Model-independent search for neutrino sources with the ANTARES neutrino telescope

Language
en
Document Type
Doctoral Thesis
Issue Date
2016-03-24
Issue Year
2016
Authors
Geißelsöder, Stefan
Editor
Abstract

The origin of high energetic cosmic rays has been puzzling since their discovery. Many theories about the sources of these cosmic rays also predict a flux of high energetic cosmic neutrinos. Recently, the existence of such a high energetic neutrino flux has been confirmed, but the location and nature of its sources remains unknown. The ANTARES neutrino telescope was built in the Mediterranean Sea, 40 km off the French coast near Toulon in a depth of 2475 meters to help answer this and other questions. It consists of a three dimensional array of 885 photomultiplier tubes that detect the Cherenkov light emitted by secondary particles, which are produced in interactions between neutrinos and nuclei in the water. The identification and reconstruction of the observed neutrino events constitute challenging tasks. Parts of this thesis deal with algorithmic approaches to improve these tasks using pattern recognition. The first application is the suppression of undesired background by a classification algorithm. The second approach is the selection of the best available direction reconstruction for each neutrino. The main focus of this thesis lies on a new method to evaluate the spatial distribution of the observed neutrinos. While most approaches test one specific hypothesis for a specific source, derived from theory or other measurements, this search refrains from optimizing for individual source hypotheses and tries to detect the most pronounced density fluctuation in the spatial distribution, regardless of its specific position, size, shape or internal distribution as unbiased as possible. To achieve this, the statistical likelihood for the observed neutrino density is evaluated in multiple scales up to distances between events of 180°. To recognize a potential cosmic neutrino signal, regions with the most pronounced deviations are identified and compared to the expectations from a random background hypothesis. The strength of such a flexible, model-independent search is not the sensitivity for a specific source hypothesis, but instead to detect also unexpected hypotheses that can then be analyzed in more detail. In the data recorded from 2007 to 2012 this search found a very large structure close to the direction of the center of our galaxy with a post-trial significance of 2.52σ. It can therefore be explained best by a statistical fluctuation. As a simple crosscheck this method has been applied to a publicly available data sample recorded independently by the neutrino telescope IceCube. This evaluation also resulted in an overfluctuation at the location where the most significant structure from ANTARES data overlaps with the field of view of IceCube. With the devised analysis method the found structure in the IC40 data has a significance of 2.14σ. While this is intriguing, ultimately, a dedicated follow-up analysis that is optimized for the derived hypothesis is necessary to find unambiguous evidence for its true nature. Since, despite further studies, no unambiguous explanation could be found for the obtained results, a follow-up analysis is recommended, that can be adapted specifically to the results and therefore has a higher chance to provide unambiguous insights. Nevertheless this result constitutes the most significant spatially resolved hypothesis for the sources of high energetic astrophysical neutrinos so far.

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