An Investigation of Matter Enhanced Neutrino Oscillation ... · PDF fileAn Investigation of...

Click here to load reader

  • date post

    05-Feb-2018
  • Category

    Documents

  • view

    213
  • download

    0

Embed Size (px)

Transcript of An Investigation of Matter Enhanced Neutrino Oscillation ... · PDF fileAn Investigation of...

  • An Investigation of Matter Enhanced Neutrino Oscillation

    with the Sudbury Neutrino Observatory

    Miles Walter Eldon Smith

    A dissertation submitted in partial fulfillment of

    the requirements for the degree of

    Doctor of Philosophy

    University of Washington

    2002

    Program Authorized to Offer Degree: Physics

  • University of Washington

    Graduate School

    This is to certify that I have examined this copy of a doctoral dissertation by

    Miles Walter Eldon Smith

    and have found that it is complete and satisfactory in all respects,

    and that any and all revisions required by the final

    examining committee have been made.

    Chair of Supervisory Committee:

    Steve Elliott

    Reading Committee:

    Peter Doe

    Steve Elliott

    Hamish Robertson

    Date:

  • In presenting this dissertation in partial fulfillment of the requirements for the Doc-

    toral degree at the University of Washington, I agree that the Library shall make

    its copies freely available for inspection. I further agree that extensive copying of

    this dissertation is allowable only for scholarly purposes, consistent with fair use as

    prescribed in the U.S. Copyright Law. Requests for copying or reproduction of this

    dissertation may be referred to Bell and Howell Information and Learning, 300 North

    Zeeb Road, Ann Arbor, MI 48106-1346, to whom the author has granted the right

    to reproduce and sell (a) copies of the manuscript in microform and/or (b) printed

    copies of the manuscript made from microform.

    Signature

    Date

  • University of Washington

    Abstract

    An Investigation of Matter Enhanced Neutrino Oscillation

    with the Sudbury Neutrino Observatory

    by Miles Walter Eldon Smith

    Chair of Supervisory Committee:

    Professor Steve ElliottDepartment of Physics

    Previous experiments have detected fewer electron neutrinos coming from the Sun

    than are predicted by the Standard Solar Model (SSM). While the Sun makes only e,

    these can change into other flavors through neutrino oscillation, a favored hypothesis

    for explaining the deficit. The Sudbury Neutrino Observatory (SNO) was designed to

    measure both the flux of electron neutrinos e and the total flux of active neutrinos

    tot (where tot = e + + ). This allows one to separate out the + component

    by = tot e. Doing this, SNO measures a flux of non-electron neutrinos = 3.410.45(stat.)+0.480.45(syst.)106cm2s1, providing evidence of neutrino flavortransformation with 5.3 significance. By refining the treatment of systematic errors,

    this improves to 7.4. Although this shows that flavor transformation is occurring, it

    does not identify a specific mechanism such as neutrino oscillation.

    Neutrino oscillation can be enhanced by the presence of matter in the Sun and

    the Earth. This predicts a possible modulation of the flux of electron neutrinos with

    solar zenith angle, as they transit through varying amounts of matter. However, we

    do not see a significant difference between the day and night measurements for either

  • the electron or total neutrino flux. By assuming the total neutrino flux is constant,

    as predicted by the simplest models, we measure the difference between the day and

    night electron neutrino flux to be +7.0 4.9 0.9% of the average electron neutrinoflux. This result is weak at 1.4. By combining this result with that of the Super-

    Kamiokande experiment, we measure a difference of 6.0 3.2%, or 1.9.By examining a specific oscillation model, we are able to identify allowed regions

    for the oscillation parameters m2, tan2 . The measured values of m2, tan2 pre-

    dict an asymmetry in the day to night flux which is consistent with the measurements

    above. This specific model also allows for a distortion of the neutrino spectrum. How-

    ever, this additional observable does little to improve the identification of neutrino

    oscillation as the cause of flavor transformation in solar neutrinos.

    We conclude that, while flavor transformation is definitely occurring for solar

    neutrinos, we can not specifically identify the mechanism to be matter enhanced

    neutrino oscillation.

  • TABLE OF CONTENTS

    List of Figures iv

    List of Tables viii

    List of Abbreviations x

    Mathematical Objects xi

    Chapter 1: Introduction 1

    1.1 A Little History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    1.2 The Solar Neutrino Problem . . . . . . . . . . . . . . . . . . . . . . . 5

    1.3 Vacuum Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    1.4 The MSW Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    1.5 Earth Regeneration and the Day-Night Asymmetry . . . . . . . . . . 18

    1.6 Solar Neutrino MSW Regions Before SNO . . . . . . . . . . . . . . . 19

    1.7 Sterile Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    Chapter 2: The Sudbury Neutrino Observatory: Data 22

    2.1 Description of SNO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    2.2 Data Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

    2.3 Final Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

    2.4 Livetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

    Chapter 3: Calibration, Systematics and Backgrounds 46

    i

  • 3.1 Optical Calibration and the SNOMAN Monte Carlo . . . . . . . . . . 46

    3.2 Energy Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

    3.3 Neutron Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

    3.4 Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

    3.5 Instrumental Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 68

    3.6 Summary of Systematics and Creating a Model for SNO . . . . . . . 83

    Chapter 4: Interpretation 90

    4.1 General Formalism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

    4.2 A Preliminary Null Hypothesis Test (Model 2a) . . . . . . . . . . . . 97

    4.3 Measuring the Day-Night Asymmetry (Models 2b and 2c) . . . . . . 101

    4.4 MSW Contour Analysis (Model 3) . . . . . . . . . . . . . . . . . . . . 106

    4.5 Rethinking Hypothesis Testing . . . . . . . . . . . . . . . . . . . . . 111

    4.6 Exclusion Region from Day-Night and Spectral Shape . . . . . . . . . 115

    4.7 Combining With Other Experimental Data . . . . . . . . . . . . . . 115

    Chapter 5: Future Work 118

    5.1 Refinement of MSW Analysis . . . . . . . . . . . . . . . . . . . . . . 118

    5.2 Future Phases of the SNO Experiment. . . . . . . . . . . . . . . . . 120

    5.3 Future Neutrino Experiments . . . . . . . . . . . . . . . . . . . . . . 124

    Bibliography 125

    Appendix A: Mathematical Formalism 131

    A.1 Linear Expansion in Systematics . . . . . . . . . . . . . . . . . . . . 131

    A.2 Solving for a Linear Physics Model (Models 2a, b, c) . . . . . . . . . 133

    A.3 Combining the SNO and SK Day-Night Results . . . . . . . . . . . . 135

    A.4 Solving for the MSW Model . . . . . . . . . . . . . . . . . . . . . . . 137

    ii

  • A.5 Some Notes About the 2 Distribution . . . . . . . . . . . . . . . . . 140

    A.6 Comparing the Current MSW Analysis to Previous SNO Analyses . 141

    A.7 Comparison to Other Published MSW Analyses . . . . . . . . . . . . 144

    Appendix B: Neutral Current Detectors 147

    B.1 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

    B.2 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

    Appendix C: SNO Publications 158

    iii

  • LIST OF FIGURES

    1.1 The p-p chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.2 Solar neutrino spectra . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    1.3 Comparison of theory and experiment . . . . . . . . . . . . . . . . . . 7

    1.4 Attempted astrophysical solutions to the solar neutrino problem . . . 8

    1.5 MSW oscillations between three neutrino flavors . . . . . . . . . . . . 16

    1.6 Average survival probability < P >cc for the SNO CC reaction. . . . 17

    1.7 Percentage difference between high and low energy part of the CC

    spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    1.8 Density of the Earth as a function of distance from the center. . . . . 18

    1.9 Day-night asymmetry for the SNO CC reaction. . . . . . . . . . . . . 19

    1.10 A global fit to solar neutrino data, prior to the addition of SNO data. 20

    2.1 Definition of cos . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    2.2 232Th and 238U decay chains. . . . . . . . . . . . . . . . . . . . . . . . 25

    2.3 Schematic of SNO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

    2.4 Event from the 16N source. . . . . . . . . . . . . . . . . . . . . . . . . 31

    2.5 Example of a muon event. . . . . . . . . . . . . . . . . . . . . . . . . 32

    2.6 Example of instrumental noise (a flasher). . . . . . . . . . . . . . . . 32

    2.7 Cumulative livetime for the neutrino data set . . . . . . . . . . . . . 34

    2.8 Month by month livetime for both day and night. . . . . . . . . . . 35

    2.9 The SNO Nhit spectrum for various levels of cuts. . . . . . . . . . . . 36

    2.10 Contamination remaining after instrumental cuts. . . . . . . . . . . . 37

    iv

  • 2.11 Sacrifice from instrumental cuts. . . . . . . . . . . . . . . . . . . . . . 38

    2.12 The high level cuts IJ and IT