Electron Neutrino Appearance in the MINOS Experiment

243 Pages·2012·20.02 MB·English

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
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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
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– 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
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present – including Tricia Vahle, Mayly Sanchez, Jeff Nelson, and Greg Pawloski –
for their able leadership through our challenging undertakings.
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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.
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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.
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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