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Tomáš Nosek Study of Neutrino Oscillations at the Nova Experiment

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DOCTORAL THESIS Tom´aˇsNosek Study of Neutrino Oscillations at the NOvA Experiment Institute of Particle and Nuclear Physics Supervisor of the doctoral thesis: RNDr. Karel Soustruˇzn´ık,Ph.D. Study programme: Physics Study branch: Subnuclear physics Prague 2021 I declare that I carried out this doctoral thesis independently, and only with the cited sources, literature and other professional sources. I understand that my work relates to the rights and obligations under the Act No. 121/2000 Sb., the Copyright Act, as amended, in particular the fact that the Charles University has the right to conclude a license agreement on the use of this work as a school work pursuant to Section 60 subsection 1 of the Copyright Act. In ........ date ............ signature of the author i Title: Study of Neutrino Oscillations at the NOvA Experiment Author: Tom´aˇsNosek Institute: Institute of Particle and Nuclear Physics Supervisor: RNDr. Karel Soustruˇzn´ık,Ph.D., Institute of Particle and Nuclear Physics Abstract: Abstract. Keywords: neutrino, neutrino oscillations, ii To the Cat. iii I would like to thank my supervisor Karel Soustruˇzn´ık for the support and opportunity to collaborate with the NOvA experiment. By the same token, I thank to all my NOvA colleagues, especially to those I spent the most time with: Liudmila Kolupaeva, Gavin Davies, Louise Suter, Alex Himmel, Michael Baird, Christopher Backhouse, and Jeremy Wolcott. iv Contents Introduction 1 1 Neutrinos and neutrino oscillation phenomena 3 1.1 Standard Model and neutrinos . .3 1.1.1 Neutrino interactions . .4 1.1.2 Neutrino masses . .5 1.1.3 Lepton mixing . .8 1.1.4 CP symmetry in lepton sector . .9 1.2 Mixing and oscillations in three neutrinos paradigm . .9 1.2.1 UPMNS mixing matrix . 10 1.2.2 Neutrino oscillations in vacuum . 10 1.2.3 Neutrino oscillations in medium . 12 1.3 Experimental evidence of neutrino flavor transitions . 14 1.3.1 Solar neutrinos . 14 1.3.2 Atmospheric neutrinos . 15 1.3.3 Accelerator neutrinos, neutrino beams . 15 1.3.4 Reactor neutrinos . 16 1.4 Status of neutrino oscillation parameters measurements . 17 1.5 Oscillation probabilities for long-baseline experiments . 17 2 The NOvA experiment 20 2.1 Introduction and physics interests . 20 2.2 The NuMI beam . 20 2.3 Off-axis concept . 23 2.4 NOvA detectors . 24 2.4.1 Detectors design . 24 2.4.2 Data acquisition and triggering systems . 25 3 The NOvA neutrino oscillation analysis 27 3.1 Analysis strategy . 27 3.2 Improvements and changes in 2020 analysis . 29 3.3 Data sets . 29 3.4 NOvA simulations . 30 3.4.1 Beam simulation . 30 3.4.2 Neutrino interactions model . 31 3.4.3 Detectors simulation . 32 3.5 Event reconstruction . 35 3.5.1 Event clustering . 36 3.5.2 Vertex reconstruction . 38 3.5.3 Prongs formation . 38 3.5.4 Tracking . 39 3.6 Detectors energy calibration . 40 3.7 Particle identification algorithms . 42 3.7.1 Filters . 42 3.7.2 Reconstructed µ identification . 42 3.7.3 Cosmic rejection . 43 3.7.4 Convolutional visual networks . 43 3.8 Energy estimation . 46 v 3.8.1 νµ energy . 47 3.8.2 νe energy . 47 3.9 Event selection and analysis samples . 49 3.9.1 Basic quality cuts . 49 3.9.2 Preselection cuts . 50 3.9.3 νµ samples . 50 3.9.4 νµ PID selection . 51 3.9.5 νe core sample . 52 3.9.6 νe peripheral sample . 52 3.10 Near detector data constraints and decomposition . 53 3.10.1 Near detector νµ samples . 54 3.10.2 Near detector νe samples . 54 3.11 Near to far extrapolation technique . 59 3.11.1 Extrapolation samples . 61 3.12 Unconstrained prediction components and cosmics . 62 3.13 Far detector predictions . 63 4 Systematic uncertainties 70 4.1 Introduction . 70 4.2 Detectors calibration . 71 4.2.1 Detectors absolute energy scale . 71 4.2.2 Calibration shape . 71 4.2.3 Calibration drift . 71 4.3 Detectors response . 72 4.3.1 Detectors light levels . 72 4.3.2 Cherenkov light production . 72 4.4 Neutron uncertainty . 72 4.5 Neutrino cross sections . 73 4.5.1 Major uncertainties . 75 4.5.2 Inferior uncertainties . 76 4.6 Beam flux . 76 4.7 Lepton reconstruction . 77 4.7.1 Muon energy scale . 78 4.7.2 Primary lepton angle reconstruction . 79 4.8 Near-Far uncorrelated uncertainties . 79 4.8.1 Detectors’ νµ and νe acceptance differences . 79 4.8.2 Michel e tagging . 80 4.8.3 Normalization uncertainties . 81 4.8.4 Cosmic and rock prediction scales . 82 4.9 Summary and notes . 83 5 Results and constraints on neutrino oscillation parameters 84 5.1 Best-fit estimates . 84 5.2 Far detector data . 85 5.3 Overview of systematic uncertainties . 85 5.3.1 Uncertainties on far detector predictions . 86 5.3.2 Uncertainties on neutrino oscillation parameters . 87 5.4 Confidence regions with Feldman-Cousins corrections . 88 5.5 Constraints on neutrino oscillation parameters . 89 Conclusion 103 vi Appendices 104 A Event selections details 105 B Principal component analysis 109 B.1 Overview . 109 B.2 Application in the NOvA analysis . 109 B.2.1 General approach and building the covariance matrix . 110 B.2.2 Cross section uncertainties . 111 B.2.3 Flux uncertainties . 111 C Detailed overview of systematic uncertainties 114 C.1 Disappearance νµ channel, ν-beam . 115 C.2 Disappearanceν ¯µ channel,ν ¯-beam . 125 C.3 Appearance νe channel, ν-beam . 135 C.4 Appearanceν ¯e channel,ν ¯-beam . 143 References 151 List of Figures 165 List of Tables 168 List of Abbreviations & Acronyms 169 Notation 172 vii Introduction Neutrinos are likely the second most abundant of all known particles. There are at least three different neutrino types or “flavors” as identified in (and only in) the charged currentweak interactions: electron, muon, and tau (e, µ, τ). It is a well-established experimental fact that neutrino flavor is not conserved in space-time propagation, and the neutrino flavors “oscillate”. A possible explanation requires neutrino (lepton) mixing and their non-vanishing mass. With the paramount evidence of neutrinos being massive, neutrino oscillation experiments are the foremost witnesses of the physics beyond the Standard Model, at the frontier of the human perception of the universe. Neutrinos have been studied intensively for several decades. The simplest mixing model of three neutrino mass states in three interaction (flavor) states described by three mixing angles (θ12, θ23, θ13), a complex phase, and two differences of squared neutrino masses seems to be very well understood. However, there are still many questions whose answers are of utmost importance to finally provide a satisfactory theory of (elementary) particles. • Are neutrinos their own antiparticles? Are they Majorana or Dirac particles? • Are there lepton number violating processes? At what energy scale? • What is the nature of neutrino masses? Do they fit into the Standard Model framework? • What are the absolute values of neutrino masses? Why are they so tiny compared to other elementary particles? • How many massive neutrinos are there? Is there the same number of massive and active flavor neutrino states? • What is the ordering of the neutrino mass spectrum? Is it “normal” with smaller differences between lighter neutrinos or “inverted” with smaller differences between heavier ones? • Is the CP symmetry violated in the weak leptonic interactions? If so, how much? • Is the mixing model of three massive neutrinos a good effective description of physical reality?.
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