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Institute
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.
The IceCube neutrino observatory is the largest operating neutrino telescope at the moment.
It consists of 5160 Digital Optical Modules (DOMs) on 86 vertical strings buried in a
depth of 1.5km to 2.5km within the Antartic ice instrumenting a volume of approximately
1 cubic km . An upgrade of the in-ice array to a volume of almost 10 cubic km, called IceCube Gen2 high-
energy array, is the subject of current research. The multi photomultiplier (PMT) Digital Optical
Module (mDOM), which consists of 24 symmetrically distributed 3-inch PMTs, is considered
as detection unit for the Gen2 high-energy array. Alternatively, an upgraded version of the
IceCube DOM, called PINGU Digital Optical Module (PDOM), containing only one 10-inch PMT
facing downwards, is also considered as detection unit. This work analyzes the effect of the
sensor segmentation of the mDOM on the angular resolution of through-going muon tracks in
comparison to the angular resolution obtained with the PDOM within the context of a Gen2
high-energy array geometry.
In order to eliminate the effect of different photon detection efficiencies of the two sensor designs,
the quantum efficiencies of the respective PMTs are scaled in the simulation to ensure an
equalized effective photocathode area per module. For down-going and horizontal through-going
muons with an energy between 3TeV and 70PeV a detector equipped with mDOMs yields between
10% and 40% better angular resolution in almost all energy regimes after sensor-independent
quality cuts (based on Monte Carlo information) have been applied.
For up-going muons with energies below 1PeV the upscaled PDOM yields between 7% and 13% lower angular errors.
Finally, estimations of the 90% exclusion limits and the 5σ discovery fluxes of neutrino point
sources are conducted for both sensors in a Gen2 high-energy array.
For sources with a declination below 5° the upper limits and discovery fluxes obtained with the mDOM are 8 − 11% lower.
The upscaled PDOM leads to 4 − 12% lower exclusion limits and discovery fluxes for sources with a declination above 33°.
Inspired by the discovery of a comogenic high-energy neutrino flux in IceCube, plans are under way for the construction of new neutrino observatory at the South pole. This next-generation neutrino telescope, IceCube-Gen2, will expand the sensitivity range to lower as well as towards higher neutrino energies with respect to the currently operating IceCube detector.
While the compact and densely instrumented low-energy component will, amongst other questions, address neutrino mass hierarchy, the large-volume high-energy array primarily aims at the first detection of neutrino point sources.
A substantial increase of detector sensitivity is expected from the employment of novel optical sensors. One of the most auspicious of the new designs is based on the multi-PMT concept first introduced in the KM3NeT detector. As indicated by its name, a multi-PMT optical module houses, inside a transparent pressure vessel, an array of small-size PMTs. The effective segmentation of the sensitive area entails some attractive advantages compared to the conventional layout featuring a single large-area PMT. Most prominent among those advantages are an increased effective area uniformly covering the solid angle, an extended dynamic range, as well as intrinsic directional sensitivity of the module.
Within the scope of this thesis, the development of a dedicated multi-PMT Digital Optical Module for IceCube-Gen2 has been advanced from the original ideas to a stage where the realization of a first fully featured and working prototype is expected within a year. The reported work includes testing and detailed characterization of key hardware components, simulation studies concerning pressure stability and performance capabilities, as well as first steps towards the construction of a prototype of the novel module.