Exclusive Backward-Angle Omega Meson Electroproduction A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfilment of the Requirements for the Degree of Doctor of Philosophy in Physics University of Regina By Wenliang Li Regina, Saskatchewan October, 2017 c 2017: Wenliang Li ° UNIVERSITY OF REGINA FACULTY OF GRADUATE STUDIES AND RESEARCH SUPERVISORY AND EXAMINING COMMITTEE Wenliang Li, candidate for the degree of Doctor of Philosohy in Physics, has presented a thesis titled, Exclusive Backward-Angle Omega Meson Electroproduction, in an oral examination held on October 20, 2017. The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: *Dr. Richard Jones, University of Connecticut Supervisor: Dr. Garth Huber, Department of Physics Committee-Member: *Dr. David Hornidge, Adjunct Committee Member: Dr. Zisis Papandreou, Department of Physics Committee Member: Dr. George Lolos, Adjunct Committee Member: **Dr. Allan East, Department of Chemistry & Biochemistry Chair of Defense: Dr. Karen Meagher, Faculty of Graduate Studies & Research *Participated via video conference **Not present at defense Abstract Exclusive meson electroproduction at different squared four-momenta of the exchanged virtual photon, Q2, and at different four-momentum transfers, t and u, can be used to probe QCD’s transition from hadronic degrees of freedom at the long distance scale to quark-gluon degrees of freedom at the short distance scale. Backward-angle meson electroproduction was previously ignored, but is anticipated to offer complimentary information to conventional forward-angle meson electroproduction studies on nucleon structure. This work is a pioneering study of backward-angle ω cross sections through the exclusive 1H(e,e′p)ω reaction using the missing mass reconstruction technique. The extracted cross sec- tions are separated into the transverse (T), longitudinal (L), and LT, TT interference terms. The analyzed data were part of experiment E01-004 (Fπ-2), which used 2.6-5.2 GeV electron beams and HMS+SOS spectrometers in Jefferson Lab Hall C. The primary objective was to detect coincidence π in the forward-angle, where the backward-angle ω events were fortuitously detected. The experiment has central Q2 values of 1.60 and 2.45 GeV2, at W = 2.21 GeV. There was significant coverage in φ and ǫ, which allowed separation of σT,L,LT,TT. The data set has a unique u coverage of u 0, which corresponds to t> 4 GeV2. − ∼ − The separated σ result suggest a flat 1/Q1.33±1.21 dependence, whereas σ seems to hold T ∼ L 9.43±6.28 2 2 a stronger 1/Q dependence. The σL/σT ratio indicate σT dominance at Q = 2.45 GeV at the 90% confidence level. ∼ After translating the results into the t space of the published CLAS data, our data show − evidence of a backward-angle ω electroproduction peak at both Q2 settings. Previously, this phenomenon showing both forward and backward-angle peaks was only observed in the meson photoproduction data. Through comparison of our σT data with the prediction of the Transition Distribution Ampli- tude (TDA) model, and signs of σT dominance, promising indications of the applicability of the TDA factorization are demonstrated at a much lower Q2 value than its preferred range of Q2 > 10 GeV2. These studies have opened a new means to study the transition of the nucleon wavefunction through backward-angle experimental observables. Acknowledgements I would like to express my sincere gratitude to my supervisor Prof. Garth Huber for encouraging me to undertake this Ph.D. project and for his continuous support through the development of this work, and for his meticulous and patient guidance. Working with him has been a deeply ed- ucational and challenging experience. I also cherish the personal bond that we manage to create along these years. I am also very grateful to Henk Blok and Dave Gaskell for their extremely valuable suggestions, comments and supports during this work. A special thanks to Tanja Horn for her great work on generating data Ntuples, this has significantly simplified the analysis. Great appreciation to Jean-Phillipe Lansberg, Bernard Pire, Krill Semenov and Lech Szy- manowski, for providing the invaluable theoretical (TDA model) calculations. Visions offered by Christian Weiss and Mark Strikman have played a critical role throughout the thesis writing. Furthermore, I want to thank the research funding provided by NSERC of Canada and the FGSR of the University of Regina. Since the beginning, Department of Physics has provided amazing supports for my education and research. I would like to thank all members of the depart- ment, include Nader Mobed, George Lolos, Zisis Papandreou, Mauricio Barbi, Pierre Ouimet, Andrei Semenov, Cheryl Risling, Derek Gervais and others. I am also grateful to all the friends and colleagues met at the university: Dilli Paudyal, Ahmed Zafar, Ahmed Foda, Sameep Basnet, Tegan Beattie, Ryan Ambrose, Rory Evans, Nathanael Hogan and others. Together, we have created a pleasant working environment and they really had to put up with my loudness. Finally I would like to dedicate this work to my parents, the Morrison family and the relatives for their kind support and hospitality throughout these years, and most importantly to my wife who cooks wonderfully. This thesis is dedicated to my family Contents Abstract Acknowledgements Dedication Table of Contents i List of Tables vii List of Figures ix 1 Introduction 1 1.1 Dynamical Properties of Hadrons . ...... 4 1.2 Electron Scattering: Access to Hadron Structure . ............ 6 1.3 Experimental Kinematics and Methodology . ........ 8 1.3.1 InteractionReferenceFrame . 8 1.3.2 MandelstamVariables . 8 1.3.3 Exclusive ω Meson Electroproduction . 10 1.3.4 L/TSeparation ............................... 13 1.4 The Fπ-2Experiment ................................ 15 1.5 Past Exclusive ω Electroproduction Experiments . 16 2 Literature review on Backward-Angle ω Meson Production 20 i 2.1 u-ChannelPhysicsOverview . 21 2.1.1 Gateway to u-Channel Physics: t-ChannelPhysics . 23 2.2 ReggeTrajectoryModel ............................. 26 2.2.1 ReggeTrajectory .............................. 26 2.2.2 u and t KinematicLimits.......................... 28 2.2.3 Regge Trajectory in Meson Production . 31 2.2.4 VGLandJMLModels ........................... 32 2.3 GPDandTDA ................................... 35 2.3.1 Two Predictions from TDA Collinear Factorization . ........ 39 3 Experimental Apparatus 41 3.1 Overview ...................................... 41 3.2 Accelerator ..................................... 42 3.3 HallC........................................ 43 3.4 Beamline ...................................... 44 3.4.1 BeamPositionMonitors . 44 3.4.2 BeamEnergyMeasurement . 49 3.4.3 BeamCurrentMonitors . 50 3.4.4 ModificationtoBeamline . 51 3.5 Targets........................................ 52 3.5.1 Cryogenic Targets: LH2 .......................... 53 3.5.2 TargetThickness .............................. 55 3.6 Detectors ...................................... 57 3.6.1 DriftChamber ............................... 57 3.6.2 Hodoscopes................................. 59 3.6.3 CherenkovDetectors ............................ 60 3.6.4 LeadGlassCalorimeter. 62 3.6.5 HMS Aerogel Cherenkov Detector . 63 ii 3.7 TriggerSystem ................................... 64 3.7.1 HMSPre-trigger .............................. 65 3.7.2 SOSPre-trigger............................... 67 3.8 Spectrometer Acceptance and Optics . ....... 67 3.9 Determination of the Spectrometer Kinematic Offsets . ............. 69 4 SIMC and New Analysis Software 71 4.1 SIMC ........................................ 71 4.1.1 SpectrometerModels. 72 4.1.2 IonizationEnergyLoss. 73 4.1.3 MultipleScattering . 74 4.1.4 RadiativeProcess.............................. 75 4.1.5 MonteCarloYield ............................. 77 4.2 TheNewC++AnalysisPlatform . 78 5 Heep Data Analysis 82 5.1 DataSelectionandCorrection . 83 5.1.1 Particle Identification in SOS and HMS . 83 5.1.2 Coincidence Timing vs.Particle Speed in the HMS . ...... 84 5.1.3 EventSelectionCriteria . 87 5.2 BackgroundSubtraction . 88 5.2.1 Random Coincidence Background Subtraction . 88 5.2.2 Cell Wall Contribution and Dummy Target Data Subtraction....... 88 5.3 EfficiencyStudy.................................. 89 5.3.1 Analysis Information . 89 5.3.2 Computer and Electronic Live Time . 90 5.3.2.1 Electronic Live Time . 91 5.3.2.2 Computer Live Time . 95 5.3.3 Spectrometer Tracking Efficiencies . ..... 97 iii 5.3.3.1 Choice of HMS Tracking Efficiency . 99 5.3.3.2 Choice of SOS Tracking Efficiency . 101 5.3.4 CarbonTargetRateStudy . .102 5.3.5 LH2 TargetBoilingStudy . .107 5.3.6 SOS Coincidence Blocking . 108 5.3.7 HMS Aerogel Cherenkov Detector Threshold Cuts . 111 5.3.8 HMS ACD Cut Study for the ω Analysis .................112 5.3.9 hsbeta Distribution and Proton Interaction Correction . 114 5.4 Missing mass and Energy Distributions . .......120 5.5 SimulatingtheHeepReaction . 123 5.6 HeepStudyResults ................................. 125 5.6.1 HeepCoincidenceStudy . .125 5.6.2 HeepSinglesStudy.............................126 5.6.3 Heep with Different Parametrizations . 127 5.7 Results........................................128 6 Omega Analysis 130 6.1 Overview and Introduction to the Iterative Procedure . .............130 6.2 PhysicsSimulationModelinSIMC . 133 6.2.1 ω ProductionModel ............................134 6.2.2 ρ0