Electron Scattering from an Almost Free Neutron in - Jefferson Lab

213 Pages·2009·9.34 MB·English

Electron Scattering from an Almost Free Neutron in Deuterium

Nathan Kidd Baillie

Burke, Virginia

B.S. Physics, Mary Washington College, 2002
M.S. Physics, The College of William and Mary, 2004

A Dissertation presented to the Graduate Faculty
of the College of William and Mary in Candidacy for the Degree of
Doctor of Philosophy

Department of Physics

The College of William and Mary
January 2010
(cid:13)c CopyrightbyNathanKiddBaillie2010
AllRightsReserved
APPROVAL PAGE

This Dissertation is submitted in partial fulfillment of
the requirements for the degree of

Doctor of Philosophy

Nathan Kidd Baillie

Approved by the Committee, July, 2009

Committee Chair
Professor Keith Griffioen, Physics
The College of William and Mary

Professor David Armstrong, Physics
The College of William and Mary

Professor Carl Carlson, Physics
The College of William and Mary

Dr. Howard Fenker
Jefferson Laboratory

Professor Sebastian Kuhn, Physics
Old Dominion University
ABSTRACT PAGE

The Barely Off-Shell Nucleon Structure (BoNuS) experiment measured electron scattering
from neutrons bound in deuterium nuclei at Jefferson Lab’s Hall B with the intent of
obtaining the ratio F n / F p at high Bjorken x. The F n structure function is difficult to obtain
2 2 2
due to nature’s lack of a free neutron target. Previous experiments have measured
inclusive scattering from atomic nuclei, but extracting F n from these data requires
2
knowledge of inclusive scattering from the proton and corrections for nucleon binding and
Fermi motion. In BoNuS we restrict our analysis to neutron scattering events tagged by a
backward-going low-energy recoil proton. This selects loosely bound neutrons that are
nearly on their mass-shell and have few final state interactions with the recoil proton. The
recoil protons were detected in a Radial Time Projection Chamber (RTPC) using gas
electron multipliers (GEMs) in a cylindrical geometry around the target. The production
data were taken at three beam energies: 2.140 GeV, 4.223 GeV and 5.262 GeV with an
additional set of calibration data taken at 1.099 GeV. The kinematic coverage includes the
invariant mass, W, from the quasi-elastic peak up to 3.0 GeV and momentum transfer, Q2,
from 0.2 GeV2 up to approximately 3.0 GeV2. The final experimental observations
presented here are the tagged / untagged cross section ratios, corrected for a number of
physical processes and backgrounds. The ratio enables us to ignore many complications
arising from a typical absolute cross section measurement. An extraction of F n / F p for x
2 2
up to 0.75 is made and compared with previous data. These data are important because
they can provide a model independent measurement of neutron structure.
DEDICATION
Thisdocumentisdedicatedtomytwofavoritepeople,mylittlebrotherJoelandmysweetwife
Karen. Theyinspireanddelightmeeverydayandmylifehasbeensupremelyenrichedbytheir
presence. IamsothankfulforyouandIloveyoubothaheartfull.
i
Contents
ACKNOWLEDGEMENTS iv
LISTOFFIGURES vi
LISTOFTABLES x
1 INTRODUCTION 1
2 FORMALISM 4
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 KinematicsofElectronScattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 DeepInelasticScattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 TheQuark-PartonModel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5 QuantumChromodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.6 NucleonStructureFunctionRatioFn/Fp . . . . . . . . . . . . . . . . . . . . . . 12
2 2
2.7 BoundNucleonStructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.7.1 NuclearEffects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.7.2 SpectatorTagging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.8 Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.8.1 TargetFragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.8.2 Off-shellCorrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.8.3 FinalStateInteractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.9 AnalysisObjectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3 PRE-EXISTINGEXPERIMENTALAPPARATUS 27
3.1 ContinuousElectronBeamAcceleratorFacility . . . . . . . . . . . . . . . . . . . 27
3.2 CEBAFLargeAcceptanceSpectrometer. . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 TorusMagnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2.2 DriftChambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2.3 CherenkovCounters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2.4 ScintillationCounters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.5 ElectromagneticCalorimeter . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3 SolenoidMagnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 NEWEXPERIMENTALAPPARATUS 48
4.1 RadialTimeProjectionChamber . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1.1 DesignMotivationsandConstraints . . . . . . . . . . . . . . . . . . . . . 48
4.1.2 GasElectronMultipliers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.1.3 Prototypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.1.4 ProductionChamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.1.5 HighVoltageDividerCircuit . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.1.6 AnalogandReadoutElectronics . . . . . . . . . . . . . . . . . . . . . . . 63
4.2 TargetStraw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
ii
5 DATAPROCESSING 71
5.1 RTPCTrackReconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1.1 DeterminingLocalE~ andB~ . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.1.2 MAGBOLTZ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.1.3 DriftElectronPaths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.1.4 OffsetCorrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.1.5 ReconstructingHits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.1.6 ChainLinking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.1.7 HelixFitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.1.8 TrackParameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.2 RTPCCalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.2.1 ElectronDriftVelocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.2.2 ChannelbyChannelGainParameters . . . . . . . . . . . . . . . . . . . . 109
5.3 DataFileSelection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
5.3.1 ElectronsperChargeMeasured . . . . . . . . . . . . . . . . . . . . . . . 119
5.3.2 HighMass(A>1)BackgroundintheRTPC . . . . . . . . . . . . . . . . 119
5.4 ElectronIdentificationinCLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.4.1 CherenkovDetectorFiducialCuts . . . . . . . . . . . . . . . . . . . . . . 122
5.4.2 ElectromagneticCalorimeterCuts . . . . . . . . . . . . . . . . . . . . . . 127
5.4.3 Solenoidθ−z Cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.5 RTPCProtonIdentification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.5.1 TimingCuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.5.2 VertexCutandBackgroundSubtraction . . . . . . . . . . . . . . . . . . . 136
5.5.3 dQ/dxvs. p/z Cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.5.4 OtherRTPCTrackQualityCuts . . . . . . . . . . . . . . . . . . . . . . . . 140
5.6 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
5.6.1 ElectronMomentum,Angle,andVertex . . . . . . . . . . . . . . . . . . . 141
5.6.2 RTPCSimulationandProtonEnergyLossCorrection . . . . . . . . . . . 146
5.6.3 RadiativeCorrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
5.6.4 AcceptanceandEfficiency . . . . . . . . . . . . . . . . . . . . . . . . . . 157
5.6.5 PionandChargeSymmetricBackgroundContamination . . . . . . . . . . 161
6 RESULTSANDCONCLUSIONS 167
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
6.2 StructureFunctionRatioExtraction . . . . . . . . . . . . . . . . . . . . . . . . . . 167
6.3 ErrorEstimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
6.4 Ratios(Fn/Fd,Fn/Fp,d/u)andFn . . . . . . . . . . . . . . . . . . . . . . . . . 170
2 2 2 2 2
6.5 ConclusionandSummary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
BIBLIOGRAPHY 194
iii
ACKNOWLEDGEMENTS
Tobegin,Ihumblythankandappreciatethesteadfastsupportandguidanceofmypatientand
wiseadvisor,KeithGriffioen. Hisadviceandexpertiseplayedakeyroleinmysuccessthrough-
outmygraduatecareer. HefirstgavemetheopportunitytoworkatSLACforasummer,kindly
funded my attendance of many enriching conferences, and patiently guided me through the
rough waters of this research project. There were many days when his even-temper provided
me with reassurance during stressful times and I consider myself very lucky to have had him
alongforthisride.
I would like to especially acknowledge Howard Fenker for the role that he played as my
mentor for the many years that we have been involved with the BoNuS experiment. From that
first day in the summer of 2003 setting up the “clean room” lab, to our long days of detector
development, to his currently ongoing guidance and extremely helpful suggestions during the
analysis phase. I am certainly grateful for his willingness to share his expertise through every
stage. Itishasbecomemyprofessionalgoaltoonedaybeadetectorexpertnearthelevelthat
hehasattained.
My fellow BoNuS graduate students deserve my utmost thanks. Thank you Jixie Zhang,
Slava Tkachenko, and Narbe Kalantarians for the valuable work that you performed during
these many years. I certainly would not have been able to do this without you guys and I
appreciateyourconstantefforts.
TherearemanyotherpeopleIwouldliketothankfortheirgracioussupportandhelpfulness
in one way or another: Stephen Bueltmann, Sebastian Kuhn, Thia Keppel, Eric Christy, Peter
Bosted, VladasTvaskis, DipangkarDutta, ChandanaJayalath, SuchetaJawalker, Gabrieland
Ioana Nicuelescu, Michael Ispiriyan, Jaison Novick, Rob Fersch, Cornel Butuceanu, Stepan
Stepanyan,GailDodge,RolfEnt,WallyMelnitchouk,JulieRoche,MarcVanderhaegen,Steve
Christo, PeterBradshaw, JoshEvans, TheHall-Bengineeringcrew(especiallyJillGram), and
JohnDomingo.
I want to thank my dissertation committee: Carl Carlson (W&M), David Armstrong (W&M),
Sebastian Kuhn (ODU), Howard Fenker (Jefferson Lab), and Keith Griffioen (W&M) for the
crucialrolethatyou’veplayedinthisprocess.
iv
I’d like to thank my undergraduate physics professors at Mary Washington College, Bu-
lent Atalay and George King, III. They helped spark my interest in physics and gave me the
foundationformygraduateexperience.
I must express gratitude to my entire supportive family and my parents, Larry and Becky
Baillie,especially. Icouldnotaskforbetterrolemodelsorsincerefriends. Youhavebeenthere
formeeverystepofthewayandIwillalwaysrememberit.
My wife Karen deserves a massive amount of credit for the number of ways that she con-
tributed to this endeavor. She has always provided me with a great deal of perspective and
triedtoinstillinmea“can-do”attitudewhenthingsseemedbleak. Notonlydidshehandleall
aspects of our day-to-day lives so that I could focus exclusively on finishing graduate school,
butasafellowphysicist,shewasabletogivemevaluablecommentsandobservationsasshe
helped to edit the entire thesis. She is an amazing woman with a huge heart and the value of
hersupporttomecannotbeoverstated.
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List of Figures
2.1 Diagramofinclusivelepton-nucleonscattering . . . . . . . . . . . . . . . . . . . 5
2.2 AcompilationofFp datafromthePDG . . . . . . . . . . . . . . . . . . . . . . . 9
2
2.3 Quarkdistributionsversusx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 ASLACmeasurementofFn/Fp . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2 2
2.5 AschematicoftheEMCeffect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.6 AcalculationofTargetFragmentationeffects . . . . . . . . . . . . . . . . . . . . 20
2.7 Comparingboundandfreeneutronstructurefunctions . . . . . . . . . . . . . . . 22
2.8 Comparingboundandfreeneutronstructurefunctions . . . . . . . . . . . . . . . 23
2.9 Comparingboundandfreeneutronstructurefunctions . . . . . . . . . . . . . . . 24
2.10 Theeffectoffinalstateinteractionsonthespectralfunction . . . . . . . . . . . . 25
2.11 Theeffectoffinalstateinteractionsonthespectralfunction . . . . . . . . . . . . 26
3.1 AschematicofCEBAF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 AschematicoftheCLASdetector . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3 Aparallelcross-sectionofCLAS . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.4 Aperpendicularcross-sectionofCLAS. . . . . . . . . . . . . . . . . . . . . . . . 30
3.5 Aschematicofthemagneticfieldofthetorus . . . . . . . . . . . . . . . . . . . . 30
3.6 Themagneticfieldvectorsofthetorus . . . . . . . . . . . . . . . . . . . . . . . . 32
3.7 TheCLASdriftchambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.8 TrackingintheCLASdriftchambers . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.9 Resultofthefinaldriftchambercalibration . . . . . . . . . . . . . . . . . . . . . . 35
3.10 Resultofthefinaldriftchambercalibration . . . . . . . . . . . . . . . . . . . . . . 36
3.11 Driftchamberresidualspre-alignment . . . . . . . . . . . . . . . . . . . . . . . . 36
3.12 Driftchamberresidualspost-alignment . . . . . . . . . . . . . . . . . . . . . . . . 37
3.13 TheCLASCherenkovCounter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.14 TheCLASScintillationCounter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.15 Time-of-flightresolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.16 Time-of-flightresolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.17 TheCLASElectromagneticCalorimeter . . . . . . . . . . . . . . . . . . . . . . . 41
3.18 PositionreconstructionintheEC . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.19 TimingcalibrationintheRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.20 Aschematicofthesolenoidmagnet . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.21 TheMøllerelectronsimulationshowingtheθ coverage. . . . . . . . . . . . . . . 45
3.22 Møllerelectrondistributioninther vs.z plane . . . . . . . . . . . . . . . . . . . . 45
3.23 Thesolenoidfieldstrengthversusz . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1 AschematicdiagramofaTPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.2 SamplingofspacepointsinaTPC . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3 AtypicalGEMfoilimagedwithanelectronmicroscope. . . . . . . . . . . . . . . 52
4.4 ThetypicalenergyresolutionofaGEM-basedTPC. . . . . . . . . . . . . . . . . 53
4.5 AschematicrepresentationofeachlayerinastandardtripleGEMdetector. . . . 53
4.6 TheeffectivegainvsthevoltageacrosstheGEMfoils. . . . . . . . . . . . . . . . 54
4.7 AnelectricfieldmapsurroundingaGEMfoil. . . . . . . . . . . . . . . . . . . . . 55
4.8 AphotographofacustomGEMfoil. . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.9 AcrosssectionalviewoftheflatprototypeTripleGEMTPC . . . . . . . . . . . . 56
4.10 Explodeddiagramofthe105degreeprototype . . . . . . . . . . . . . . . . . . . 57
4.11 FlatandCurvedGEMx-rayenergydistributions. . . . . . . . . . . . . . . . . . . 58
vi