A Measurement of the Parity-Violating Asymmetry in Aluminum and Its Contribution to a Measurement of the Proton’S Weak Charge
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A Measurement of the Parity-Violating Asymmetry in Aluminum and its Contribution to a Measurement of the Proton's Weak Charge Joshua Allen Magee Bridgewater, MA MS Physics, College of William and Mary, 2010 BS Mechanical Engineering, University of Hartford, 2007 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 May 2016 © 2016 Joshua Allen Magee All Rights Reserved APPROVAL PAGE This dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Joshua Allen Magee Approved by the Committee February 2016 Committee Chair Chancellor Professor David Armstrong, Physics The College of William and Mary Assistant Professor Wouter Deconinck, Physics The College of William and Mary Professor Keith Griffioen, Physics The College of William and Mary Professor Carl Carlson, Physics The College of William and Mary David Gaskell, Staff Scientist Thomas Jefferson National Accelerator Facility ABSTRACT The Qweak experiment, which ran at the Thomas Jefferson National Accelerator p Facility, made a precision measurement of the proton's weak charge, QW . The weak charge is extracted via a measurement of the parity-violating asymmetry in elastic electron-proton scattering from hydrogen at low momentum transfer (Q2=0.025 GeV2). This result is directly related to the electroweak mixing 2 angle, sin (θW ), a fundamental parameter in the Standard Model of particle physics. This provides a precision test sensitive to new, as yet unknown, fundamental physics. This dissertation focuses on two central corrections to the Qweak measurement: the target window contribution and sub-percent determination of the electron beam polarization. The aluminum target windows contribute approximately 30% of the measured asymmetry. Removal of this background requires precise measurements of both the elastic electron-aluminum scattering rate and its parity-violating asymmetry. The results reported here are the most precise measurement of the Qweak target dilution and asymmetry to date. The parity-violating asymmetry for the aluminum alloy was found to be 1.6174 ± 0.0704 (stat.) ± 0.0113 (sys.) parts-per-million. The first sub-percent precision polarization measurements made from the Hall C Møller polarimeter are also reported, with systematic uncertainties of 0.84%. Table of Contents Table of Contentsi Acknowledgements vi Dedication viii List of Tables ix List of Figures xi 1 The Electroweak Standard Model1 1.1 Introduction to the Standard Model.................... 2 1.2 Symmetries.................................. 4 1.2.1 Discrete symmetries......................... 5 1.3 Electroweak unification........................... 7 1.3.1 Electroweak gauge structure..................... 7 1.3.2 Spontaneous symmetry breaking and the Higgs mechanism... 9 1.3.3 Neutral current interactions..................... 11 1.3.4 Nucleon weak charges ........................ 12 1.4 Electroweak radiative corrections...................... 13 2 Parity-Violating Electron Scattering and the Qweak Experiment 18 2.1 Electromagnetic form factors........................ 18 2.1.1 Neutral weak form factors...................... 20 2.2 Cross sections for e + p and e + Al scattering............... 22 2.3 Parity-violating asymmetries and electron scattering........... 23 i 2.4 Qweak introduction.............................. 26 3 Experimental Setup 32 3.1 Experimental apparatus........................... 33 3.2 Beam delivery................................. 34 3.2.1 Photocathode and beam generation ................ 35 3.2.2 Injector and slow helicity reversals................. 38 3.2.3 Accelerator.............................. 39 3.3 Beam diagnostics............................... 39 3.3.1 Beam position monitors....................... 40 3.3.2 Beam current monitors ....................... 41 3.3.3 Beam energy determination..................... 42 3.3.4 Beam modulation .......................... 43 3.4 Polarimetry.................................. 45 3.4.1 Møller Polarimeter.......................... 45 3.4.2 Compton polarimeter ........................ 48 3.5 Targets.................................... 49 3.5.1 Primary LH2 target ......................... 49 3.5.2 Aluminum targets .......................... 52 3.6 Collimators.................................. 53 3.7 Spectrometer magnet............................. 55 3.8 Detectors................................... 57 3.8.1 Main detectors............................ 57 3.8.2 Focal plane scanner ......................... 59 ii 3.8.3 Luminosity monitors......................... 60 3.9 Tracking system............................... 61 3.9.1 Horizontal drift chambers...................... 62 3.9.2 Vertical drift chambers........................ 62 3.9.3 Trigger scintillators.......................... 63 3.10 Data acquisition............................... 63 4 The Møller Polarimeter Analysis 65 4.1 Configuration during Qweak ......................... 65 4.2 Analyzing power determination and uncertainty.............. 69 4.2.1 Nominal target polarization..................... 70 4.2.2 Target temperature ......................... 71 4.2.3 Solenoid field uncertainty ...................... 72 4.2.4 Quadrupole currents......................... 74 4.2.5 Position and angle systematic uncertainty............. 75 4.2.6 Møller Run 1 optics effect...................... 78 4.3 Data and results............................... 82 4.3.1 Leakage corrections.......................... 82 4.3.2 Effect of the insertable half wave plate............... 85 4.4 Møller summary............................... 86 5 The Aluminum Window Contribution 90 5.1 Aluminum overview............................. 90 5.2 Dilution method............................... 93 5.2.1 Measurement conditions....................... 94 iii 5.2.2 Reference target normalization ...................104 5.3 Data and analysis............................... 106 5.4 Radiative corrections............................ 109 5.4.1 Simulation model uncertainties...................111 5.5 Dilution final results............................. 113 6 The Parity-Violating Asymmetry of Aluminum 115 6.1 Aluminum asymmetry overview....................... 115 6.2 Data quality.................................. 116 6.3 Asymmetry analysis............................. 122 6.3.1 Raw asymmetry and background corrections . 124 6.3.2 Extracting the Qweak correction................... 131 7 Concluding discussions 136 7.1 Summary of findings............................. 136 7.2 Current Run 2 Qweak Results........................ 137 7.3 Future work.................................. 142 Appendices 145 A Møller-Compton Cross Calibration 145 A.1 Systematics and method........................... 145 A.2 Data, analysis, and results.......................... 148 B Alternate Rate Determinations 152 B.1 Scaler based main detector deadtime determination............ 152 iv B.2 Event-mode technique............................ 155 B.2.1 Rates in event-mode.........................156 B.2.2 Detector deadtimes.......................... 158 B.2.3 Prescaling, DAQ deadtime, and time determination . 159 B.3 Data analysis................................. 160 Bibliography 162 v Acknowledgements This milestone was made possible in large-part due to the wonderful people surrounding me at William and Mary and Jefferson Lab. Without the generous gift of their time and help, this document would be, in some way, incomplete. More importantly, without them my life would be less colorful, rich, and fufilling. I will do my best to continue the grand tradition of naming everybody, but there is a near uncountable set, and I fear my attempts may be incomplete at points. First I should thank everyone at William and Mary, in particular Marc Sher, who encouraged my application and acceptance into the program. As an engineer with little previous formal physics training, it was often overwhelming, but worth it. Everybody was so supportive, and I am grateful for the nuturing environment provided. Thank you to Paula, Carol, Elle, and Bonnie for putting up with my antics and crazy sense of humor, and for making my time there so much more enjoyable (and for always sharing the best sandwich in town). I am indebted to my fellow Qweak graduate students. Many, many owl shifts (and late-night \shifts" at the bar) were spent with Manolis, Scott, Adesh, Wade, Anna, and Nur. Amrendra brought much needed joy and peace during many stressful times. The first Qweak graduate students to graduate, in particular John Leacock, Rakitha, and Buddhini, paved the way for the rest of us. Even though not a grad student, Jean-Francois deserves a shout-out for all of his cunning plans and willingness to share his wisdom. Everybody's hard work and clear descriptions made understanding this experiment fun and doable. My fellow William and Mary students spent more time with me than anybody should have to. John Leckey taught me much about project management and that I had to concentrate on what's important in my analysis - not every technique needs refinement. Valerie was always willing to help in my analysis and with the farm. Jim Dowd always brought quick wit, lively debate, and thoughtful discussion. Rachel brought much joy to our meetings and became a dear friend. Josh Hoskins needs to be thanked for the constant trouble I caused him - he has an uncanny ability to put with with my bad jokes about being my namesake. Without his guidance and friendship