Indication of Electron Neutrino Appearance in an Accelerator-Produced Muon Neutrino Beam

236 Pages·2012·10.44 MB·English

by  Joshua Benjamin Albert

Indication of Electron Neutrino Appearance in an
Accelerator-Produced Muon Neutrino Beam
by
Joshua Benjamin Albert
Department of Physics
Duke University
Date:
Approved:
Christopher W. Walter, Supervisor
Kate Scholberg
Mark Kruse
Tom Mehen
Henry Greenside
Dissertation submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in the Department of Physics
in the Graduate School of Duke University
2012
Abstract
Indication of Electron Neutrino Appearance in an
Accelerator-Produced Muon Neutrino Beam
by
Joshua Benjamin Albert
Department of Physics
Duke University
Date:
Approved:
Christopher W. Walter, Supervisor
Kate Scholberg
Mark Kruse
Tom Mehen
Henry Greenside
An abstract of a dissertation submitted in partial fulfillment of the requirements for
the degree of Doctor of Philosophy in the Department of Physics
in the Graduate School of Duke University
2012
Copyright (cid:13)c 2012 by Joshua Benjamin Albert
All rights reserved except the rights granted by the
Creative Commons Attribution-Noncommercial Licence
Abstract
T2K (Tokai to Kamioka) is a long baseline neutrino experiment with the primary
goal of measuring the neutrino mixing angle θ . It uses a muon neutrino beam,
13
produced at the J-PARC accelerator facility in Tokai, sent through a near detector
complex on its way to the far detector, Super-Kamiokande. Appearance of electron
neutrinos at the far detector due to oscillation is used to measure the value of θ .
13
This dissertation describes the experimental setup, analysis methods, and results
from the analysis of T2K data taken from January 2010 through March 2011. Six
signal candidate events were observed on an expected background of 1.5 0.3. The
(cid:8)
probability to see six or more such events is 0.7% under the θ 0 hypothesis. This
13
(cid:16)
is the first experiment to exclude θ 0 at the 90% confidence level. The 90%
13
(cid:16)
confidence level allowed region is 0.03 0.04 sin22θ 0.28 0.34 with a best fit
13
p q (cid:160) (cid:160) p q
point of sin22θ 0.11 0.14 for δ 0 and ∆m2 2.4 10 3 eV2 in the normal
13 (cid:16) p q CP (cid:16) | 32| (cid:16) (cid:2) (cid:1)
(inverted) hierarchy.
iv
For my family.
v
Contents
Abstract iv
List of Tables xii
List of Figures xiii
List of Abbreviations and Symbols xvii
Acknowledgements xix
1 Introduction 1
1.1 Dissertation Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Author Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Neutrino Physics 4
2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 Assembling the Neutrino Family . . . . . . . . . . . . . . . . . 4
2.1.2 Symmetry (or lack thereof) . . . . . . . . . . . . . . . . . . . 6
2.1.3 Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 The Standard Model Neutrino . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.1 Two Flavor Model . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.2 Three Flavor Vacuum Oscillations . . . . . . . . . . . . . . . . 22
2.3.3 Matter Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4 CP Violation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
vi
2.5 Recent Neutrino Oscillation Experiments . . . . . . . . . . . . . . . . 30
3 T2K 33
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Basic Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3 Off-Axis Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.4 J-PARC Accelerator . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.5 Neutrino Beamline . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.6 ND280 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.6.1 INGRID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.6.2 ND280 Off-Axis . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.7 Far Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4 Super-Kamiokande 52
4.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2 Detector Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.3 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.4 Water and Air Purification . . . . . . . . . . . . . . . . . . . . . . . . 61
4.5 Atmospheric Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.6 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.6.1 Vertex Finding . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.6.2 Ring Counting . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.6.3 Particle ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.6.4 Decay Electron Tagging . . . . . . . . . . . . . . . . . . . . . 66
4.6.5 Momentum Reconstruction . . . . . . . . . . . . . . . . . . . . 67
4.6.6 Ring Number Correction . . . . . . . . . . . . . . . . . . . . . 68
4.6.7 POLfit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
vii
4.7 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.7.1 Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.7.2 PMT Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.7.3 Water Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.7.4 Auto-Calibration . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.8 Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.8.1 Multi-GeV Stopping Muons . . . . . . . . . . . . . . . . . . . 77
4.8.2 Sub-GeV Stopping Muons . . . . . . . . . . . . . . . . . . . . 77
4.8.3 π0 Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.8.4 Decay Electrons . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.8.5 Absolute Scale . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.8.6 Time Variation . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.9 Additional Decay Electron Studies . . . . . . . . . . . . . . . . . . . 79
4.10 T2K-Specific Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5 Analysis Overview and Development 86
5.1 Analysis Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.2 Far Detector Selection Cuts . . . . . . . . . . . . . . . . . . . . . . . 88
5.2.1 Fiducial Volume Cut . . . . . . . . . . . . . . . . . . . . . . . 89
5.2.2 Fully Contained Cut . . . . . . . . . . . . . . . . . . . . . . . 90
5.2.3 Single Ring Cut . . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.2.4 E-like Ring Cut . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2.5 Visible Energy Cut . . . . . . . . . . . . . . . . . . . . . . . . 92
5.2.6 µ e Decay Cut . . . . . . . . . . . . . . . . . . . . . . . . . 93
(cid:209)
5.2.7 POLfit π0 Mass Cut . . . . . . . . . . . . . . . . . . . . . . . 94
5.2.8 Reconstructed Neutrino Energy Cut . . . . . . . . . . . . . . 95
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5.3 Sensitivity and Discovery Potential . . . . . . . . . . . . . . . . . . . 97
5.3.1 Upper Limit Sensitivity . . . . . . . . . . . . . . . . . . . . . 98
5.3.2 Discovery Potential . . . . . . . . . . . . . . . . . . . . . . . . 99
5.3.3 Sensitivity and Discovery Potential vs. Exposure . . . . . . . . 100
5.4 Far Detector Cut Optimization . . . . . . . . . . . . . . . . . . . . . 100
5.4.1 Rough Sensitivity Calculation . . . . . . . . . . . . . . . . . . 100
5.4.2 Background Rejection Cut Candidates . . . . . . . . . . . . . 102
5.4.3 Optimization Technique . . . . . . . . . . . . . . . . . . . . . 103
5.4.4 Unnecessary Cut Rejection . . . . . . . . . . . . . . . . . . . . 104
5.4.5 Systematic Considerations . . . . . . . . . . . . . . . . . . . . 107
5.4.6 Cut Value Optimization . . . . . . . . . . . . . . . . . . . . . 112
5.4.7 Robustness of Cut Optimization . . . . . . . . . . . . . . . . . 112
6 Analysis Inputs 117
6.1 MC Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.1.1 Beam Simulation . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.1.2 Neutrino Interactions . . . . . . . . . . . . . . . . . . . . . . . 120
6.1.3 NEUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
6.1.4 SKdetsim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6.1.5 Additional Corrections . . . . . . . . . . . . . . . . . . . . . . 125
6.2 Beam Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.3 Near Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.4 Far Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
7 Systematic Errors 127
7.1 Neutrino Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
7.2 Near Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
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7.3 Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
7.4 Far Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
8 ν Appearance Analysis 134
e
8.1 T2K Data Taking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
8.2 Normalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
8.2.1 Far Detector MC Normalization . . . . . . . . . . . . . . . . . 137
8.2.2 Near/Far Ratio Normalization . . . . . . . . . . . . . . . . . . 138
8.3 Statistical Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
8.3.1 Feldman-Cousins Method . . . . . . . . . . . . . . . . . . . . 139
8.3.2 Systematic Error Incorporation . . . . . . . . . . . . . . . . . 143
8.3.3 Method Check and Comparison . . . . . . . . . . . . . . . . . 144
8.4 Analysis Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
8.4.1 Oscillation Computation . . . . . . . . . . . . . . . . . . . . . 146
8.4.2 Oscillation Grids . . . . . . . . . . . . . . . . . . . . . . . . . 147
8.4.3 Calculation of N . . . . . . . . . . . . . . . . . . . . . . . . 147
exp
8.4.4 Systematic Error Details . . . . . . . . . . . . . . . . . . . . . 150
8.4.5 Toy MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
8.4.6 Feldman Cousins Ordering . . . . . . . . . . . . . . . . . . . . 152
8.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
8.6 Analysis Checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
9 Conclusions 167
A Decay Electron Studies 169
A.1 Decay Electron Calibration Details . . . . . . . . . . . . . . . . . . . 169
A.2 Muon Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
A.3 Position and Direction Dependence . . . . . . . . . . . . . . . . . . . 175
x