ACTA UNIVERSITATIS UPSALIENSIS Uppsala Dissertations from the Faculty of Science and Technology 143 Alexander Burgman Bright Needles in a Haystack A Search for Magnetic Monopoles Using the IceCube Neutrino Observatory Dissertation presented at Uppsala University to be publicly examined in Polhemsalen, 10134, Uppsala, Wednesday, 3 February 2021 at 13:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor David Milstead (Stockholm University, Stockholm, Sweden). Abstract Burgman, A. 2020. Bright Needles in a Haystack. A Search for Magnetic Monopoles Using the IceCube Neutrino Observatory. (Ljusstarka Nålar i en Höstack. En Sökning efter Magnetiska Monopoler med Neutrino-Observatoriet IceCube). Uppsala Dissertations from the Faculty of Science and Technology 143. 166 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-1083-1. The IceCube Neutrino Observatory at the geographic South Pole is designed to detect the light produced by the daughter-particles of in-ice neutrino-nucleon interactions, using one cubic kilometer of ice instrumented with more than 5000 optical sensors. Magnetic monopoles are hypothetical particles with non-zero magnetic charge, predicted to exist in many extensions of the Standard Model of particle physics. The monopole mass is allowed within a wide range, depending on the production mechanism. A cosmic flux of magnetic monopoles would be accelerated by extraterrestrial magnetic fields to a broad final velocity distribution that depends on the monopole mass. The analysis presented in this thesis constitutes a search for magnetic monopoles with a speed in the range [0.750;0.995] in units of the speed of light. A monopole within this speed range would produce Cherenkov light when traversing the IceCube detector, with a smooth and elongated light signature, and a high brightness. This analysis is divided into two main steps. Step I is based on a previous IceCube analysis, developed for a cosmogenic neutrino search, with similar signal event characteristics as in this analysis. The Step I event selection reduces the acceptance of atmospheric events to lower than 0.1 events per analysis livetime. Step II is developed to reject the neutrino events that Step I inherently accepts, and employs a boosted decision tree for event classification. The (astrophysical) neutrino rate is reduced to 0.265 events per analysis livetime, corresponding to a 97.4 % rejection efficiency for events with a primary energy above 1E+5 GeV. No events were observed at final analysis level over eight years of experimental data. The resulting upper limit on the magnetic monopole flux was determined to 2.54E–19 per square centimeter per second per steradian, averaged over the covered speed region. This constitutes an improvement of around one order of magnitude over previous results. Keywords: magnetic monopole, IceCube, astroparticle physics, neutrino telescope Alexander Burgman, Department of Physics and Astronomy, High Energy Physics, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden. © Alexander Burgman 2020 ISSN 1104-2516 ISBN 978-91-513-1083-1 urn:nbn:se:uu:diva-425610 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-425610) This thesis is dedicated to my children Olivia and Victor, who are all that is best in me. Contents 1 Units and Conventions .............................................................................. 13 1.1 High Energy Physics Natural Units .............................................. 13 1.2 Speed and Relativistic Lorentz Factor .......................................... 13 2 Magnetic Monopoles ................................................................................. 15 2.1 Electric-Magnetic Duality ............................................................. 15 2.2 The Dirac Monopole ...................................................................... 16 2.3 The ’t Hooft-Polyakov Monopole ................................................. 18 2.4 The Cosmic Monopole Population ............................................... 20 2.5 Magnetic Monopole Search Methods ........................................... 21 2.6 Monopole Search Results from Neutrino Telescopes .................. 23 3 The IceCube Neutrino Observatory .......................................................... 26 3.1 The Detector ................................................................................... 26 3.1.1 The Detector Constituent Arrays .................................... 27 3.1.2 The DOM ......................................................................... 29 3.1.3 The Detector Medium ..................................................... 31 3.1.4 Coordinate System .......................................................... 32 3.2 Data Acquisition and Triggering ................................................... 33 3.2.1 Data Filter Stream ............................................................ 34 3.3 Typical Events ................................................................................ 34 3.4 High Energy Neutrinos in IceCube ............................................... 35 3.4.1 Typical Event Signatures ................................................. 36 3.5 Interpreting an Event View ............................................................ 38 4 Magnetic Monopoles in IceCube .............................................................. 40 4.1 Energy Loss in Matter .................................................................... 40 4.2 Light Production ............................................................................. 41 4.2.1 Cherenkov Radiation ....................................................... 42 4.2.2 Indirect Cherenkov Radiation ......................................... 44 4.3 Magnetic Monopole Signatures in IceCube ................................. 45 5 Magnetic Monopole Event Simulation in IceCube ................................. 48 5.1 Magnetic Monopole Generation ................................................... 48 5.1.1 Generation Disk Radius .................................................. 50 5.1.2 Generation Disk Distance ............................................... 51 5.2 Magnetic Monopole Propagation .................................................. 54 5.3 Light Production and Detection .................................................... 54 5.4 Simulation Validation .................................................................... 55 5.4.1 Validation with Experimental Data ................................ 56 5.4.2 Magnetic Monopole Light Yield Validation .................. 58 6 Event Cleaning and Reconstruction Methods .......................................... 60 6.1 Event Cleaning Algorithms ........................................................... 60 6.1.1 The SeededRadiusTime Cleaning Method .................... 60 6.1.2 The TimeWindow Cleaning Method .............................. 61 6.2 Event Reconstruction Algorithms ................................................. 62 6.2.1 A Particle Track Representation ..................................... 62 6.2.2 The LineFit Track Reconstruction Method .................... 62 6.2.3 The EHE Reconstruction Suite ....................................... 63 6.2.4 The CommonVariables Event Characterization Suite ... 66 6.2.5 The Millipede Track Reconstruction Method ................ 67 6.2.6 The BrightestMedian Track Reconstruction Method .... 68 7 Data Analysis and Statistical Tools .......................................................... 72 7.1 Analysis Strategies ......................................................................... 72 7.1.1 Cut-and-Count Analyses ................................................. 72 7.1.2 Multi-Variate Analyses .................................................... 73 7.1.3 Analysis Blindness .......................................................... 73 7.2 Determining an Upper Limit ......................................................... 74 7.2.1 Effective Area .................................................................. 74 7.2.2 Upper Limit ...................................................................... 74 7.2.3 Sensitivity ......................................................................... 75 7.2.4 Including Uncertainties in the Upper Limit ................... 75 7.3 Model Rejection and Discovery Potentials .................................. 76 7.3.1 Model Rejection Potential ............................................... 77 7.3.2 Model Discovery Potential .............................................. 77 7.4 Boosted Decision Trees ................................................................. 78 8 Analysis Structure, Exposure and Assumptions ...................................... 80 8.1 Analysis Structure .......................................................................... 80 8.1.1 Step I ................................................................................ 80 8.1.2 Step II ............................................................................... 80 8.2 Analysis Exposure .......................................................................... 81 8.2.1 Livetime ........................................................................... 81 8.2.2 Solid Angle ...................................................................... 82 8.3 Signal and Background Parameter Space ..................................... 82 8.3.1 Magnetic Monopole Flux Assumptions ......................... 82 8.3.2 Astrophysical Neutrino Flux Assumptions ...................