-8. ThetransMAXandtransMINregions AsillustratedinFigure3-8,wedeneMAXandMINtransverseregionswhichhelptoseparatethehardcomponent(initialandnal-stateradiation)fromthebeam-beamremnantcomponent.MAX(MIN)refertothetransverseregioncontaininglargest(smallest)numberofchargedparticlesortotheregioncontainingthelargest(smallest)scalarpTsumofchargedparticles,onaneventbyeventbasis.Foreventswithlargeinitialornal-stateradiationthe`transMAX'regionwouldcontainthethirdjetinhighpTjetproductionorthesecondjetinDrell-Yanproductionwhileboththe`transMAX'and`transMIN'regionsreceivecontributionsfromthebeam-beamremnants.HenceoneexpectsthatthetransMAXregionwillpickupthehardestinitialornal-stateradiationwhileboththetransMAXandtransMINregionsshouldreceivebeam-beamremnantcontributions.HenceoneexpectsthetransMINregiontobemoresensitivetothebeam-beamremnantcomponentoftheunderlyingevent,whilethetransMAXminusthetransMIN(i.e.,transDIF)isverysensitivetohardinitialandnal-stateradiation.Thisidea,wasrstsuggestedbyBryanWebberandPinoMarchesini[ 31 ],andimplementedinapaperbyJonPumplin[ 32 ]. 42

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Figure4-1. SchematicrepresentationoftheDrell-Yanleptonpairproduction TheinitialstudiesonmuonpairproductioninhadronhadroncollisionswasrstdoneatBNL(BrookhavenNationalLaboratory)byChristensonetal.andtheirresultsareshowninFigure4-2[ 33 ].Acoupleofinterestingfeatureswereobserved.Theshoulderlikestructurenearthemuonpairmassof3GeV=c2,whichwaslater[ 34 ]discoveredtobetheJ=particle.Therapidfallinincrosssectionwithincreasingdileptonmasswasnotconsistentwiththepointlikecrosssectionsobservedindeepinelasticelectronscattering.CalculationsofDrellandYan(1970,1971)[ 35 ]explainedmostfeaturesoftheprocessbyextendingthethepartonmodel[ 36 ]developedtoexplaindeepinelasticleptonscatteringandthisprocesscametobeknownastheDrell-Yanprocess.Howevertheoverallratewasunderestimatedbyafactorofaround2,whichwaslaterpredictedbytheQCDcalculations,takingintoeectthegluonemissionandabsorption.Alsoresultinglargeaveragetransversemomentaofdileptonswasinitiallynotwellunderstood-QCDeectsofgluonemissionandgluonscatteringprovidedtheexplanation.Wewillusnowdescribetheprocessindetail,following[ 37 ]. 43

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DimuonspectrumfromtheBNL(BrookhavenNationalLaboratory)experiment

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FeynmandiagramfortheDrell-Yanprocess productofprobabilitytondtheparticularpartoncongurationandthecrosssectionforfreepartons.SofortheDrell-Yanprocess, 45

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38 ], dM2=42 2ln(x1 dM2dy=42 46

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2 39 ], dM2dy=F()(4{8)Sointhepartonmodel,thecrosssectiondependsonlyonthescalingvariable=Q2 40 41 ]. Figure4-4. QCDdiagramsfortheDrell-Yan(a)Leadingorderdiagramsforquark-antiquarkannihilationsubprocess(b)LeadingorderdiagramsforComptonsubprocess TheQCDcorrections[ 2 ]resultinlogarithmiccorrectionsinQ2whichcanbeabsorbedinQ2-dependentquarkandantiquarkdistributionfunctionofthehadrons.Analyticcontinuationfromspace-likeq2(deepinelasticscattering)totime-likeq2(leptonpairproduction)andthedierenceinkinematicsbetweenthetwoprocessesproduceanon 47

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42 ].Inpractice,leptonpairproductionwithMMZisanalyzedintermsoftheproductioncrosssectionforZbosons(qq!Z),multipliedbythebranchingratiofordecayintoleptonicnalstates(Z!l+l).SingleZbosonsareproducedwithlargepTviatheordinaryQCDsubprocessesqg!Zq,q q!Zg,q g!Z q.Theygenerateadditionalgluonsviabremsstrahlungresultinginmultipartonnalstatesfragmentingintohadronsandformingaway-sidejets.asinFig4-5. Figure4-5. IllustrationoftherecoilfromtheZ-bosonproductionandformationofawaysidejet 48

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37 ].Thesedataalsoshowedthatlogarithmicviolationofscalingdatawasnotverysignicant.ThegoodagreementbetweenthetheoreticalpredictionsandmeasuredDrell-Yancrosssections,asseeninTable4-1providedconrmationofthispartonmodelapproach.TheQCDimprovedversionofthepartonmodelhasbeenconrmedbytheexperimentscarriedovertheyears. Table4-1. Drell-Yancrosssections CDF(pb)[ 43 ]NNLOtheory(pb)[ 44 ] 254:93:3(stat)4:6(sys)15:2(lum)252:35:0 Mostoftheimportanthardscatteringprocesseshavebeencalculatedtonexttoleadingorder(NLO)inperturbationtheory-whiletheDrell-Yanprocessitselfhasbeencalculatedtonexttonextleadingorder(NNLO).AsaresultithasbeenanimportanttheoreticaltooltoexploredierentaspectssuchasinfrareddivergencesandcollineardivergencesleadingtothefactorizationtheoreminQCD.Theprocessissowellunderstoodtheoreticallythatithasbecomeatoolforprecisionmeasurements,asexempliedbydiscoveryandmeasurementofWandZ.Bymeasuringthedistributioninrapidityandmassoftheleptonpaironecaninprincipledirectlymeasurethequarkandantiquarkdistributionfunctionofcollidinghadrons.ForleptonpairproductionabovetheZmasstheelectroweakinterferenceisimportantandtheforwardbackwardasymmetriescanbeaneectivetooltondZ0bosonsiftheyexist[ 45 ].However,thereareafewsourcesofbackgroundwhichcometodominatethedileptoncontinuumatlowdileptonmasssuchasdileptondecaysofthecloselyspaced(neutral)vectormesonsandBethe-Heitlerpairs[ 46 ].Alsotheaccidentalcoincidencesofleptonicdecayssuchas+!++mubetweentwohadronsproducedinanyhadronicinteractioncanactasabackground.AtlowmassitisthereforediculttoextracttheDrell-Yansignalfrombackgroundandgenerallydataondileptonswithmassesaboveatleast2GeV=c2areused. 49

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Figure4-6. UnderlyingeventinDrell-Yanproduction-everythingexceptthenalstatelepton-antileptonpairandinitialstateradiation Bylookingatthediagramwecanseethatessentiallyeverythingotherthanthenalleptonantileptonpairistheunderlyingevent.InhighpTDrell-Yan,wecanhavetwoleptonsonthesameside,balancedbyajetontheotherside,asinFigure4-5. Figure4-7. Drell-Yanproductionwithhightransversemomentum,withtwoleptonsareonthesameside ForDrell-Yanitseasytoidentifyandremoveleptons(sincetheyarethecolorlesscomponents)fromthetransverseandtoward(whichcannotbedonefordijetevents,astheleadingjetisitselfintowardregion)regionsandusethemtostudytheunderlying 50

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Figure4-8. EventdisplayofaDrell-Yanprocess.COTviewonleft,whileEMcalorimeterviewonright.FigurescourtesyCDF. 51

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47 ].Asopposedtodeterministicsimulationmethods,theyuserandomnumbers(pseudo-randomnumbers,inpractice)andhencearestochastic.Itisusefulinmanyinterestingcalculations,suchasdeterminingthecrosssectionforascatteringprocesshavetoomanydegreesoffreedomfordirectnumericalintegration.MonteCarlocalculatestheseintegralsbygeneratingarandomsampleofcongurationsandaveragingtheintegrand,i.e.,bygeneratingarandomsampleof`real'eventsandaveragingtheirweights.Asthemethodisbasedonrandomchance,itwasnamedafteragamblingresort.InordertondnewphysicsatahadronhadroncollideritisessentialtounderstandandmodeltheordinaryQCDeventswell.Todothisonemustnotonlyhaveagoodmodelofthehardscatteringpartoftheprocess,butalsoofunderlyingevent.HowevermanyaspectsofnonperturbativeQCDphysics,likehadronizationandmultipleinteractions,cannotbederivedfromrstprinciples.Theonlytoolthatwehaveinourdisposalforthesestudiesisacross-comparisonofthedataandvariousMonteCarlogenerators.ByadjustingmanyparametersthatrepresentatrueuncertaintyinourunderstandingofnatureintheseMonteCarlogenerators,wetrytomatchthesimulationtothedatainthebestpossiblewayinordertogaindeeperinsightsintotherelativeimportanceifthevariouscontributingsub-processes.Aneventgeneratorisalsohelpfulingivingafeelforexpectedrealdata,leadingtoimprovementdetectordesignandanalysisstrategies.AlsoMonteCarlocanbeusedasamethodforestimatingdetectoracceptancecorrectionsthathavetobeappliedtorawdata,inordertoextractthetruephysicssignal. 52

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53 ].1.Startsobytwoparticlescomingtowardeachotherandoneradiatedparticlefromeachoftheincomingparticlestartsoasequenceofbranchings,whichbuildupaninitial-stateshower.2.Oneincomingpartonfromeachofthetwoshowersparticipateinthehardprocess,whichdeterminesthemaincharacteristicsoftheeventwhereanumberofoutgoingpartonsareproduced,usuallytwo.3.Thehardprocessmayproduceasetofshort-livedresonances,liketheZ0orWgaugebosons,whosedecaytonormalpartonshastobeconsideredincloseassociationwiththehardprocessitself.4.Theoutgoingpartonsusuallybranch,analogouslytotheincomingones,tobuildupnal-stateshowers.5.Inadditiontothehardprocessconsideredabove,therecanbefurthersemihardinteractionsbetweentheotherpartonsoftwoincominghadrons,termedmultipleparton-partoninteraction.6.Aftertheparticleparticipatinginthehardscatteringprocessistakenoutofabeamparticle,abeamremnantisleftbehind.Thisremnantmayhaveanetcolorchargethatrelatesittotherestofthenalstate.(5)and(6)mostlyconstitutetheunderlyingevent.7.TheQCDconnementmechanismensuresthattheoutgoingquarksandgluonsarenotobservable,butinsteadhadronizetocolorneutralhadrons.8.Manyoftheproducedhadronsareunstableandcandecayfurther.Howeverthedatacomingfromarealexperimentalsointeractswithacomplexdetectorsystem.Thebehaviorofthedetectorshowparticlesproducedbytheeventgeneratortraversethedetector,spiralinmagneticelds,showerincalorimeters,orsneakoutthroughcracks,etc.,whichallaectthedistributionsproduced,issimulatedin 53

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Structureofthebasiceventgenerationprocess programssuchasGeant[ 48 ].TheCDFdetectorsimulationpackage,CDFSIMisbasedonsimilarprograms. 49 ]andHERWIG[ 50 ]aretwoofthemostpopularMonteCarloeventgeneratorsforhighenergyphysics.Theycontaintheoryandmodelsforallthestepsdescribedabove.Theyarelargelybasedonoriginalresearch,butalsoborrowmanyformulaeandotherknowledgefromtheliterature.SincethisanalysismainlyusesPYTHIA,wewillfocusmostlyonit.PYTHIAcontainsrichselectionofaround240dierenthardprocesses,classiedaccordingtothenumberofnal-stateobjects.Thebulkoftheprocessesareofthe2-to-2type,whichisnotamajorlimitation,sinceshowersaddtherequiredextraactivity.Ineveryprocessthatcontainscoloredand/orchargedobjectsintheinitialornalstate,gluonand/orphotonradiationmaygivelargecorrectionstotheoverallevent.Twotraditionalapproaches 54

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51 ]isimplementedinPYTHIA.Theassumptionoflinearconnementprovidesthestartingpointforthestringmodel.Astheqand q0andq0 52 ]. 55

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Atsmalldistancesthecolorforcesareweakandtheoutgoingpartonsmoveawayfromthebeam-beamremnantsandatlargedistancesthecolorforcesbecomestrongandquark-antiquarkpairsarepulledoutofthevacuumandhadronsareformed. Inahadroniccollision,thecollidingpartononlytakessomefractionofthetotalbeamenergy,leavingbehindabeamremnantwhichtakestherest.Foraprotonbeam,auquarkcollidingwouldleavebehindauddiquarkbeamremnant,withanantitripletcolorcharge.Theremnantisthereforecolor-connectedtothehardinteraction,andformspartofthesamefragmentingsystem.Oftentheremnantismorecomplicated,e.g.agluonparticipatinginthehardscatteringwouldleavebehindauudprotonremnantsysteminacoloroctetstate,whichcanconvenientlybesubdividedintoacolortripletquarkandacolorantitripletdiquark,eachofwhicharecolor-connectedtothehardinteraction.Theenergysharingbetweenthesetworemnantobjects,andtheirrelativetransversemomentum,introducesadditionaldegreesoffreedom,whicharenotunderstoodfromrstprinciples.Alsototakeintoaccountthemotionofquarksinsidetheoriginalhadron,asrequiredbytheuncertaintyprinciplebytheprotonsizeaprimordialtransversemomentumisassignedtothecollidingparton.ThisprimordialkTisselectedaccordingtosomesuitabledistribution,andtherecoilisassumedtobetakenupbythebeamremnant.Eachofthebeamparticlescontainsanumberofpartons,andsotheprobabilityforseveralinteractionsinoneandthesameeventisnotbenegligible.Inprincipletheseadditionalinteractionscouldarisebecauseonesinglepartonfromonebeamscattersagainstseveraldierentpartonsfromtheotherbeam,orbecauseseveralpartonsfromeachbeamtakeplaceinseparate2-to-2scatterings.Bothareexpected,butcombinatorics 56

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53 ]-herewewouldpointoutthesomeoftherelevantonesdescribingtheunderlyingeventinTable5-1.Technically,PYTHIAparameterscanbevariedindependentlyofeachother,butthephysicalrequirementofasensibledescriptionofasetofdataleadstocorrelationsandanticorrelationsbetweentheparameters.Henceweneedtoproducetunes,notofoneparameteratatime,butsimultaneouslyforagroupofthem.GiventhemanyPYTHIAparameterstobetuned,itisconvenienttodividethetaskintosubtasks.Firstly,ifweassumejetuniversality,hadronizationandnal-statepartonshowersshouldbetunedtoe+eannihilationdata,notablyfromLEP1(TheLargeElectron-Positroncollider,formerlyatCERN)sincethisoersthecleanestenvironment.Secondly,withsuchparametersxed,hadroncolliderdatashouldbestudiedtopindownmultipleinteractionsandotherfurtheraspects,suchasinitial-stateradiation.WewillfocusonPYTHIAtuneswhicharerelevanttounderlyingeventstudies.PYTHIAtuneAwasdeterminedbyttingtheCDFRun1underlyingeventdata[ 54 ],bymostlyadjustingmultiplepartoninteraction.Figure5-3showsthatPYTHIAtuneAdoesnotttheCDFRun1Z-bosonpTdistribution[ 55 ],sinceatthattimetheZbosondatawasnotconsidered.PYTHIAtuneAW,mostlybyadjustingtheinitialstateradiationtstheZ-bosonpTdistributionaswellastheunderlyingeventattheTevatron. 57

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SomeofthePYTHIAparametersdescribingtheunderlyingevent ParameterDenition MSTP(81)MPIon/oMSTP(82)3/4:respectively-denotingsingleordoublegaussianhadronicmatterdistributioninthep/ Thevaluesoftherelevantparameters 56 ].BothplotsinFigure5-3revealaremarkablygoodagreementofthedataandPYTHIA,which,however,wasachievedonlyaftertuninganumberofMonteCarlogeneratorparameters,asfollows.Theinitialstateradiationhadtobesignicantlyintensied.Thedependenceoftheprobabilityofmulti-parton(secondary)interactionsontheimpactparameterhadtobesmoothedout.Theprobabilityofdi-gluonproductioninmulti-partonsecondaryinteractionshadtobesubstantiallyenhancedoverdi-quarkproduction.Theprobabilityofcolorconnectionsofproductsofsecondaryinteractionswithpp-remnantshadtobeincreased.

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Z-bosonpTdistributionfromRun1andPYTHIAtuneAandAWpredictions ThisexerciseshowsthatPYTHIAcanbebroughtintoagoodagreementwithdata,withpropertuning[ 57 ].For`leadingjet'productionTuneAandTuneAWarenearlyidentical.WewillconcludethissectionbybrieymentioningsomeofthePYTHIARun2tunes[ 58 ].Table5.1showstheparametersforseveralPYTHIA6.2tunes.PYTHIATuneDWisverysimilartotuneAexceptthatitalsotstheCDFRun1Z-bosonpTdistributionwhichtuneAdoesnott.TuneDWhastheDpreferredvalueoftheparameter 59 ].TuneDWandtuneDWTareidenticalat1.96TeV,buttuneDWandDWTextrapolatedierentlytotheLHC.TuneDWTusestheATLASenergydependence,whiletuneDWusesthetuneAvalue 60 ].Therst9parametersinTable3.1tunethemultiplepartoninteractions(MPI).PARP(62),PARP(62),andPARP(62)tunetheinitial-stateradiation

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Table5-2. ParametersforseveralPYTHIA6.2tunes.TuneAisaCDFRun1`underlyingevent'tune.TuneAWandDWareCDFRun2tuneswhichttheexistingRun2`underlyingevent'dataandttheRun1Z-bosonpTdistribution.TheATLAStuneisthedefaulttunecurrentlyusedbyATLASattheLHC.TuneDWTusetheATLASenergydependencefortheMPI,PARP(90).Therst9parameterstunethemultiplepartoninteractions.PARP(62),PARP(62),andPARP(62)tunetheinitial-stateradiationandthelastthreeparameterssettheintrinsickTofthepartonswithintheincomingprotonandantiproton. ParameterTuneATuneAWTuneDWTuneDWTATLAS PDFCTEQ5LCTEQ5LCTEQ5LCTEQ5LCTEQ5L MSTP(81)11111MSTP(82)44444PARP(82)2.02.01.91.94091.8PARP(83)0.50.50.50.50.5PARP(84)0.40.40.40.40.5PARP(85)0.90.91.01.00.33PARP(86)0.950.951.01.00.66PARP(89)18001800180019601000PARP(90)0.250.250.250.160.16PARP(62)1.01.251.251.251.0PARP(64)1.00.20.20.21.0PARP(67)4.04.02.52.51.0MSTP(91)11111PARP(91)1.02.12.12.11.0PARP(93)5.015.015.015.05.0 Table5-3. ComputedvalueofthemultiplepartonscatteringcrosssectionforthevariousPYTHIA6.2tunes Tune(MPI)at1.96TeV(MPI)at14TeV A,AW309.7mb484.0mbDW351.7mb549.2mbDWT351.7mb829.1mbATLAS324.5mb768.0mb Table5.2showsthecomputedvalueofthemultiplepartonscatteringcrosssectionforthevarioustunes.Themultiplepartonscatteringcrosssection(dividedbythetotalinelasticcrosssection)determinestheaveragenumberofmultiplepartoncollisionsperevent. 60

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61 ].HERWIGdoesafairlygoodjobttingtheZ-bosonpTdistributionwithoutadditionaltuning,butdoesnotttheCDFunderlyingeventdata.Minimumbiascollisionsareamixtureofhardprocesses(perturbativeQCD)andsoftprocesses(non-perturbativeQCD)andare,hence,verydiculttosimulate.Min-biascollisionscontainsoft`beam-beamremnants',hardQCD2-to-2parton-partonscattering,andmultiplepartoninteractions(softandhard).Tocorrectlysimulatemin-biascollisionsonemusthavethecorrectmixtureofhardandsoftprocessestogetherwithagoodmodelofthemultiple-partoninteractions.Therstmodelthatevencameclosetocorrectlymodelingmin-biascollisionsatCDFisPYTHIAtuneA.TuneAwasnottunedtotmin-biascollisions.Itwastunedtottheactivityinthe`underlyingevent'inhightransversemomentumjetproduction.However,PYTHIAusesthesamepTcut-ofortheprimaryhard2-to-2parton-partonscatteringandforadditionalmultiplypartoninteractions.Hence,xingtheamountofmultiplepartoninteractions(i.e.settingthepTcut-o)allowsonetorunthehard2-to-2parton-partonscatteringallthewaydowntopT(hard)=0withouthittingadivergence.ForPYTHIAtheamountofhardscatteringinmin-biasis,therefore,relatedtotheactivityofthe`underlyingevent'inhardscatteringprocesses.NeitherHERWIG(withoutMPI)orHERWIG(withJIMMYMPI)canbeusedtodescribemin-biaseventssincetheydivergeaspT(hard)goestozero. 61

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62 ].Alsobylookingatthemeasurementssensitivetotheunderlyingevent,wewouldbeabletobetterconstrainourunderlyingeventmodels. 56 ]samples,takenbetweenFebruary2002andApril2008,asshowninTable1

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DataandMonte-Carlosamplesusedinthisanalysis LeptonMonteCarloData ElectronDrell-YanZ/gamma*!eesampleHigh-pTcentralelectronsMuonDrell-YanZ/gamma*!sampleHigh-pTCMUPandCMXmuons pcollisioneventvertexfromthecenterofthedetectorinzdirection.Toensurethatatrackforeachchargedparticleiswellmeasuredbythetrackingsystem,weneedthisrequirement.

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64

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63 64 ],withtightandloosecutsdescribedinTable6-2.Thetightandloosecutsaresimilartillthelastfourvariablesinthetable,i.e.looseelectronsaretightelectronswithouttheLShr,E=p,signedCESX,CESZandCESstrip2cuts. Table6-2. Electronselection VariableLooseTight RegionCEMCEMCESducial11ET20GeV20GeVTrackz060cm60cmTrackpT10GeV=c10GeV=cCOTaxial3AxialSLswith5hits/SL3AxialSLswith5hits/SLCOTstereo2StereoSLswith5hits/SL2StereoSLswith5hits/SLIsolation(R=0.4)/ET(withleakagecorrection)0:10:1EHad=EEM(3towers)(0:055+(0:00045E))(0:055+(0:00045E))LShr(3towers,track)...0:2E=p...2:0(unlesspT50GeV=c)CESZ...3:0cmSignedCESX...3:0qX1:5CESstrip2(scaledwithE)...10:0 Photonconversions,viapairproductionoccurthroughoutthedetectormaterialandareamajorsourceofelectronsandpositronsthatpasstheaboveselectioncriteria.Theyareidentiedbythecharacteristicsmallopeninganglebetweentwooppositelychargedtracksthatareparallelattheirdistanceofclosestapproachtoeachother[ 66 ].Electroncandidateswithanoppositelychargedpartnertrackmeetingtheserequirementsarerejected. 65

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Photonconversionintoanelectronpositronpair chambers)eitherinboththeCMU(CentralMuonsystem)andCMP(CentralMuonUpgrade)muondetectors(`CMUP'muon)orintheCMX(CentralMuonExtension)system(`CMX'muon).Goodcentralmuonsarerequiredtohaveatrack-stubmatchingdistancelessthan3cmforCMU,lessthan5cmforCMP,andlessthan6cmforCMX.Thevariablesneededforreconstructingamuonareasfollows.Trackz0.Distanceofwherethemuontrackextrapolatestobeamlinefromthecenterofthedetectorinzdirection.Toensurethattrackforeachmuoniswellextrapolatedtothecalorimeteranddriftchamber,weneedjzvertexjtobelessthan60cm.Thisrequirementhelpstoinsurethattheparticlepassesthroughasignicantportionofthedetectorsothatwecanobtainenoughinformationabouttheevent.TrackpT.Speciesthevalueofthetransversemomentumoftheassociatedtrack(transversetothebeamline).Weplaceaminimum20GeV/ccutonmuonmomentuminordertoremovelowenergybackgroundmuonscomingfromdecaysotherthanZ-bosons

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63 65 ].WelookattheCMUPandCMXmuons,withducialcuts

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Muonselection VariableMuon Forallmuontypes,RegionCMUPandCMXpT20GeV=cEEM2GeV+Max(0,(0.0115(p-100))EHad6GeV+Max(0,(0.028(p-100))Isolation(totalETinR=0.4aroundmuon)/pT0:1COTaxial3AxialSLswith5hits/SLCOTstereo2StereoSLswith5hits/SLTrackjz0j60cmTrackjd0j(beamcorrected)0:2cmAdditionallyforCMUPmuons,jXCMUj7cmjXCMPj5cmXFIDCMU0cmXFIDCMP0cmZFIDCMU3cmZFIDCMP0cmAdditionallyforCMXmuons,jXCMXj<6cmForrunnumber>150144,XFIDCMX<0cmZFIDCMX<3cmToremovecosmicmuons,jzj3cm getridofcosmicmuons,wealsousea`timeofight'(ToF)cosmiclter[ 67 ],whichwillbedescribedindetailinthenextsection. 68

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T=TupperTlower=(L1+L2)=c2L1=c2L2=cwhereL1andL2arethedistancestraveledbythecosmicrayintheupperandthelowerhalfofthedetector.Fortwomuonsoriginatingatthecenterofthedetector, T=TupperTlower=(L1L2)=c0Soideallythemuonsnotcomingfromthecosmicrayswouldhaveverylittletimedierence,andthisprincipleisusedtoeliminatecosmicraymuons.Themassrangeoftheleptonpairisdividedinto3regionsforthisanalysis,asshowninTable6-4.Theregionofleptoninvariantmassbetween70and110GeV=c2,termedtheZ-region,isusedforthisanalysis. Table6-4. Massranges MassregionMassrange LowLessthan70GeV=c2Z70110GeV=c2HighAbove110GeV=c2 68 ]haveshownthatthesebackgroundsarenegligibleattheregionofZ-boson.Approximately65,000electronandmuonpairseachpassedourselectioncriteriaandareusedintheanalysis.WeusethesamekinematiccutsonboththeparticlelevelMonteCarloandthedetectorlevelMonteCarloanddata-werequirethat, 69

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ShowsthatthebackgroundattheregionofZ-bosonisverylow IndividualleptonpT>20GeV/c, Individualleptonjj<1,and Leptonpairjj<6. 70

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66 ],tomakesurenoneofthechargedparticlesareelectronscomingfrompairproductionfromphoton.ThetrackselectioncriteriaisgiveninTable6-5. Table6-5. Chargedtrackselection VariableLooseTight TrackregionCOTCOTTrackpTmin0:5GeV=c0:5GeV=cTrackpTmax150GeV=c150GeV=cTrackjj11Trackz0<60cm<60cmTrackjd0j(beamcorrected)1cm0:5cmTrackjzj3cm2cmCOTaxial2AxialSLswith10hits/SL2AxialSLswith10hits/SLCOTstereo2StereoSLswith10hits/SL2StereoSLswith10hits/SLTrackt2/DoF1010 71

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Observablesexaminedinthisanalysisastheyaredenedattheparticlelevelandthedetectorlevel.Chargedtracksareconsidered`good'iftheypasstheselectioncriteriongiveninTable6-5.Themeanchargedparticle\pTiisconstructedonanevent-by-eventbasisandthenaveragedovertheevents.FortheaveragepTandthepTmaxwerequirethatthereisatleastonechargeparticlepresent.ThepTsumdensityistakentobezeroiftherearenochargedparticlespresent. ObservableParticlelevelDetectorlevel LeptonpTpToftheleptonpairpToftheleptonpair,formedbyatleastonetightleptonChargeddensityNumberofchargedparticlesperunitNumberof`good'chargedtracksperunitpTsumdensityScalarpTsumofchargedparticlesperunitScalarpTsumof`good'chargedtracksperunithpTiAveragepTofchargedparticlesAveragepTof`good'chargedtrackspTmaxMaximumpTofchargedparticlesMaximumpTofgoodchargedtracks ThemeanchargedparticlehpTiandthepTmaxareconstructedonaneventbyeventbasis.FortheaveragepTandpTmax,werequirethatthereisatleastonechargedparticlepresent.ThepTsum(hencethepTsumdensity)ustakentobezeroiftherearenochargedparticlepresent. 72

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69 ]todenejetsandcomparedtheobservablescorrectedbacktotheparticlelevelwiththeQCDMonteCarlomodels[ 70 ].Thethebehaviorofchargedparticles(pT>0.5GeV/c,jj<1)producedinassociationwithlargetransversemomentumjetswerestudied.ThemodelsincludesPYTHIAtuneAandHERWIG.Theeventsinwhichthereisaleadingcalorimeterjet(MidPointR=0:7)intheregionjj<2andtherearenorestrictionsplacedonthesecondandthirdhighestpTjets(jet#2andjet#3)arereferredtoasleadingjetevents.TheconclusionwasthatPYTHIAtuneAdoesnothavequiteenoughactivityinthetransverseregion.IthoweverdoesamuchbetterjobthanHERWIG,whichproducesapTdistributionofchargedparticlesthatistoo`soft'.Forallthedensities(number,pTsum)PYTHIAtuneAislow,butinallthecasesitagrees,ifwetakethetransDIF(i.e.transMAXtransMIN).ThisindicatesthattheexcessactivityseeninthedataoverPYTHIAtuneAarisesfromthesoftcomponentoftheunderlyingevent(i.e.beam-beamremnantsand/ormultiplepartoninteractions)thatcontributesequallytobothtransMAXandtransMIN.Wewouldcompareourresultswiththeleadingjetunderlyingeventresults. 73

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Illustrationofdataattheparticlelevelandatthedetectorlevel. anexampleforchargedparticledensityintransverseregioninFigure7-2.Therestoftheplotsarearrivedatsimilarly. 74

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Stepbystepdescriptionofhowthedataarecorrectedbacktoparticlelevel,fortransverseregionchargedparticledensity,asanexample.Therstrowshowsuncorrecteddata,detectorlevelMonteCarloandparticlelevelMonteCarlo.ThesecondrowshowsthecorrectionfactorobtainedbydividingtheparticlelevelMonteCarlobydetectorlevelMonteCarlo.Thethirdrowshowsthedatacorrectedbacktoparticlelevelbymultiplyingthecorrectionfactorobtainedinthepreviousstep.Theleftsideisforelectrondataandrightsideisformuondata. 75

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Systematicuncertainties ElectrondataMuondata Binbybindierencebetweenthecorrecteddatafortight-looseelectronpairwithtighttrackcutandthecorrecteddatafortight-looseelectronpairwithloosetrackcutBinbybindierencebetweenthecorrecteddatawithtighttrackcutandthecorrecteddatawithloosetrackcut Binbybindierencebetweenthecorrecteddatafortight-looseelectronpairwithtighttrackcutandthecorrecteddatawithtight-tightelectronpairandtighttrackcut Figure7-3. Showstheoriginofsystematicuncertaintiesinuncorrecteddata,fortransverseregionchargedparticledensity,asanexample.Theleftsideisforelectrondataandrightsideisformuondata. 7.4.1UnderlyingEventObservablesWepresenttheresultsontheunderlyingeventobservablesintheeventswiththeleptonpairinvariantmassintheZ-bosonregion,i.e.70-110GeV/c2,withchargedparticleshavingpT>0.5GeV/candjj<1.Wehavecombinedourelectronandmuon 76

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Showstheoriginofsystematicuncertaintiesincorrecteddata,fortransverseregionchargedparticledensity,asanexample.Theleftsideisforelectrondataandrightsideisformuondata. results.WepresentresultsforleptonpairpT<100GeV=c,abovewhichwedonothaveenoughstatistics.Whenllingthehistogram,alltheeventsineachbinareaveragedover.Figures7-5,7-6and7-7showthedataonthedensityofchargedparticlesforthetransverse,towardandtheawayregionsfortheZ-bosonevents,respectively.ThedensitiesareplottedasafunctionofthepT(Z).ThedataarecorrectedtotheparticlelevelandcomparedwithPYTHIAtuneAW(thesolidblackline)andHERWIG,withoutMPIaddedthroughJIMMY(thedottedblueline),attheparticlelevel.ThetowardregioncorrecteddataarealsocomparedwithHERWIGwithMPIaddedthroughJIMMY(thedashedgreenline).InFigures7-8wedividethetransverseregionintotransMAX(red)andtransMIN(blue).InFigure7-9,weshowthetransDIF(i.e.transMAXtransMIN)result.Figures7-10,7-11and7-12showthedataonthescalarpTsumdensityforthetransverse,towardandtheawayregionsfortheZ-bosonevents,respectively.ThedensitiesareplottedasafunctionofthepT(Z).Thedataarecorrectedtotheparticleleveland 77

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70 ].Figure7-33,7-34,7-35and7-36showrespectivelythetransverseregionchargedparticledensity,scalarpTsumdensity,averagechargedparticlepTandtheaveragemaximumchargedparticlepTforDrell-Yandata(theblackdatapoints)andPYTHIAtuneAW(thesolidblackline)predictionscomparedwiththeleadingjetdata(thebluedatapoints)andPYTHIAtuneA(thebrokenblueline)predictions.ForlargepT(jet#1)thetransversedensitiesaresimilarforleadingjetandZ-bosoneventsasonewouldexpect.Iftheleadingjethasnotransversemomentumthentherearenochargedparticles,wejustgetmin-biasevents.TherearealotoflowtransversemomentumjetsandforpT(jet#1)<30GeV=ctheleadingjetisnotalwaysthejetresultingfromthehard2-to-2scattering.Thisproducesisa`bump'inthetransversedensityatlowpT. 80

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72 ],andtheninvestigatedatTevatronRun1[ 73 ].ThemeanpT(tobedistinguishedfromthemeaneventpT)isobtainedbysummingthepTofallchargedtracksinaneventthendividingbythenumberofsuchtracks,asinTable6-6.Thisisanimportantobservable.TherateofchangeofhpTiversuschargedmultiplicityisameasureoftheamountofhardversussoftprocessescontributingtocollisionsanditissensitivethemodelingofthemultiplepartoninteractions[ 75 ].Ifonlythesoftbeam-beamremnantscontributedtomin-biascollisionsthenhpTiwouldnotdependonchargedmultiplicity.Ifonehastwoprocessescontributing,onesoft(beam-beamremnants)andonehard(hard2-to-2partonpartonscattering),thendemandinglargemultiplicitywouldpreferentiallyselectthehardprocessandleadtoahighhpTi.However,weseethatwith 81

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Drell-Yantransverseregionchargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-6. Drell-Yantowardregionchargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentsPYTHIAtuneAW,HERWIGandHERWIG+JIMMYpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 83

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Drell-Yanawayregionchargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-8. Drell-YantransMAXandtransMINregionschargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 84

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Drell-YantransDIFregionchargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-10. Drell-YantransverseregionchargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 85

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Drell-YantowardregionchargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAW,HERWIGandHERWIG+JIMMYpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-12. Drell-YanawayregionchargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentsPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 86

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Drell-YantransMAXandtransMINregionchargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-14. Drell-YantransDIFregionchargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWandHERWIGpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 87

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Drell-YantransverseregionchargedpTaverage,electronandmuondatacombined(pT>0:5GeV=candjj<1).SolidlinerepresentsPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-16. Drell-YantowardregionchargedpTaverage,electronandmuondatacombined(pT>0:5GeV=candjj<1).SolidlinerepresentsPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 88

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Drell-YanawayregionchargedpTaverage,electronandmuondatacombined(pT>0:5GeV=candjj<1).SolidlinerepresentsPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-18. Drell-YantransverseregionchargedpTmaximum,electronandmuondatacombined(pT>0:5GeV=candjj<1).SolidlinerepresentsPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 89

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Drell-YantowardregionchargedpTmaximum,electronandmuondatacombined(pT>0:5GeV=candjj<1).SolidlinerepresentsPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-20. Drell-YanawayregionchargedpTmaximum,electronandmuondatacombined(pT>0:5GeV=candjj<1).SolidlinerepresentsPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty) 90

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OverlayingDrell-Yantransverseandtowardregionschargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-22. OverlayingDrell-YantransMAX,transMINandtowardregionschargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 91

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OverlayingDrell-Yanallthreeregionschargedmultiplicitydensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-24. OverlayingDrell-YantransverseandtowardregionschargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 92

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OverlayingDrell-YantransMAX,transMINandtowardregionschargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-26. OverlayingDrell-YanallthreeregionschargedpTsumdensity,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 93

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OverlayingDrell-YantransverseandtowardregionschargedpTaverage,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-28. OverlayingDrell-YanallthreeregionschargedpTaverage,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 94

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OverlayingDrell-YantransverseandtowardregionschargedpTmaximum,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-30. OverlayingDrell-YanallthreeregionschargedpTmaximum,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 95

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Drell-Yanallthreeregionschargedmultiplicitydensityaddedup,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-32. Drell-YanallthreeregionschargedpTsumdensityaddedup,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 96

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Drell-Yantransverseregionchargedmultiplicitydensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-34. Drell-YantransverseregionchargedpTsumdensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 97

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Drell-YantransverseregionchargedpTaverage,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-36. Drell-YantransverseregionchargedpTmaximum,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 98

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Drell-YantransMAXandtransMINregionschargedmultiplicitydensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-38. Drell-YantransMAXandtransMINregionschargedpTsumdensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 99

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Drell-YantransDIFregionchargedmultiplicitydensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-40. Drell-YantransDIFregionchargedpTsumdensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 100

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Drell-Yanawayregionchargedmultiplicitydensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-42. Drell-YanawayregionchargedpTsumdensity,comparedwithleadingjetresult(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAandtuneAWpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 101

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Drell-YanchargedpTaverageandchargedmultiplicitycorrelation,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAW,HERWIGandHERWIG+JIMMYpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). Figure7-44. Drell-YanpairpTaverageandchargedmultiplicitycorrelation,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneA,HERWIGandHERWIG+JIMMYpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 102

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Drell-YanchargedpTaverageandchargedmultiplicitycorrelation,withZ-pT<10GeV=c,electronandmuondatacombined(pT>0:5GeV=candjj<1).LinesrepresentPYTHIAtuneAW,HERWIGandHERWIG+JIMMYpredictionsandthedataarecorrectedbacktoparticlelevel(witherrorsthatincludeboththestatisticalerrorandthesystematicuncertainty). 103

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74 ].Wealsoseethatthetunesroughlyagreeforlow-multiplicityevents,howevernoneoftheMonteCarloeventgenerators,includingPYTHIAcouldnotreproducethefullnalstatecorrelationbefore,althoughagreatprogresshasbeendonesinceRun1.Thenaiveexpectationfromanuncorrelatedsystemofstringsdecayingtohadronswouldbethathp?ishouldbeindependentofNChg.Tomaketheaveragep?risesucientlytoagreewithTevatrondata,tuneAincorporatestrongcolorcorrelationsbetweennal-statepartonsfromdierentinteractions,choseninsuchawayastominimizetheresultingstringlength[ 75 ].PYTHIAtuneAandtuneAWdoagoodjobindescribingthedataonhp?iversusmultiplicityformin-bias[ 77 ]andZ-bosonevents,respectively,althoughagaintheagreementbetweentheoryanddataisnotperfect.IthasbeenseenthatthatthebehaviorofhpTiversusmultiplicityisremarkablysimilarformin-biaseventsandZ-bosoneventswithpT(Z)<10GeV/csuggestingthatMPIareplayinganimportantroleinboththeseprocesses.Measurementsofthesedistributions,bothatpresentandfuturecolliders,wouldthereforeaddanotherhighlyinterestingandcomplementarypieceofinformationonthephysicsprocesses. 105

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76 ],wecanseethattheunderlyingeventismuchmoreactiveattheLHC.AsshowninFigure.8.1,PYTHIAtuneDWTpredictaboutafactoroftwoincreaseinchargedparticledensityingoingfromtheTevatrontotheLHCinthetowardregion.ForHERWIG(withoutMPI)thetowardregionofZ-bosonproductiondoesnotchangemuchingoingfromtheTevatrontotheLHC. Figure8-1. ExtrapolatingchargedparticledensitytoLHCenergies,10TeVand14TeVandtheyarecomparedwithTevatrondata. However,comparingtheunderlyingeventpredictionsfortheLHCgeneratedbymodels,tunedtotheavailabledata,dramaticdisagreementsintheirpredictionsatLHCenergieshasbeenobserved[ 75 ].Thattellsusthatimprovedmodelsforthesoftcomponentofhadroniccollisionsareneeded.Futurestudiesshouldfocusontuningtheenergydependencefortheeventactivityinbothminimumbiasandtheunderlyingevent,whichatthemomentseemstobeoneoftheleastunderstoodaspectsofallthemodels.Goingbeyondtheunderlyingeventstudies,animportantrststepinLHCwouldbeto`rediscover'thestandardmodel,Drell-Yanprocessisoneofthecleanestsignatureforthat.TheprocedurefollowedusesapartoftheDrell-Yanmassspectrumwherenoevidenceofnewphysicsisexpectedtobeobservedasacontrolregion.Thisisused 107

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TableA-1. Chargedmultiplicitydensity,PYTHIAtuneAW(againstlepton-pairpT) BinTransverseTowardAway GeV=c 109

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Chargedmultiplicitydensitydata(againstlepton-pairpT) BinTransverseTowardAwayValueErrorValueErrorValueError GeV/c 2.50.4298840.0381570.3944970.0368940.4868820.0337127.50.4645330.0374870.4108790.0409250.6649620.03020112.50.4967810.0359230.4189550.0413760.8511130.02781117.50.5162050.0411020.4376910.0387581.0228720.02765522.50.5441450.0395040.4431630.0427261.1674220.03357427.50.5785530.0404710.4619980.0380991.3109170.03223932.50.5763290.0394920.4772000.0498881.4271070.04202637.50.5827380.0465510.4748450.0409701.5372290.04810442.50.6118540.0441200.5030030.0391151.5987800.05469347.50.5955560.0534810.4889120.0639721.7395240.06510052.50.6577540.0663250.5319480.0776301.7810170.07403757.50.6182620.0589640.4978310.0549621.8776470.08722562.50.6968170.0710290.5634990.0708582.0224020.09789367.50.6845870.1104650.5190120.0964352.0590670.11277072.50.6259260.0913690.4609390.1260922.1217360.13160477.50.7242130.0827030.5391870.0870762.3131250.15327582.50.7310660.1682240.5895330.0997742.3462080.18823487.50.6271640.0981950.5724040.0875892.2236880.19862192.50.6326670.1081490.6220340.1169742.3919680.23121997.50.7149890.1216840.4701670.1600052.6747080.259181 110

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ChargedpTsumdensity,PYTHIAtuneAW(againstlepton-pairpT) BinTransverseTowardAway GeV=cGeV=cGeV=cGeV=c 111

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ChargedpTsumdensitydata(againstlepton-pairpT) BinTransverseTowardAwayValueErrorValueErrorValueError GeV=cGeV=cGeV=cGeV=cGeV=cGeV=cGeV=c 112

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TransMAXandtransMINchargedmultiplicitydensityandchargedpTsumdensity,PYTHIAtuneAW(againstlepton-pairpT) BinTransMAXNChgTrasMINNChgTrasMAXpTsumTrasMINpTsum GeV=cGeV=cGeV=c 113

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TransMAXandtransMINchargedmultiplicitydensityandchargedpTsumdensity,data(againstlepton-pairpT) BinTransMAXNChgTransMINNChgTransMAXpTsumTransMINpTsumValueErrorValueErrorValueErrorValueError GeV=cGeV=cGeV=cGeV=cGeV=c 114

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ChargedpTaverage,PYTHIAtuneAW(againstlepton-pairpT) BinTransverseTowardAway GeV=cGeV=cGeV=cGeV=c 115

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ChargedpTaveragedata(againstlepton-pairpT) BinTransverseTowardAwayValueErrorValueErrorValueError GeV=cGeV=cGeV=cGeV=cGeV=cGeV=cGeV=c 116

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ChargedpTmaximum,PYTHIAtuneAW(againstlepton-pairpT) BinTransverseTowardAway GeV=cGeV=cGeV=cGeV=c 117

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ChargedpTmaximumdata(againstlepton-pairpT) BinTransverseTowardAwayValueErrorValueErrorValueError GeV=cGeV=cGeV=cGeV=cGeV=cGeV=cGeV=c 118

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Correlation,PYTHIAtuneAW(againstchargedmultiplicity) BinChargedpTaverageZ-bosonpTChargedpTaverage(Z-pT<10GeV/c)(Z-bosonpT<10GeV/c) GeV=cGeV=cGeV=c 119

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Correlations,data(againstchargedmultiplicity) BinChargedpTaverageZ-bosonpTChargedpTaverage(Z-bosonpT<10GeV/c)ValueErrorValueErrorValueError GeV=cGeV=cGeV=cGeV=cGeV=cGeV=c 120

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DeepakKarwasborninSilverspring,MDin1979.HereturnedtoIndiawithhisparents,NikhileshandRamalaKarsoonafterthat,growingupalongwithhisyoungersisterNandiniinthequietlittletownofSiliguri,onthefoothillsoftheHimalayas.Aftercompletinghighschoolthere,hemovedtoCalcutta(nowknownasKolkata),oneofthemajorIndianmetropolisestonishbothhisthree-yearB.Sc(in2001)andtwo-yearM.Sc(in2003),fromJadavpurUniversity,majoringinPhysics.Hisinterestinthisbranchofsciencepartlygrewfromhisfather,aprofessorofPhysicshimselfandfurtherdevelopedundertheexcellentteachersatJadavpur.Besidecompletinghiscoursework,DeepakworkedatHarishchandraResearchInstitute(HRI),Allahabad,in2001,atSahaInstituteofNuclearPhysics(SINP),Kolkata,in2002andattheIndianAssociationfortheCultivationofSciences(IACS)in2003,asasummerresearchprojectstudent.HewasoneoftherecipientsofthehighlycompetitiveSummerResearchFellowshipofIndianAcademyofSciencesin2001,andinthesameyearwasalsoawardedaGoldMedalforbeingplacedinthenationaltopveintheNationalGraduatePhysicsExaminationorganizedbyIndianAssociationofPhysicsTeachers.DeepakjoinedtheUniversityofFlorida,Gainesvilleforhisgraduatestudyinthefallof2003leadingtoadoctoraldegree.Apartfromhisresearchwork,heworkedasateachingassistant,wherehisresponsibilitiesincludedteachinglabclassesinmechanicsandelectricity-magnetismatthebeginningundergraduatelevelwithfullresponsibilityovergrades.HewasaparticipantinProspectsinTheoreticalPhysicsSummerSchool'TheStandardModelandBeyond'heldinJuly2007attheInstituteofAdvancedStudies,PrincetonandthesecondandthirdCERN-FNALHadronColliderPhysicsSummerSchoolheldrespectivelyatGeneva,SwitzerlandandatFermilabduringthesummerof2007and2008.Hisresearchwasfocusedonbettermodelingofthesocalledunderlyingeventsathadroncolliders,whichisanunavoidablebackgroundtoallthecolliderobservables.HepresentedhisresearchatAmericanPhysicalSociety's(APS)annualmeetingatSt.Louis, 127

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