ACTA UNIVERSITATIS UPSALIENSIS Uppsala Dissertations from the Faculty of Science and Technology 117 Exploring the Universe Using Neutrinos A Search for Point Sources in the Southern Hemisphere Using the IceCube Neutrino Observatory Rickard Ström Dissertation presented at Uppsala University to be publicly examined in Ångströmlaboratoriet, Polhemsalen, Lägerhyddsvägen 1, Uppsala, Friday, 18 December 2015 at 13:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Lawrence R. Sulak (Boston University, Boston, USA). Abstract Ström, R. 2015. Exploring the Universe Using Neutrinos. A Search for Point Sources in the Southern Hemisphere Using the IceCube Neutrino Observatory. Uppsala Dissertations from the Faculty of Science and Technology 117. 254 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9405-6. Neutrinos are the ideal cosmic messengers, and can be used to explore the most powerful accelerators in the Universe, in particular the mechanisms for producing and accelerating cosmic rays to incredible energies. By studying clustering of neutrino candidate events in the IceCube detector we can discover sites of hadronic acceleration. We present results on searches for point- like sources of astrophysical neutrinos located in the Southern hemisphere, at energies between 100 GeV and a few TeV. The data were collected during the first year of the completed 86- string detector, corresponding to a detector livetime of 329 days. The event selection focuses on identifying events starting inside the instrumented volume, utilizing several advanced veto techniques, successfully reducing the large background of atmospheric muons. An unbinned maximum likelihood method is used to search for clustering of neutrino-like events. We perform a search in the full Southern hemisphere and a dedicated search using a catalog of 96 pre- defined known gamma-ray emitting sources seen in ground-based telescopes. No evidence of neutrino emission from point-like sources is found. The hottest spot is located at R.A. 305.2° and Dec. -8.5°, with a post-trial p-value of 88.1%. The most significant source in the a priori list is QSO 2022-077 with a post-trial p-value of 14.8%. In the absence of evidence for a signal, we calculate upper limits on the flux of muon-neutrinos for a range of spectra. For an unbroken E-2 neutrino spectrum, the observed limits are between 2.8 and 9.4×10-10 TeV cm-2 s-1, while for an E-2 neutrino spectrum with an exponential cut-off at 10 TeV, the observed limits are between 0.6 and 3.6×10-9 TeV cm-2 s-1. Keywords: astroparticle physics, neutrino sources, neutrino telescopes, IceCube Rickard Ström, Department of Physics and Astronomy, High Energy Physics, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden. © Rickard Ström 2015 ISSN 1104-2516 ISBN 978-91-554-9405-6 urn:nbn:se:uu:diva-265522 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-265522) “The probability of success is difficult to estimate but if we never search the chance of success is zero.” Cocconi and Morrison [1] Table of Contents Page Acknowledgments .............................................................................................. 9 About this Thesis ............................................................................................. 13 Abbreviations and Acronyms .......................................................................... 17 1 Introduction ................................................................................................ 23 1.1 Physics Motivation ......................................................................... 24 1.2 The Multi-Messenger Approach ................................................... 26 2 The Standard Model .................................................................................. 29 2.1 Matter Particles and Force Carriers .............................................. 30 2.2 Interaction Strengths ...................................................................... 32 2.3 Symmetries ..................................................................................... 33 2.4 The Weak Interaction ..................................................................... 35 2.5 The Higgs Mechanism ................................................................... 37 2.6 Fermion masses .............................................................................. 38 2.7 The Parameters of the Standard Model ........................................ 39 2.8 Beyond the Standard Model .......................................................... 40 3 The Cosmic Ray Puzzle ............................................................................ 47 3.1 Energy Spectrum ............................................................................ 50 3.2 The Origin of High-Energy Cosmic Rays .................................... 53 3.3 Astrophysical Neutrinos ................................................................ 55 3.4 Atmospheric Backgrounds ............................................................ 60 3.5 Acceleration Mechanisms ............................................................. 63 3.6 Potential Acceleration Sites ........................................................... 67 4 Neutrino Detection Principles ................................................................... 75 4.1 Neutrino Cross-Section .................................................................. 76 4.2 Cherenkov Radiation ..................................................................... 78 4.3 Energy Loss Mechanisms .............................................................. 80 4.4 Event Topologies ............................................................................ 82 5 The IceCube Neutrino Observatory .......................................................... 85 5.1 The IceCube In-Ice Detector ......................................................... 88 5.2 Optical Properties of the Ice at the South Pole ............................ 90 5.3 The Digital Optical Module .......................................................... 93 5.4 Data Acquisition System and Triggering ..................................... 96 5.5 Processing and Filtering ................................................................ 97 6 Event Simulation ....................................................................................... 99 6.1 The IceCube Simulation Chain ..................................................... 99 6.2 Neutrino Event Weighting ........................................................... 104 7 Reconstruction Techniques ..................................................................... 107 7.1 Noise Cleaning ............................................................................. 107 7.2 Particle Direction ......................................................................... 108 7.3 Angular Uncertainty .................................................................... 118 7.4 Interaction Vertex ......................................................................... 121 7.5 Energy ........................................................................................... 124 8 Opening Up a Neutrino Window to the Southern Hemisphere ............ 129 8.1 The FSS Filter .............................................................................. 132 9 Point Source Analysis Methods .............................................................. 139 9.1 Hypothesis Testing ....................................................................... 139 9.2 Likelihood and Test Statistic ....................................................... 141 10 A Search for Low-Energy Starting Events from the Southern Hemisphere .............................................................................................. 147 10.1 Analysis Strategy ......................................................................... 149 10.2 Experimental Data ....................................................................... 151 10.3 Simulated Data ............................................................................. 154 10.4 Event Selection ............................................................................. 154 10.5 Event Selection Summary ........................................................... 189 10.6 Likelihood Analysis ..................................................................... 192 10.7 Results ........................................................................................... 201 10.8 Systematic Uncertainties ............................................................. 209 11 Summary and Outlook ............................................................................ 213 Summary in Swedish ..................................................................................... 217 Appendix A: BDT Input Variables ............................................................... 223 Appendix B: Result Tables ............................................................................ 227 References ...................................................................................................... 235 Acknowledgements During my time as a graduate student at Uppsala University I have had the privilege to work with many dedicated, helpful and experienced scientists. We’ve organized meetings and workshops, had lengthy discussions on pecu- liar features of fundamental physics, and had a lot of fun during lunches and fika breaks, discussing everything from foreign and domestic politics to candy