ABSTRACT Title of Document: THE SEARCH FOR NEUTRALINO DARK MATTER WITH THE AMANDA NEUTRINO TELESCOPE Ralf Ehrlich, Doctor of Philosophy, 2009 Directed By: Professor Gregory Sullivan Department of Physics There is convincing indirect evidence based on cosmological data that approximately one quarter of the universe is made of dark matter. However, to this date there is no direct detection of the dark matter and its nature is unknown. Most theories suggest that this dark matter is made of Weakly Interacting Massive Particles (WIMPs), or more specifically: supersymmetric particles. The most promising candidate out of the supersymmetric particles is the lightest neutralino. These neutralinos can get trapped in the gravitational field of the Earth, where they accumulate and annihilate. The annihilation products decay and produce neutrinos (among other particles). These neutrinos (the focus is on muon-neutrinos here) can be detected with the AMANDA neutrino telescope located between one and two kilometers deep in the ice of the glacier near the South Pole. Neutrinos cannot be detected directly. However, there is a small possibility that they interact with nuclei of the ice and create charged leptons. These charged leptons continue to travel in the same direction as the neutrinos (accompanied by electromagnetic/hadronic cascades, and δ electrons). As long as their speed is higher than the speed of light of the ice, they emit Cherenkov radiation which can be captured by photomultipliers installed inside the ice. The signals collected by the photomultipliers can be used to reconstruct the track of the lepton. AMANDA – the Antarctic Muon and Neutrino Detector Array – makes use of the unique properties of the neutrino: Since neutrinos interact only weakly, they can travel through the Earth without being stopped. Therefore all detected particles which have been identified as upward going (i.e. through the Earth coming) must have been produced by charged leptons originating from neutrinos after they reacted with the nuclei of the ice. All other particles which do not come from below are rejected. If the neutrino flux coming from the neutralino annihilation inside Earth is strong enough to be detected with AMANDA, it should show up as an excess over the expected neutrino flux, which comes from the atmospheric neutrinos produced in the northern hemisphere. This analysis which used data from 2001 and 2002 showed that there is no significant excess, yielding an upper limit on the neutrino flux that could have come from WIMP annihilation. THE SEARCH FOR NEUTRALINO DARK MATTER WITH THE AMANDA NEUTRINO TELESCOPE By Ralf Ehrlich Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park, in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2009 Advisory Committee: Professor Gregory Sullivan, Chair Professor Kara Hoffman Professor Rabindra Mohapatra Professor Sarah Eno Professor Coleman Miller © Copyright by Ralf Ehrlich 2009 Dedication Für Mama und Papa To Eddie ii Content Dedication........................................................................................................................... ii Content...............................................................................................................................iii List of Tables ...................................................................................................................... v List of Figures.................................................................................................................... vi Chapter 1: Introduction....................................................................................................... 1 Chapter 2: Astrophysical Motivations ................................................................................ 3 2.1 The dark matter problem........................................................................................... 3 2.2 Relic abundance of the neutralinos......................................................................... 14 Chapter 3: Particle Physics Motivation............................................................................. 20 3.1 Why do we need supersymmetry? .......................................................................... 20 3.2 The Wess-Zumino model........................................................................................ 23 3.3 Supermultiplets ....................................................................................................... 28 3.4 Minimal Supersymmetric Standard Model (MSSM).............................................. 33 3.5 Mass eigenstates of the neutralinos......................................................................... 37 Chapter 4: General concepts of the Neutralino detection ................................................. 42 4.1 Neutralino capture rates .......................................................................................... 42 4.2 Neutralino annihilation rates................................................................................... 49 4.3 Neutrino fluxes from neutralino annihilations........................................................ 53 4.4 Other detection methods ......................................................................................... 56 Chapter 5: The experiment................................................................................................ 60 5.1 The idea behind the AMANDA detector................................................................ 60 5.2 General remarks about neutrinos ............................................................................ 63 5.3 Background neutrinos ............................................................................................. 64 5.4 Other background particles..................................................................................... 67 5.5 The detection of neutrinos ...................................................................................... 68 5.6 Muon propagation in the ice ................................................................................... 72 5.7 Cherenkov Radiation .............................................................................................. 74 5.8 Photomultiplier Tubes............................................................................................. 76 5.9 AMANDA’s hardware and DAQ ........................................................................... 77 5.9.1 Outlook for IceCube ........................................................................................ 83 Chapter 6: Data Processing............................................................................................... 85 6.1 Calibration............................................................................................................... 85 6.2 Hit Cleaning............................................................................................................ 86 6.3 Retriggering ............................................................................................................ 88 6.4 Reconstructions – First guess methods................................................................... 88 6.5 Reconstructions – Maximum Likelihood method................................................... 90 Chapter 7: The initial analysis .......................................................................................... 98 7.1 Data Sets ................................................................................................................. 99 7.1.1 Experimental Data ........................................................................................... 99 7.1.2 Simulated Data............................................................................................... 100 7.2 Overview of the analysis....................................................................................... 103 7.3 Data Filtering ........................................................................................................ 104 7.3.1 L1/L2.............................................................................................................. 104 7.3.2 Initial zenith angle cut.................................................................................... 105 7.3.3 Flare checking................................................................................................ 105 iii 7.4 Analysis................................................................................................................. 107 7.4.1 Cut Parameters............................................................................................... 107 7.4.2 Upper Limit.................................................................................................... 113 7.4.3 Model rejection factor.................................................................................... 115 7.4.4 Finding the cut settings .................................................................................. 117 7.4.5 Some comments regarding previous unblinding proposals ........................... 123 Chapter 8: Unblinding..................................................................................................... 127 8.1 Error Calculations ................................................................................................. 129 8.1.1 Systematic uncertainties of the background .................................................. 129 8.1.2 Statistical uncertainties