Searches for Point-like Sources of Astrophysical Neutrinos with the IceCube Neutrino Observatory By Jacob Feintzeig Adissertationsubmittedinpartialfulfillmentof the requirements for the degree of Doctor of Philosophy (Physics) at the UNIVERSITY OF WISCONSIN–MADISON 2014 Date of final oral examination: August 22, 2014 The dissertation is approved by the following members of the Final Oral Committee: Amy Connolly, Assistant Professor, Physics John S Gallagher, Professor, Astronomy Francis Halzen, Professor, Physics Albrecht Karle, Professor, Physics Dan McCammon, Professor, Physics i ACKNOWLEDGMENTS Iamincrediblyfortunatetohavemanysupportivementorsandpeerswhomadethis work possible. I’d like to first thank my advisor Albrecht for giving me the opportunity to work on IceCube, for providing valuable guidance and advice throughout this project, and for giving me the independence to pursue ideas I found interesting. I’d like to thank Naoko for helping me troubleshoot analysis problems and brainstorm ideas when I was stuck, and providing advice from all issues large to small. Thanks to Chad for encouraging me to think in new ways and approach problems from di↵erent angles. I’d like to express my appreciation for Chris Wendt and Gary Hill for teaching me how to do statistics, dig into the details of the data, and complete a rigorous analysis. Thanks to John Kelley for helping me get to Pole and for teaching me how to do everything once we were there. Thanks to Dima and Juan Carlos for explaining the technical details of reconstruction and simulation in many times of need. Many thanks to Markus for our many valuable physics discussions. Ioweadebtofgratitudetothelargenumberofstudentsandpostdocswhohelpedme debug my code, brainstorm ideas, develop analyses, and o↵ered their support in a myriad of small, invisible ways (not to mention provided entertaining office banter). Thanks Mike, Nathan, Claudio, Marcos, Jakob, Chris, Laura, Ben, Kyle, Moriah, Melanie, Greg, Rameez, Juanan, Kai, Rob, Mike, Leif, Ian, Frank, Zig, Dan, and undoubtedly many others. I’d like to thank my parents, Elise and Irwin, and my siblings, Ben and Rachel, for fostering my interest in science, encouraging me to pursue grad school, and supporting me along the way. Finally, I’d like to thank Deirdre for moving to Madison, o↵ering so much support and encouragement at every step, and now leading the way for the next adventure. DISCARD THIS PAGE ii TABLE OF CONTENTS Page LIST OF TABLES .................................... v LIST OF FIGURES ................................... vi 1 Neutrino Astronomy ................................ 1 1.1 The Origin of the Cosmic Rays . 5 1.2 High-energyAstrophysicalMessengers . 7 1.2.1 Neutrinoproduction . 11 1.3 PotentialSourcesofHigh-EnergyNeutrinos . 12 1.3.1 Galactic Sources . 12 1.3.2 Extragalactic Sources . 14 2 High-Energy Neutrino Detection ........................ 17 2.1 Neutrino-nucleonInteractions . 17 2.2 Detecting Charged-current ⌫µ Interactions . 19 2.3 Detecting Charged-current ⌫e interactions . 21 2.4 Detecting Charged-current ⌫⌧ interactions . 22 2.5 Detecting Neutral-current ⌫x interactions . 23 2.6 Cherenkov Radiation . 23 2.7 Backgrounds to detecting astrophysical neutrinos . 26 3 The IceCube Neutrino Observatory ...................... 27 3.1 Optical Properties of the Glacial Ice . 29 3.2 The Digital Optical Module (DOM) . 31 3.3 DataAcquisition,Triggers,andFilters . 33 3.4 Reconstructing Particle Directions and Energies . 35 3.4.1 LineFit................................... 35 3.4.2 Likelihood-based Angular Reconstruction - SPE and MPE . 36 3.4.3 MuEXAngularReconstruction . 39 3.4.4 ParaboloidAngularUncertaintyEstimator . 40 iii Page 3.4.5 MuEX Energy Reconstruction . 40 3.4.6 Millipede Energy Unfolding - Energy and Direction . 41 3.5 Detector Simulation . 42 4 Detector Calibration ................................ 45 4.1 Verification of the optical properties of the ice using atmospheric muons . 46 4.2 Measuring the DOM efficiency using minimum-ionizing muons . 49 5 Point Source Searches: Introduction ...................... 63 6 Search for Point Sources using Four Years of Throughgoing Muon Data 66 6.1 NeutrinoEventSelection. 66 6.1.1 Preliminary Data Reduction: The Level 3 Filter . 67 6.1.2 FinalEventSelection. 71 6.2 Characteristics and Performance of the Final Event Sample . 83 6.3 PointSourceAnalysisMethod . 85 6.3.1 Likelihood and Test Statistic . 86 6.3.2 Observables used in the Likelihood . 92 6.3.3 Sensitivity and Discovery Potential . 95 6.3.4 HypothesisTestsPerformed . 95 6.3.5 SystematicUncertainties . 97 6.4 Results . 98 7 Search for Point Sources using Three Years of High-Energy Contained- Vertex Event Data .................................. 106 7.1 Introduction . 106 7.2 Event Selection . 107 7.3 Reconstruction . 114 7.4 EvidenceforAstrophysicalOriginoftheEvents . 114 7.5 AnalysisMethod ................................. 117 7.5.1 SearchusingAll-SkyLikelihoodScan . 120 7.5.2 SearchusingMarginalization. 121 7.5.3 Galactic Plane Search . 122 7.5.4 Source List Search . 123 7.6 AnalysisPerformance............................... 124 7.7 Results . 125 iv Page 8 Search for Point Sources using Three Years of Medium-Energy Starting Track Events ..................................... 137 8.1 Introduction . 137 8.2 Event Selection and Performance of the Final Sample . 138 8.3 AnalysisMethodandPerformance . 142 8.4 Results . 148 8.4.1 Discussion on the Starting Track Event at the Hottest Spot . 153 9 Astrophysical Implications of Point Source Results ............. 161 9.1 Constraints on Single Sources . 161 9.1.1 Model-dependent Tests for Specific Sources . 161 9.1.2 Constraints on Hadronic Emission from Gamma-ray Sources . 166 9.2 Constraints on Populations of Sources . 170 9.2.1 Model-independent Constraints on the Total Number of Sources . 171 9.2.2 Constraints on the Density of Uniformly Distributed Sources . 176 10 Conclusion ...................................... 181 LIST OF REFERENCES ............................... 185 DISCARD THIS PAGE v LIST OF TABLES Table Page 4.1 Single muon selection criteria for the DOM efficiency measurement. 54 5.1 Summary of live-time and event numbers for each point source analysis . 65 6.1 Signal and data efficienciesfortheLevel3filter . 71 6.2 Summary for four di↵erent IceCube configurations for point source analyses . 85 6.3 Results for Galactic objects on the a priori search list. 101 6.4 Results for extragalactic objects on the a priori search list. 104 7.1 Properties of the 37 high-energy contained-vertex events . 113 7.2 Point source fit results for 36 Galactic sources . 133 7.3 Point source fit results for 42 extragalactic sources . 135 8.1 Results for objects on the a priori source list for the starting track analysis . 153 DISCARD THIS PAGE vi LIST OF FIGURES Figure Page 1.1 Expected and measured fluxes of neutrinos . 2 1.2 ImageofneutrinoemissionfromtheSun. 4 1.3 The cosmic ray energy spectrum . 6 1.4 Hillas diagram illustrating astrophysical objects capable of containing cosmic rays 8 1.5 Diagram of multi-messenger astronomy . 10 1.6 Electromagnetic observations of Tycho’s supernova remnant . 14 2.1 Feynman diagrams for neutrino-quark interactions . 18 2.2 Neutrino-nucleon (left) and antineutrino-nucleon (right) cross-section as a func- tion of energy . 19 2.3 Muon energy loss as a function of muon energy . 20 2.4 Mean length of charged particles in water as a function of energy. 21 2.5 Diagram of Cherenkov radiation . 24 2.6 Diagram of a cosmic-ray air shower . 25 3.1 DiagramoftheIceCubeNeutrinoObservatory . 28 3.2 Bird’s eye view of IceCube . 28 3.3 Optical properties of the deep glacial ice at the South Pole . 30 3.4 Diagram of a Digital Optical Module (DOM) . 32 3.5 Photon timing distributions for muon angular reconstructions . 39 vii Figure Page 4.1 Diagram of muon hit timing on adjacent DOMs . 47 4.2 DeltaT distribution for muon data. 48 4.3 WidthofdeltaTdistributionsasafunctionofdepth. 49 4.4 DeltaT distribution for SPICE2x . 50 4.5 DeltaT distribution for hole ice models . 50 4.6 DeltaT distribution for di↵erent scattering functions . 51 4.7 DeltaT distribution for SPICE Mie . 51 4.8 DOM angular efficiency curve for di↵erentholeicemodels . 52 4.9 Diagram of track-DOM geometry used for the DOM efficiency analysis . 55 4.10 Distributions of variables for data and atmospheric muon simulation . 56 4.11 Energyandmuonmultiplicityofsimulatedmuons . 57 4.12 Accuracy of reconstructed track-DOM distance and reconstructed muon direction 58 4.13 Histogramofobservedchargesformuon-DOMpairs . 59 4.14 Average observed charge vs. distance from the DOM to the reconstructed muon track.......................................... 60 4.15 The average charge vs. track-DOM distance, normalized to the observed charge in data. 60 4.16 Scaled average charge as a function of the simulated DOM efficiency . 61 5.1 Diagram of event topologies used in each point source analysis . 64 6.1 Charge-weighted average distance for data and simulated signal . 68 6.2 Distributions of eight BDT variables for signal and background in the horizontal region . 77 6.3 DistributionsofBDTscoresfortheupgoingregion . 78 viii Figure Page 6.4 HitdistributionsfortheIceTopsurfaceveto. 79 6.5 Probability distribution function for the energy loss likelihood . 82 6.6 Probability distribution function for the time residual likelihood . 83 6.7 Distributions of BDT scores for the downgoing region . 84 6.8 Median neutrino angular resolution as a function of neutrino energy . 86 6.9 Neutrino e↵ective area and central 90% energy region for simulated