Measurement of the energy spectrum of astrophysical muon-neutrinos with the IceCube Observatory

The acceleration of high-energetic cosmic rays belongs to the fundamental open questions of modern physics. In order to study the mechanisms behind it and the astrophysical environments which provide the enormous power required for the acceleration, it is crucial to use complementary measurements, i.e. to investigate the fluxes of different messenger particles that reach the Earth. Since their first discovery in 2013, high-energetic astrophysical neutrinos have been established as additional messengers. In this thesis, a measurement of the cumulative flux of these neutrinos and of their energy spectrum is presented. Data has been collected with the IceCube Neutrino Observatory, which instruments approximately one cubic kilometer of glacial ice deep below the South Pole. Using more than 650.000 observed muon-track events from nearly ten years of operation, an improved measurement of the astrophysical muon-neutrino flux has been performed: The increased statistics compared to previous publications (' 2 ) and an improved treatment of systematic uncertainties lead to a more precise measurement of the astrophysical flux properties. The observed energy spectrum can be described by a power-law with normalization +¯ @100TeV = 1.36+0.24 -0.25 10-18 GeV-1cm-2s-1sr-1 and spectral index SPL = -2.37+0.08 -0.09. Additionally, a wide range of other parameterizations for the energy spectrum of the astrophysical neutrinos have been tested, including model-independent approaches to enable an easy comparison to theory predictions and other measurements. These tests show first hints for spectral features beyond the single power-law: The experimental data is better described by parameterizations for the astrophysical neutrino spectrum with a changing slope, i.e. with a steeper spectrum at highest energies.