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Search for Neutrinos from Annihilating Dark Matter in Galaxies and Galaxy Clusters with the Icecube Detector

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Search for Neutrinos from Annihilating Dark Matter in Galaxies and Galaxy Clusters with the Icecube Detector Search for neutrinos from annihilating dark matter in galaxies and galaxy clusters with the IceCube detector Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium ( Dr. rer. nat.) im Fach Physik eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät der Humboldt-Universität zu Berlin von M.Sc. Meike Machieltje de With Präsidentin der Humboldt-Universität zu Berlin: Prof. Dr.-Ing. Sabine Kunst Dekan der Mathematisch-Naturwissenschaftlichen Fakultät: Prof. Dr. Elmar Kulke Gutachter: 1. Prof. Dr. Elisa Bernardini 2. Prof. Dr. Marek Kowalski 3. Prof. Dr. Carlos de los Heros Tag der mündlichen Prüfung: 16.02.2018 iii Abstract The identity of dark matter is one of the largest unsolved problems in modern-day (particle) physics. There is a significant number of observations that cannot be explained using current models and dark matter could be a solution for this problem. However, so far all evidence for dark matter is based on gravitational effects. To shed light on the existence and characteristics of dark matter, many experiments are searching for additional evidence. One method for doing this is to search for particles which could have been produced in self-annihilations of dark matter particles. In this thesis, three years worth of data from the completed IceCube detector is used to search for neutrinos produced in dark matter annihilations in five nearby dwarf galaxies (Segue 1, Ursa Major II, Willman 1, Coma Berenices and Draco), the M31 galaxy and the Virgo cluster. To do this, an event selection which was developed for this analysis is applied to the data sample to reduce the atmospheric background rate from approxi- mately 100 Hz to less than 0.5 mHz. Then, an unbinned maximum likelihood method is used to determine whether there is an excess of neutrinos from the direction of the con- sidered galaxies or galaxy cluster that has an energy spectrum that matches the spectrum expected from dark matter annihilations. For the M31 galaxy and the Virgo cluster an extended signal with a two-dimensional Gaussian shape and width up to 5 degrees is also considered, in addition to the search for a point-like signal. In all cases, the results of the analysis are compatible with the background-only hy- pothesis and limits are set on the velocity-averaged dark matter annihilation cross section for different annihilation channels. For the combination of the five dwarf galaxies, these limits (including systematic uncertainties) go down to 3.9 · 10−22 cm3 s−1 for a dark mat- ter mass of 3 TeV and annihilation to muons. For high dark matter masses there is an excess of neutrinos from three of the five dwarf galaxies. This excess has a global 푝-value of 4.9%, so it is not statistically significant. The search for an extended emission from the direction of the M31 galaxy and the Virgo cluster also did not result in an excess: in both cases the global 푝-value is larger than 50%. The limits on the velocity-averaged dark matter annihilation cross section have im- proved significantly (up to an order of magnitude) with respect to the previous IceCube analysis considering these same targets. This is partially due to general improvements, for example the increased data sample, larger detector and improved direction reconstruc- tion. In addition, there were improvements to this analysis specifically: an improved event selection was used to select the final data sample and an unbinned maximum likelihood method was used for the final analysis instead of a binned analysis method. v Zusammenfassung Die Identität der Dunklen Materie ist eines der größten ungelösten Probleme in der modernen Physik. Es gibt eine große Anzahl von Beobachtungen die mann nicht mit ak- tuellen Modellen erklären kann, und für Dunkle Materie eine Erklärung liefern könnte. Bislang jedoch kommen alle Hinweise für Dunkle Materie aus der Beobachtung von Gra- vitationseffekte. Um Aufschluss zu geben über die Existenz und die Eigenschaften von Dunkler Materie, suchen viele Experimente nach zusätzlichen Hinweisen. Eine Methode dafür besteht darin nach Teilchen zu suchen, die als Resultat von Selbst-Annihilationen von Teilchen der Dunklen Materie produziert worden sein könnten. In dieser Dissertation werden Daten aus drei Jahren vom vollständig fertiggestell- ten IceCube-Detektor benutzt um nach Neutrinos zu suchen , die in Dunkle-Materie- Annihilationen in fünf nahegelegen Zwerggalaxien (Segue 1, Ursa Major II, Willman 1, Coma Berenices und Draco), der M31 Galaxie und dem Virgo Galaxienhaufen produ- ziert wurden. Um die Messung durchzuführen, wurde zunächst eine Ereignis-Selektion angewandt, die es ermöglicht, die von aus der Atmosphäre stammenden Teilchen domi- nierte Rate der Ereignisse von zirka 100 Hz auf 0.5 mHz zu reduzieren. Danach wird eine Maximum-Likelihood-Schätzer eingesetzt um zu bestimmen ob es ein Überschuss von Neutrinos aus der Richtung der jeweiligen Quellen gibt, der mit einen Energie-Spektrum übereinstimmt das mann für Dunkle-Materie-Annihilationen erwartet. Für die M31 Ga- laxie und den Virgo Galaxienhaufen wurde zusätzlich zu dieser Suche nach einer Punkt- quelle auch eine Suche für ein erweitertes Signal mit einer zwei-dimensionalen Gaußschen Verteilung und bis zu fünf Grad Breite durchgeführt. In allen untersuchten Fällen ist das Ergebnis der Analyse vereinbar mit einer Messung der Hintergrund-Hypothese, und daraus wurden Limits für den über die Geschwindig- keit gemittelten Wirkungsquerschnitt für Dunkle-Materie-Annihilation für verschiedene Endprodukte bestimmt. Für die Kombination von den fünf Zwerggalaxien gehen diese Limits (inklusive systematischer Unsicherheiten) hinab bis 3.9 · 10−22 cm3 s−1 für Dunkle Materie mit einer Masse von 3 TeV und einer Annihilation in Myonen. Für hohe Dunkle- Materie-Massen gibt es ein Überschuss von Neutrinos aus drei der Zwerggalaxien. Dieser Überschuss hat einen globalen p-Wert von 4.9% und ist damit nicht statistisch signifikant. Die Suche für ein erweitertes Signal von der M31 Galaxie und dem Virgo Galaxienhaufen ergab keinen Überschuss: in beiden Fallen war der globale 푝-Wert mehr als 50%. Die Limits auf den über die Geschwindigkeit gemittelten Wirkungsquerschnitt für Dunkle-Materie-Annihilation haben sich im Vergleich zu vorherigen IceCube-Analysen signifikant verbessert, um bis zu einer Größenordnung. Diese ist teilweise auf Grundall- gemeine Verbesserungen, wie den großeren Datensatz, großeren Dektor und verbesserte Spurrekonstruktion. Zusätzlich gab es Verbesserungen für diese Analyse besonders: ei- ne verbesserte Ereignis-Selektion um den endgültigen Datensatz zu selektieren, und für die Analyse ist eine Maximum-Likelihood-Schätzer eingesetzt statt eine Analyse in ein Suchfenster. Contents 1. Introduction1 1.1. Dark matter . .1 1.2. This thesis . .2 2. Dark matter3 2.1. Observational evidence for dark matter . .3 2.2. The nature of dark matter . .5 2.2.1. Supersymmetry . .8 2.2.2. Extra dimensions . .8 2.3. Current limits on dark matter . .9 2.3.1. Direct detection . .9 2.3.2. Production at colliders . 10 2.3.3. Indirect detection . 13 2.4. Promising targets for indirect detection of dark matter . 18 2.4.1. Dark matter halo profiles . 19 2.4.2. The Galaxy . 22 2.4.3. Dwarf spheroidal galaxies . 23 2.4.4. Galaxies and galaxy clusters . 25 2.4.5. Sun and Earth . 28 2.5. Neutrinos from dark matter annihilations in dwarf galaxies, the M31 galaxy and the Virgo cluster . 29 2.5.1. Neutrino oscillations . 32 3. Detecting neutrinos from dark matter annihilations 35 3.1. Detection principle . 35 3.1.1. Cherenkov radiation . 35 3.1.2. Neutrino interactions with matter . 36 3.1.3. Muon energy loss . 38 3.2. Backgrounds . 39 3.2.1. Atmospheric muons . 40 3.2.2. Atmospheric neutrinos . 42 3.3. The IceCube Neutrino Observatory . 43 3.3.1. Layout . 43 3.3.2. Hardware and readout . 44 3.3.3. Triggering and filtering . 45 3.3.4. Optical properties of the South Pole ice . 46 3.4. Monte Carlo simulation . 48 3.4.1. Atmospheric muons . 49 3.4.2. Neutrinos . 49 3.4.3. Propagation . 50 3.4.4. Detector simulation . 51 viii Contents 4. Event reconstruction and selection 53 4.1. Event reconstruction . 54 4.1.1. Pulse series cleaning . 54 4.1.2. Track reconstruction . 55 4.1.3. Estimated error of the direction reconstruction . 61 4.1.4. Energy reconstruction . 62 4.2. Event variables . 64 4.2.1. Event direction variables . 65 4.2.2. First HLC hit . 66 4.2.3. Direct hits . 66 4.2.4. Track quality parameters . 66 4.2.5. Event splitting . 67 4.2.6. Hit series properties . 67 4.3. Boosted and Randomized Decision Tree Forests . 68 4.3.1. Decision trees . 68 4.3.2. Decision tree forests . 69 4.3.3. Overtraining and pruning . 70 4.3.4. Optimization . 72 4.4. Event selection . 72 4.4.1. Filters . 73 4.4.2. Good run selection . 76 4.4.3. Preliminary cuts . 77 4.4.4. IceCube-dominated and DeepCore-dominated subsamples . 78 4.4.5. Second level of cuts . 78 4.4.6. Final cut level . 83 4.5. Properties of the final sample . 89 4.5.1. Purity and signal efficiency . 89 4.5.2. Median angular resolution . 90 4.5.3. Comparison of seasons . 91 5. Analysis method 97 5.1. Unbinned maximum likelihood method . 97 5.1.1. Probability density functions . 99 5.2. Recalibration of paraboloid sigma . 101 5.3. Kernel Density Estimation . 105 5.4. Extended sources . 106 5.5. Confidence interval and sensitivity calculation . 107 5.6. Sensitivities . 108 6. Results 115 6.1. Limits . 115 6.2. Extended targets . 121 6.3. Systematic uncertainties . 122 6.4. Comparison to other limits . 122 7. Summary and outlook 129 A. Event variables, IceCube-dominated sample, after preliminary cuts 131 Contents ix B. Event variables, DeepCore-dominated sample, after second level of cuts 137 C.
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