Arthur Franke

354 Pages·2012·6.16 MB·English

Searching for Reactor Antineutrino Flavor
Oscillations with the Double Chooz Far Detector
Arthur James Franke
Submitted in partial fulfillment of the
requirements for the degree
of Doctor of Philosophy
in the Graduate School of Arts and Sciences
COLUMBIA UNIVERSITY
2012
c 2012
(cid:13)
Arthur James Franke
All Rights Reserved
ABSTRACT
Searching for Reactor Antineutrino Flavor
Oscillations with the Double Chooz Far Detector
Arthur James Franke
Thisdissertationpresentsresultsfromasearch forreactor ν¯ flavor oscillations usingthe
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Double Chooz Far Detector. The search was performed by observing the rate and energy
spectrumof ν¯ interacting viaInverseBeta Decay inaGd-dopedliquidscintillator detector,
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and comparing the observation to an expectation based on a prediction of the emitted
reactor flux. The Columbia University neutrino group was instrumental in construction
of the Double Chooz Outer Veto, as well as the analysis efforts leading to two oscillation
measurement results. The most recent analysis is presented herein, focusing on 251.27 days
of data (or 33.71 GW-ton-years of exposure). In these data, 8249 IBD candidates were
observed, compared to a signal+background prediction of 8936.8. A fit to a two-neutrino
oscillation model considering event rate, spectral shape, and time yields a best-fit value
of sin2(2θ ) = 0.109 0.030 (stat.) 0.025 (syst.) at ∆m2 = 2.32 10−3 eV2, with
13 ± ± 31 ×
χ2 /d.o.f. = 42.1/35. A frequentist method deems the null-oscillation hypothesis excluded
RS
by the data at 99.8% C.L., or 2.9σ. These results are in agreement with the measurements
of other modern reactor ν¯ experiments.
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Table of Contents
1 Introduction 1
I Neutrino Oscillations & Experimental Searches 5
2 Neutrino Flavor Oscillations 7
2.1 Neutrinos in the Standard Model of Particle Physics . . . . . . . . . . . . . 7
2.2 Neutrino Flavor Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 The PMNS Neutrino Mixing Matrix . . . . . . . . . . . . . . . . . . 9
2.2.2 Oscillation Probability Formulas . . . . . . . . . . . . . . . . . . . . 10
2.3 Possible Mechanisms for Neutrino Masses . . . . . . . . . . . . . . . . . . . 16
3 Neutrino Oscillation Experiments 19
3.1 Past Experimental Oscillation Results . . . . . . . . . . . . . . . . . . . . . 19
3.2 Modern Searches for θ -Driven Oscillations . . . . . . . . . . . . . . . . . . 23
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3.2.1 Accelerator-Based Searches . . . . . . . . . . . . . . . . . . . . . . . 24
3.2.2 Reactor-Based Searches . . . . . . . . . . . . . . . . . . . . . . . . . 25
II The Double Chooz Experiment 29
4 The Double Chooz Experiment 31
4.1 Experiment Site & Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.2 Detector Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.2.1 Inner Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
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4.2.2 Inner Veto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2.3 Steel Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2.4 Outer Veto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2.5 Calibration Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.3 Main Detector Data Acquisition Systems . . . . . . . . . . . . . . . . . . . 43
4.3.1 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5 The Double Chooz Outer Veto 47
5.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.2 Module Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.3 DAQ Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3.1 Electronics Hardware Design . . . . . . . . . . . . . . . . . . . . . . 51
5.3.2 Maroc2 Crosstalk Testing . . . . . . . . . . . . . . . . . . . . . . . . 54
5.4 OV EventBuilder and DOGSifier . . . . . . . . . . . . . . . . . . . . . . . . 64
5.5 Monitoring Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.5.1 OV Online Monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.5.2 OV Offline Monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.6 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
III Double Chooz Analyses 69
6 Neutrino Signal Flux Estimation & Uncertainties 71
6.1 Instantaneous ν¯ Rate from a Single Reactor . . . . . . . . . . . . . . . . . 71
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6.1.1 Reactor Fission Rate Calculation . . . . . . . . . . . . . . . . . . . . 72
6.1.2 Mean Cross-Section per Fission . . . . . . . . . . . . . . . . . . . . . 77
6.2 Binned Expected Neutrino Count . . . . . . . . . . . . . . . . . . . . . . . . 82
6.3 Bugey4 Anchor Point. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.4 Binned Expectation Uncertainty Propagation . . . . . . . . . . . . . . . . . 85
6.4.1 Multiple Integration Periods. . . . . . . . . . . . . . . . . . . . . . . 87
6.4.2 Covariance Matrix Components of Prediction Anchored to ILL Spectra 87
6.4.3 Covariance MatrixComponentsofPrediction Anchored toBugey4Rate 90
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6.4.4 Covariance Matrix Components of Prediction Anchored to Measured
Near Detector Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.4.5 Comparison of Uncertainty Contributions . . . . . . . . . . . . . . . 93
7 Second Double Chooz Publication 95
7.1 Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
7.2 Candidate Selection Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
7.2.1 Second Publication Analysis . . . . . . . . . . . . . . . . . . . . . . . 95
7.2.2 Peripheral Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.2.3 Effects of IBD Selection on Detector Livetime . . . . . . . . . . . . . 104
7.3 Signal Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.3.1 Reactor Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3.2 Signal Selection Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 108
7.3.3 Signal Prediction Summary . . . . . . . . . . . . . . . . . . . . . . . 116
7.4 Background Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.4.1 Accidental Background . . . . . . . . . . . . . . . . . . . . . . . . . 118
7.4.2 Cosmogenic Lithium-9 . . . . . . . . . . . . . . . . . . . . . . . . . . 120
7.4.3 Fast Neutron & Stopping µ Backgrounds . . . . . . . . . . . . . . . 123
7.4.4 Cross-checks of Background Measurements . . . . . . . . . . . . . . 130
7.5 Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
7.5.1 Per-channel Gain vs. Charge . . . . . . . . . . . . . . . . . . . . . . 133
7.5.2 Detector Response Spatial Correction . . . . . . . . . . . . . . . . . 134
7.5.3 Detector Response Time Dependence Correction . . . . . . . . . . . 134
7.5.4 Absolute Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . . 137
7.5.5 Uncertainty Propagation . . . . . . . . . . . . . . . . . . . . . . . . . 137
7.6 Multiple Integration Periods . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
7.7 Oscillation Fit & Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
7.7.1 Parameter-Dependent Covariance Matrix . . . . . . . . . . . . . . . 145
7.7.2 Definitions of χ2 Statistics . . . . . . . . . . . . . . . . . . . . . . . . 147
7.7.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7.7.4 Frequentist Confidence Intervals . . . . . . . . . . . . . . . . . . . . 154
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7.7.5 Fit Without MINOS ∆m2 Constraint . . . . . . . . . . . . . . . . . 157
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7.8 More Two-Reactor-Off Data . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
IV Context & Conclusions 165
8 Double Chooz Measurements in Context 167
8.1 Winter 2011/Spring 2012: First Oscillation Results . . . . . . . . . . . . . . 167
8.2 Summer 2012: Updated Results . . . . . . . . . . . . . . . . . . . . . . . . . 168
8.3 The Post-θ Era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
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8.3.1 Outlook on the PMNS Matrix . . . . . . . . . . . . . . . . . . . . . 170
8.3.2 Outlook on Reactor ν¯ Experiments . . . . . . . . . . . . . . . . . . 172
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9 Conclusions 177
V Appendices 179
A First Double Chooz Publication 181
A.1 Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
A.2 Candidate Selection Cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
A.2.1 First Publication Analysis . . . . . . . . . . . . . . . . . . . . . . . . 181
A.2.2 Effects of IBD Selection on Detector Livetime . . . . . . . . . . . . . 182
A.3 Signal Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
A.3.1 Reactor Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
A.3.2 Signal Selection Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 184
A.3.3 Signal Prediction Summary . . . . . . . . . . . . . . . . . . . . . . . 189
A.4 Background Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
A.4.1 Accidental Background . . . . . . . . . . . . . . . . . . . . . . . . . 191
A.4.2 Cosmogenic Lithium-9 . . . . . . . . . . . . . . . . . . . . . . . . . . 192
A.4.3 Fast Neutron & Stopping µ Backgrounds . . . . . . . . . . . . . . . 193
A.4.4 Cross-checks of Background Measurements . . . . . . . . . . . . . . 195
A.5 Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
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A.5.1 Detector Response Correction Functions . . . . . . . . . . . . . . . . 197
A.5.2 Uncertainty Propagation . . . . . . . . . . . . . . . . . . . . . . . . . 198
A.6 Oscillation Fit & Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
A.6.1 Parameter-Independent Covariance Matrix . . . . . . . . . . . . . . 202
A.6.2 Definitions of χ2 Statistics . . . . . . . . . . . . . . . . . . . . . . . . 202
A.6.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
A.6.4 Frequentist Confidence Intervals . . . . . . . . . . . . . . . . . . . . 206
pred DC,far
A.6.5 Synthesized Quantities: R , σ , and σ . . . . . . . . . . . 208
DC f f
B CUfits 211
B.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
B.2 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
C The MultiSim Method 215
C.1 General Description of MultiSim Method . . . . . . . . . . . . . . . . . . . 215
C.2 Applied to Neutrino Reference Spectra . . . . . . . . . . . . . . . . . . . . . 217
C.2.1 Application & Results . . . . . . . . . . . . . . . . . . . . . . . . . . 219
C.2.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
C.3 Applied to Reactor Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . 221
C.4 Applied to Energy Scale Uncertainties . . . . . . . . . . . . . . . . . . . . . 224
C.4.1 First Double Chooz Publication . . . . . . . . . . . . . . . . . . . . . 225
C.4.2 Second Double Chooz Publication . . . . . . . . . . . . . . . . . . . 226
D DCRxtrTools Neutrino Event Generator & Uncertainty Calculator 229
D.1 Description of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
D.1.1 Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
D.1.2 Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
D.1.3 General Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
D.2 Monte Carlo Event Generation . . . . . . . . . . . . . . . . . . . . . . . . . 234
D.3 Uncertainty Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
D.3.1 Propagation from Etrue to Ereco . . . . . . . . . . . . . . . . . . . . 236
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D.4 Rebinning of Reference Spectra and Uncertainties . . . . . . . . . . . . . . . 236
D.4.1 Rebinning Reference Spectra . . . . . . . . . . . . . . . . . . . . . . 237
D.4.2 Rebinning Covariance Matrices . . . . . . . . . . . . . . . . . . . . . 237
D.5 Power-Scaling of Fission Rates . . . . . . . . . . . . . . . . . . . . . . . . . 238
E OV Online Monitor 241
E.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
E.2 Data Handling & Visualization . . . . . . . . . . . . . . . . . . . . . . . . . 245
F Drawing Confidence Intervals using a Frequentist Method 247
F.1 Procedure for Drawing Confidence Intervals . . . . . . . . . . . . . . . . . . 247
F.1.1 Generation of Pseudoexperiments . . . . . . . . . . . . . . . . . . . . 248
F.1.2 Goodness-of-Fit Statistic Comparison . . . . . . . . . . . . . . . . . 249
F.1.3 Drawing Confidence Intervals . . . . . . . . . . . . . . . . . . . . . . 249
F.2 Procedure for Testing the Null-Oscillation Hypothesis . . . . . . . . . . . . 250
G Pulls vs. Covariance χ2 251
G.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
G.2 Tests With Two Simple χ2 Statistics . . . . . . . . . . . . . . . . . . . . . . 252
G.2.1 Uncertainties on Linear Parameters . . . . . . . . . . . . . . . . . . 252
G.2.2 Uncertainties on Multiplicative Parameters . . . . . . . . . . . . . . 254
G.3 Commentary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
G.4 Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
G.4.1 Linear Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
G.4.2 Multiplicative Uncertainties . . . . . . . . . . . . . . . . . . . . . . . 258
G.4.3 Single Multiplicative Uncertainty . . . . . . . . . . . . . . . . . . . . 258
H Covariance Matrix Component Break-Out 259
H.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
H.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
I Second Publication Data Release 261
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