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Muon Neutrino to Electron Neutrino Oscillation in Noνa

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Muon Neutrino to Electron Neutrino Oscillation in Noνa Muon Neutrino to Electron Neutrino Oscillation in NOνA A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Kanika Sachdev IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy Prof. Daniel Cronin-Hennessy & Prof. Gregory Pawloski August, 2015 c Kanika Sachdev 2015 ALL RIGHTS RESERVED Acknowledgements I want to begin with thanking my undergraduate teacher, Adarsh Shroff- I would not be a physicist had it not been for your encouragement. Thank you, to my advisers, Daniel Cronin-Hennessy and Gregory Pawloski, for your guidance, support, and patience, and for sharing your intuition in Physics with me. Thank you, Mayly Sanchez and Patricia Vahle, for your trust and encouragement. Thank you, NOνA graduate students at Minnesota- Dominick, Jan, Minerba, Nick and Susan, for helping me learn the ropes early on and teaching me how to stand on my own two feet. Thank you, Gavin Davies, Chris Backhouse and Evan Niner, with whom I shared this nerve-wracking, life-changing and thrilling experience of bringing a new experiment online and extracting the first physics results from it. I wouldn't choose different friends if I had to do it all over again. Thank you, mom, dad, Maggi and Larry, for everything; this journey would be unimaginable without you. i Dedication To my parents, Ravi and Archana, my sister, Maggi, and Larry. ii Abstract NOvA is a long-baseline neutrino oscillation experiment optimized for electron neu- trino (νe) appearance in the NuMI beam, a muon neutrino (νµ) source at Fermilab. It consists of two functionally identical, nearly fully-active liquid-scintillator tracking calorimeters. The near detector (ND) at Fermilab is used to study the neutrino beam spectrum and composition before oscillation, and measure background rate to the νe appearance search. The far detector, 810 km away in Northern Minnesota, observes the oscillated beam and is used to extract oscillation parameters from the data. NOνA's long baseline, combined with the ability of the NuMI beam to operate in the anti- neutrino mode, makes NOνA sensitive to the last unmeasured parameters in neutrino oscillations- mass hierarchy, CP violation and the octant of mixing angle θ23. This thesis presents the search for νe appearance in the first data collected by the NOνA detectors from October 2013 till May 2015. Studies of the NuMI neutrino data collected in the NOνA near detector are also presented, which show large discrepancies between the ND simulation and data. Muon- removed electron (MRE) events, constructed by replacing the muon in νµ charged cur- rent interactions by a simulated electron, are used to correct the far detector νe appear- ance prediction for these discrepancies. In the analysis of the first data, a total of 6 νe candidate events are observed in the far detector on a background of 1, a 3.46 σ excess, which is interpreted as strong evidence for νe appearance. The results are consistent with our expectation, based on constraints from other neutrino oscillation experiments. The result presented here differs from the officially published νe appearance result from the NOνA experiment where the systematic error is assumed to cover the MRE correction. iii Contents Acknowledgementsi Dedication ii Abstract iii List of Tablesx List of Figures xii 1 Neutrino Physics1 1.1 Neutrinos in the Standard Model.....................1 1.1.1 Weak Interaction...........................2 1.1.2 Free Neutrinos............................3 1.2 Extending the Standard Model.......................4 1.2.1 Majorana Mass of Neutrinos....................4 1.3 Neutrino Oscillations.............................6 1.3.1 Oscillations in Vacuum.......................6 1.3.2 Oscillations in Matter........................8 2 Status of Neutrino Oscillation Measurements 12 2.1 Conventions and Nomenclature....................... 12 iv 2.2 Oscillation Parameter Measurements.................... 13 2 2.2.1 θ12 and ∆m21 Measurements.................... 13 2 2.2.2 θ23 and j∆m32j Measurements................... 16 2.2.3 θ13 Measurements.......................... 17 2.3 Unknown Parameters............................. 18 2.3.1 Mass Hierarchy............................ 18 2.3.2 CP Violation............................. 20 2.3.3 θ23 Octant............................... 21 2.4 νµ ! νe on Long Baseline.......................... 21 2.4.1 Experimental Observation...................... 24 3 The NOνA Experiment 25 3.1 The NuMI Beam............................... 27 3.1.1 The Accelerator Complex...................... 30 3.1.2 Slip-Stacking............................. 31 3.1.3 The NuMI Beamline......................... 31 3.2 The NOνA Detector Design......................... 32 3.2.1 The NOνA Unit Cell......................... 32 3.2.2 Liquid Scintillator.......................... 33 3.2.3 Optical Fiber............................. 35 3.2.4 Avalanche Photodiode (APD).................... 36 3.2.5 PVC Modules............................. 37 3.2.6 Detector Assembly.......................... 40 4 Data Acquisition and Triggering Systems 43 4.1 Overview of NOνA DAQ System...................... 43 4.2 Clock and Triggering............................. 44 4.3 Readout.................................... 46 4.3.1 APD Sag and Ringing........................ 47 v 4.4 Timing Peak................................. 48 4.4.1 Far Detector............................. 48 4.4.2 Near Detector............................. 50 5 νe Appearance Analysis Overview 52 5.1 Oscillation Analysis Steps.......................... 52 5.1.1 Blind Analysis............................ 52 5.1.2 Cosmic Background Estimate.................... 53 5.1.3 Beam Background Estimate..................... 54 5.1.4 Extrapolation............................. 55 5.1.5 Final Fit............................... 55 5.2 NOνA's First Data.............................. 55 5.2.1 Treatment of 64 µs Delayed Peak in Pre-Shutdown Data..... 56 5.3 Simple Oscillations.............................. 57 6 Simulation 59 6.1 Particle Simulation.............................. 59 6.1.1 Beam................................. 59 6.1.2 Neutrinos............................... 60 6.1.3 Particles in Detector......................... 61 6.2 Detector Response Simulation........................ 62 6.2.1 Photon Transport.......................... 62 6.2.2 Readout Simulation......................... 63 6.3 Tuning to Data................................ 64 6.3.1 APD Sag Simulation......................... 65 6.3.2 Scintillator Quenching........................ 65 6.3.3 Diblock and Bad Channel Masking................. 68 vi 7 Event and Energy Reconstruction 70 7.1 Calibration.................................. 70 7.1.1 Attenuation Correction....................... 71 7.1.2 Absolute Energy Scale........................ 73 7.1.3 Timing resolution.......................... 74 7.2 Reconstruction Steps............................. 75 7.3 Slicing Algorithm............................... 77 7.4 νe Reconstruction............................... 80 7.4.1 Hough Transform........................... 80 7.4.2 Elastic Arms Vertex......................... 82 7.4.3 Fuzzy-k Prong Reconstruction................... 83 7.4.4 νe Event Energy Reconstruction.................. 85 7.5 νµ Reconstruction Chain........................... 86 8 νe Event Selection 89 8.1 νe Selection Cuts............................... 89 8.1.1 Data Quality Cuts.......................... 89 8.1.2 Preselection and Cosmic Rejection Cuts.............. 92 8.1.3 Performance of Preselection..................... 98 8.2 νe Particle Identification.......................... 101 8.2.1 Likelihood-based Particle Identification.............. 101 8.3 Final Cosmic Background Prediction.................... 108 9 Near Detector Data 112 9.1 Near Detector Data-MC Comparisons................... 113 9.1.1 ND νe Event Selection....................... 113 9.1.2 ND νµ Event Selection........................ 119 9.2 Interpretation................................. 122 9.2.1 Fitting For Interaction Type.................... 123 vii 9.2.2 Fitting For Interaction Mode.................... 125 10 Muon Removed Electron Events 134 10.1 MRE Event Generation........................... 134 10.1.1 Muon Removal............................ 135 10.1.2 Muon and Hadron Energy Disentaglement............ 136 10.1.3 Performance of Muon-Removal................... 139 10.1.4 Insertion of Simulated Electron................... 141 10.2 MRE ND Data-MC Comparison...................... 141 10.2.1 MRE Event Selection........................ 141 10.2.2 MRE with Interaction Mode Scaling................ 145 10.3 MRE Correction On Signal Prediction................... 147 10.3.1 Efficiency by Shower and Hadronic Energy............ 147 11 Decomposition and Extrapolation 155 11.1 Decomposition................................ 156 11.1.1 Proportional Decomposition..................... 156 11.1.2 Decomposition, Upon Scaling by Interaction Type........ 156 11.1.3 Decomposition, Upon Scaling by Interaction Mode........ 156 11.1.4 Summary of Decomposition..................... 157 11.2 Background Extrapolation......................... 157 11.3 Signal Extrapolation............................. 160 11.4 Results of Extrapolation........................... 163 12 Error Estimates 167 12.1 Calibration Error............................... 167 12.2 Interaction Model Uncertainties....................... 169 12.2.1 Beam Flux Errors.......................... 170 12.2.2 Neutrino Interaction Model Uncertainty.............. 171 viii 12.3 Birks Suppresion..............................
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