The underlying event in hard scattering collisions of proton and antiproton at 1.8 TEV

240 Pages·2001·9 MB·English

by  Haas, Richard Martin

THEUNDERLYINGEVENTINHARDSCATTERINGCOLLISIONS
OFPROTONANDANTIPROTONAT1.8TEV
By
RICHARDMARTINHAAS
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2001
ACKNOWLEDGMENTS
Itisdifficultformetoenvisionreachingthisstagewithoutthesupportofseveral
people. Whetheritwasthelate-nightconversations,thewordsofencouragement,
orthefriendshipshownbysomany,Iappreciatedtheexperiences. Inparticular,
thereareafewindividualswhohaveplayedsignificantrolesandwhomIwishto
expresslythank.
RickFieldprovidedinvaluableadviceandguidanceoverthecourseofmygrad-
uatecareerattheUniversityofFlorida. Hecreatedanenvironment inwhichI
wasencouragedtopursuevarioustopicswhilealwayshavingrecoursetohiscoun-
sel. Instead ofproducing anautomaton, thissettingyielded aneducated and
enthusiasticscientistcapableofindependentthought. Rick’sroleasamentorand
good-naturedwillingnesstofurthermydevelopmentcontributedstronglytomy
progress. Hiswillingnesstoofferhonestwordsandopinionsconcerningthesig-
nificantopportunitiesavailabletomeinevitablyservedtoaidincastingthebest
possibledecisions. Bothfriendandadvisor,hehastaughtmemuchoverthelast
fewyears.
OriginallyIbegangraduateschoolwiththeintentofconcentratingontheoreti-
calcosmology.Whilemyowninterestschangedtoparticlephysics,hadIcontinued
withastrophysics,IknowthatIwouldhaveenjoyedworkingforJimFry. Jimwas
oneofthefirst UniversityofFloridaprofessors Imet. Healwaysofferedgood
adviceandoccasionallyprovidedhisvaluablesignatureforgrantproposals. The
choicetofollowadifferentcourseofstudycamewithsomeremorseforIconsidered
ii
Jimtobeanexcellentphysicistandaclosefriend. Fortunately,hehasremained
afriendandanindividualwhoseopinionIseekwhenmakingdifficultdecisions. I
willmisstheultra-hotThaifoodandtheinsightfulconversations.
Asathirdyearstudent,Ihadthegoodfortunetobeintroducedtoexperimental
particlephysicsbyGuenakhMitselmakherthroughacoursehetaught. Hewould
lace his descriptions ofdetector anatomywiththe stories ofvariousscientists’
missedopportunitiesforadiscoveryortheexcitementoffindingnewphenomena
(severalstoriesofPontecorvocometomind). Heandhistalesaddedanexcitement
toparticlephysicsthatattractedseveralyoungstudentstothefield. Guenakh’s
dedication,noticeablebythemanyhoursspentinthedepartmentatnightworking,
wasnotlostonme. IknowthattheCMSendcapmuonsystemwillbesuccessful
underhisguidinghandandwishhimthebest.
MyindoctrinationintothecultureofCDFwouldhaveentailedadditionalob-
staclesifnotforAndreiNomerotski. Hepatientlytaughtmetheintricaciesofthe
Run1 analysissoftwareintheunfortunatelynecessarytribalnatureoflearning
howtousetheprograms. Oneofthemostinsightfulandintuitiveexperimentalists
Iknow,heillustratedthatsimpleargumentsusingreasonableassumptionsoften
servethesamepurposeandleadtoabetterunderstandingofthephysicsthanthe
derivationofarbitrarilycomplexsetsofequations. IthankAndreiforenriching
myunderstandingofexperimentalparticlephysics.
Severalfriendshaveprovidedfondmemoriesduringmytimeasagraduatestu-
dent. Whileitseemsunfairtomentiononlycertainindividuals,nomaliceismeant
inexplicitlythankingthemandnotothers. BrianBakerinsuredthatIsawocca-
sionalsunlightwithroundsofgolfinthespringandassistedwithvariousbusiness
initiativesthatallowedforalifeoutsideofphysics. ThemonthlyspaghettiWest-
ernrenaissancefestatMikeJones’placeprovidedneededbreaks. Aftermoving
west,movienightwasreplacedwiththebiannualpilgrimagetoLoard’s. Evenings
attheFermilabUser’sCenterwithAlexeiSafonovalwaysprovedinteresting. My
experiencewiththeintermediatesiliconlayersfortheRunIIupgradewouldhave
beenlessfulfillingifnotforTonyAffolder,ChrisHill,andformerCDFgodparent
JoelGoldstein. ConferencesattheStanfordLinearAcceleratorCenterprovided
opportunitiestovisitCareyHeckman. TheweekendhikesinthePaloAltofoothills
andmemorableconversationswerealoneworththetrips.
Withoutthesupportandencouragementofbothmymotherandfather,com-
pletingmygraduatestudieswouldhavebeenmuchmoredifficultifnotimpossible.
IcherishedtheSundayphonecallsandexcursionstoFermilab. Icanonlyhope
thatmyachievementshavemadethemproud.
Sisu,thefluffiestcat,wasmycompanionthroughoutmygraduateexperience
inFlorida. Herneedsofplayingwitharibbon, awarmlaptositon, andme
scratchingherheadconstitutedtherapeuticrelaxationafterlongdaysandnights
inthedepartment.
WordsserveaspoormessengerstoexpressthefeelingsIhaveforlisaWebeck.
Herloveandencouragementhavebeenpricelessduringtheseyears.
IV
TABLEOFCONTENTS
ACKNOWLEDGMENTS
ii
LISTOFTABLES viii
LISTOFFIGURES x
ABSTRACT xxii
CHAPTERS
1 PARTICLEPHYSICS 1
1.1 HistoricalPrelude 2
1.2 ParticlesandForces 8
1.3 ParticleTheories 12
2 STANDARDMODELDYNAMICS 17
2.1 QuantumChromodynamics 17
2.1.1 AsymptoticFreedom 23
2.1.2 PartonDensities 31
2.1.3 QCDQuestions 37
2.2 HadronCollisions 38
2.3 ModelingCollisionEvents 42
2.3.1 Monte-CarloModels 42
2.3.1.1 HERWIG 45
2.3.1.2 ISAJET 47
2.3.1.3 PYTHIA 51
3 THETEVATRONANDCDF 54
3.1 TheFermilabAccelerators 55
3.2 ColliderDetectoratFermilab 62
3.2.1 SiliconVertexDetector 70
3.2.2 VertexTrackingChamber 74
3.2.3 CentralTrackingChamber 76
3.2.4 Calorimeters 81
3.2.5 MuonChambers 84
v
3.2.6 BeamBeamCounters 85
3.2.7 Triggers 88
4 DATACONSTRUCTIONANDREDUCTION 94
4.1 Min-BiasData 95
4.2 Jet20Data 97
4.3 ZBosonData 99
4.4 J/-0BData 104
4.4.1 J/?/>Selection 107
4.4.2 EventSelection 109
5 DATAANALYSISTECHNIQUES 115
5.1 JetDefinitions 117
5.2 A(pCorrelations 118
5.2.1 AngularCorrelationsinMin-BiasandJet20Data 121
5.2.2 AngularCorrelationsinZData 128
5.2.3 AngularCorrelationsinJ/ipBData 133
5.3 Systematics 142
6 EVENTSTRUCTURE 143
6.1 EventStructureinDijetData 143
6.1.1 TheDijetTransverseRegion: TheoryandData 149
6.1.1.1 SourcesofChargedParticleMultiplicity 154
6.2 EventStructureinZData 159
6.2.1 TheZTransverseRegion: TheoryandData 164
6.3 EventStructureinJ/ijjBData 170
6.3.1 EventStructureusingtheLeadingJet 171
6.3.1.1 ComparisonofJ/ipandDijetData 175
6.3.2 EventStructureusingtheJ/i\) 180
6.3.2.1 ComparisonofJ/^jjandZData 185
7 CONCLUSION 192
7.1 TheoreticalComparisons 192
7.2 TheUnderlyingEvent 194
7.3 FutureEndeavors 195
APPENDICES
A NOTATION 197
A.l Metric,FourVectors,andOperators 197
A.2 Levi-CivitaTensor 198
vi
B MATRIXPROPERTIES 199
B.l GammaMatrices 199
B.2 Gell-MannMatrices 202
C DATASYSTEMATICS 205
REFERENCES 209
BIOGRAPHICALSKETCH 215
vii
LISTOFTABLES
1.1 ThefundamentalfermionsintheStandardModel 9
1.2 StandardModelgaugebosonpropertiesandmediatedforces 11
3.1 CDFdetectorsystems. Thesystemsarecomposedofseveralele-
mentswhicharedenotedbyvariousabbreviations 69
3.2 CharacteristicsoftheCDFcalorimeters. Segmentationcorresponds
tothe tower size in At?xA(f> and energy resolution is given by
oe/\[EwithenergymeasuredinGeV.Thesymbol®intheenergy
resolutiondenotesadditionoftheconstantterminquadrature. . . 84
3.3 CDFLevel1,2,and3triggeracceptanceratesataninstantaneous
luminosityof5x1030cm-2sec-1 89
4.1 TheMin-Biasdatasampleusedintheanalysisoftheunderlying
event 96
4.2 TrackselectioncriteriafortheMin-Biasdatasample 97
4.3 Level 1, Level 2, and combined prescaling factors for the Jet20,
Jet50,Jet70,andJetlOOdatasamples 98
4.4 TheJet20datasampleusedintheanalysisoftheunderlyingevent. 98
4.5 Selectioncriteriafortheinclusiveelectrondatasample 100
4.6 SelectioncriteriaforthelooseZdatasample 101
4.7 TrackselectioncriteriafortheZdatasample 104
4.8 Numberofeventssatisfyingtheselectioncriteriausedtocreatethe
Zsample 104
4.9 CDFLevel2dimuontriggers 108
4.10 SelectioncriteriawhichformadatasamplecontainingJ/t/jcandi-
datespredominatelyfromBdecay 113
viii
4.11 SelectioncriteriatocreateadatasamplecontainingJ/ijjcandidates
withhighefficiencyintheCTCandcharacteristicsappropriateto
theunderlyingeventanalysis 114
7.1 Averagechargedmultiplicityandscalarprsuminthetransverse
regionsforthedatasamplesusedinthisanalysis 194
IX
LISTOFFIGURES
2.1 Feynmandiagramforthetreelevelquark-gluonvertex 30
2.2 Feynmandiagramsfortreelevelqqproductionfromqqandgluon
pairs 31
2.3 Illustrationofappcollisioninwhicha“hard”2-to-2partonscatter-
ingwithtransversemomentum,p^ard,hasoccurred. Theresulting
“dijet”eventcontainsparticlesthatoriginatefromthetwooutgoing
partons(plusfinal-stateradiation)andparticlesthatcomefromthe
breakupoftheprotonandantiproton(i.e. “beam-beamremnants”). 40
2.4 Illustrationofappcollisioninwhichmultiplepartoninteractions
haveoccurred. Inadditiontothe “hard” 2-to-2partonscattering
withtransversemomentump)?rd,thereisanadditional“semi-hard”
parton-partonscatteringthatcontributesparticlestotheunderlying
event. ForPYTHIA, weincludethecontributionsfrommultiple
partonscatteringinthebeam-beamremnantcomponent 40
2.5 IllustrationofappcollisioninwhichaZbosonwithlargetransverse
momentum,pr{Z),hasbeenproduced. Theresultingeventcontains
particlesthatoriginatefromthe“away-side”outgoingparton(plus
final-stateradiation)andparticlesthatcomefromthebreakupof
theprotonandantiproton(i.e. “beam-beamremnants”) 41
3.1 TheensembleofacceleratorsusedforRunIoftheTevatron. The
experimentbeginsintheCockroft-Waltongenerator(notshownin
thefigure)locatedbeforethelinearaccelerator. TheMainRingand
Tevatronphysicallysharethesametunnel 56
3.2 Mechanismtoproduceantiprotons. Protonscollidewithatarget
whichgivesrisetoabroadspectrumofsecondaryparticles. The
secondaryparticlesarefocusedbyalithiumlens. Apulseddipole
magnet selects antiprotons, while the remainingparticles are di-
rectedtowardsabeamdump 59
X
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