Improvements for IceCube Event Reconstruction through Geometry Calibration and Photon Timing Distributions

Sammanfattning: The origin of high-energy cosmic rays is an unsolved mystery because cosmic rays do not travel in a straight line but are bent by magnetic fields due to their electric charge. High energy neutrinos are predicted to be created close to these origins, and are an unambiguous probe, while gamma rays could be obscured or created by other processes. The IceCube Neutrino Observatory at the South Pole has instrumented one cubic kilometre of ice to a depth of 2.4 km by deploying digital optical modules (DOMs) in 86 drill holes. As neutrinos interact in the ice, they create secondary particles which generate Cherenkov light. The light is detected by DOMs and given the timing and the location of the DOMs the event can be reconstructed and the direction of the neutrino's origin can be determined.  So far, IceCube has used the original location of the drill towers for the positions of DOMs in the horizontal plane. Previous attempts to determine the x-y coordinates of the DOMs, relying on data from the drill head or in-situ calibration, could not improve on these generic position estimates or reduce their estimated uncertainty below a few metres. This thesis presents a new method for calibrating the positions of the DOMs. A large sample of muon tracks is used as a calibration light source throughout the detector volume, and a maximum likelihood-based approach is used to determine the positions of DOMs. The method has been applied to 18 central strings, and found statistically significant shifts of the order of 1-2 m from nominal positions for four strings. The results led to, and have been corroborated by, a new study using LED flashers,which has also found deviations in other parts of the detector. There are also indications that calibration of individual DOM positions are possible. Modelling how light propagates in the ice is of fundamental importance and over the last decade the knowledge of the ice has progressed and in particular the knowledge on how the birefringent properties of ice impact scattering of light. So in an effort to both improve sensitivity of the geometry calibration above and improve event reconstruction, this thesis also presents work done to better incorporate current knowledge of the glacial ice optical properties when reconstructing muon tracks. There is some evidence that this has resulted in a better description of the light distribution in the detector.