Electron Neutrino Appearance in the MINOS Experiment Thesis by Mhair-Armen Hagop Orchanian In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 2012 (Defended May 9, 2012) ii (cid:13)c 2012 Mhair-Armen Hagop Orchanian All Rights Reserved iii To my parents, Anna and Hagop iv v Acknowledgements Withthesuccessfulcompletionofthisthesis,Iwouldliketoexpressmydeepgratitude to the people who helped make it a reality. Despite his many commitments, my advisor, Harvey Newman, skillfully guided me through the graduate program and made sure I stayed on track. His advice along the way – particularly his careful reading of and comments on drafts of this thesis – was absolutely invaluable. I am deeply indebted to Ryan Patterson; his patience, incredible generosity with his time, and great skill in explaining difficult concepts made my education as a physicist not only possible but also quite enjoyable. My fellow graduate students on MINOS, Alex Himmel and Pedro Ochoa, were always happy to answer my questions about programming and physics and to share with me their experiences in the graduate program. I am also very grateful to Leon Mualem for helping me take over the Caltech Monte Carlo generation responsibilities and to Jason Trevor for training me in the NOνA R&D lab in the year before I started the graduate program. Working with my colleagues on MINOS has been a very rewarding experience. With her indefatigable patience and guidance, Lisa Whitehead was directly respon- sible for the timely completion of the work I did for my candidacy; her continued assistance since then has been greatly appreciated. My fellow graduate students on the ν appearance analysis – Jo˜ao Coelho, Ruth Toner, and Adam Schreckenberger e – have been wonderful collaborators; their camaraderie has made the joy of scientific discovery even sweeter. I am also thankful to all of the ν group conveners, past and e present – including Tricia Vahle, Mayly Sanchez, Jeff Nelson, and Greg Pawloski – for their able leadership through our challenging undertakings. vi My family has been a constant source of inspiration throughout my life and has served as a solid support system for as long as I can remember. My elder cousin, Karoun Bagamian, blazed the trail for graduate education in our family, setting an example that I was honored to follow. My paternal grandfather, Sarkis, was an ardent proponent of education, despite never making it past middle school in the tumult following the Armenian Genocide. He missed this milestone in my life by just four years, but I just know that he would have been so proud of me. My parents, Hagop andAnna, instilledinmetheimportanceofeducationandcharacter. Theyhavenever thought twice about sacrificing for my ultimate benefit, and their unwavering support and encouragement in all of my endeavors – particularly throughout my writing of this thesis – has been utterly indispensable. I consider myself incredibly fortunate to be their son. vii Abstract This thesis describes a search for ν appearance in the two-detector long-baseline e MINOS neutrino experiment at Fermilab, based on a data set representing an expo- sure of 8.2×1020 protons on the NuMI target. The analysis detailed herein represents an increase in sensitivity to the θ mixing angle of approximately 25% over previous 13 analyses, due to improvements in the event discriminant and fitting technique. Based on our observation, we constrain the value of θ further, finding 2sin2θ sin22θ < 13 23 13 0.12(0.20) at the 90% confidence level for δ = 0 and the normal (inverted) neutrino CP mass hierarchy. The best-fit value is 2sin2θ sin22θ = 0.041+0.047(0.079+0.071) un- 23 13 −0.031 −0.053 der the same assumptions. We exclude the θ = 0 hypothesis at the 89% confidence 13 level. viii CONTENTS ix Contents Acknowledgements v Abstract vii List of Figures xiii List of Tables xvii 1 Neutrinos: The Standard Model and Beyond 1 1.1 An Overview of the Standard Model . . . . . . . . . . . . . . . . . . 2 1.1.1 Theoretical Foundation . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Generating Neutrino Masses . . . . . . . . . . . . . . . . . . . 6 1.2 Getting to Massive Neutrinos . . . . . . . . . . . . . . . . . . . . . . 8 1.2.1 Solar Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.2 Atmospheric Neutrinos . . . . . . . . . . . . . . . . . . . . . . 12 1.2.3 Accelerator Neutrinos . . . . . . . . . . . . . . . . . . . . . . 19 1.2.4 Reactor Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . 20 1.3 Describing Massive Neutrinos . . . . . . . . . . . . . . . . . . . . . . 23 1.3.1 Formalism of Neutrino Mixing . . . . . . . . . . . . . . . . . . 23 1.3.1.1 General Probability of Flavor Transition . . . . . . . 25 1.3.1.2 The Mixing Matrix . . . . . . . . . . . . . . . . . . . 28 1.3.2 Two-Neutrino Oscillations . . . . . . . . . . . . . . . . . . . . 30 1.3.2.1 The Two-Neutrino Framework . . . . . . . . . . . . 30 1.3.2.2 Oscillation Probabilities and Parameter Measurements 31 x CONTENTS 1.3.2.3 Matter Effects . . . . . . . . . . . . . . . . . . . . . 38 1.3.3 Three-Neutrino Oscillations . . . . . . . . . . . . . . . . . . . 45 1.3.3.1 Understanding the Two-Neutrino Approximation . . 45 1.3.3.2 ν → ν Oscillation in the Three-Neutrino Framework 48 µ e 2 Experimental Setup 51 2.1 Neutrinos at the Main Injector (NuMI) . . . . . . . . . . . . . . . . . 51 2.2 The MINOS Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . 59 2.2.1 The Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . 61 2.2.2 The Scintillator Strips . . . . . . . . . . . . . . . . . . . . . . 62 2.2.2.1 The Light Injection System . . . . . . . . . . . . . . 65 2.2.3 The Photodetectors . . . . . . . . . . . . . . . . . . . . . . . . 67 2.2.4 The Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . 69 2.2.5 The Data Acquisition System (DAQ) . . . . . . . . . . . . . . 71 2.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 2.4 Event Types of Interest . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.5 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 2.6 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.6.1 Simulating the Beam . . . . . . . . . . . . . . . . . . . . . . . 83 2.6.2 Simulating Neutrino Interactions . . . . . . . . . . . . . . . . 85 2.6.3 Simulating the Detectors . . . . . . . . . . . . . . . . . . . . . 85 3 Overview of the Analysis 89 4 Event Selection 95 4.1 Fiducial Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.2 ν -like Preselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 e 4.3 Library Event Matching . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.3.1 The Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 4.3.2 Preparing Events for Matching . . . . . . . . . . . . . . . . . 104 4.3.2.1 Pulse Height Measurement . . . . . . . . . . . . . . . 104