UHE neutrino detection using the lunar Čerenkov technique

© 2009 Dr. Rebecca McFaddenOther title: UHE neutrino detection using the Lunar Cherenkov techniqueThis thesis investigates Ultra High Energy (UHE) neutrino detection using the lunar Čerenkov technique. UHE neutrinos may hold the key to understanding the origin of the most energetic particles observed in nature, the UHE Cosmic Rays (CR). UHECR traveling over distances larger than ∼80 Mpc will lose energy to photo-pion production, causing a suppression of the cosmic ray spectrum at the highest energies. However, significant information is preserved in the spectrum of neutrinos produced as a result of these interactions and UHE neutrino astronomy may therefore be able to provide more insight into the origin of the UHECR. Direct detection of UHE neutrinos is very difficult due to their extremely small interaction cross-sections. Instead, they may be detected indirectly via observation of the Askaryan effect (1) in the lunar regolith. Using the Moon as a large volume neutrino detector, coherent radio Čerenkov emission from neutrino-induced cascades in the lunar regolith can be observed with ground-based telescopes. This thesis explores detection issues associated with using this technique including an investigation into alternative planetary detectors, the phenomenology of the radio Čerenkov emission and the effect of ionospheric dispersion on Čerenkov pulse propagation. The results of this investigation were used to design a detector system for a series of experiments performed at the Australia Telescope Compact Array. The experiments made use of a 600-MHz RF signal available at the ATCA which required the development of custom-designed ionospheric dedispersion and pulse detection hardware. Approximately 36.5 hours of lunar data were taken over three observations. Earlier runs were scheduled to target a broad region surrounding the galactic centre, which was chosen as it harbors the closest super massive black hole and potential accelerator of UHECR. Scheduling of the final ATCA experiment in May 2008 was influenced by results published by the Pierre Auger Observatory in late 2007, which showed a statistical correlation between observations of the highest energy CR and the matter distribution in the local universe as represented by nearby active galactic nuclei (2). These experiments offered an increase in exposure to the regions of Sagittarius A* and Centaurus A. However, analysis of the data revealed that most of the candidate events could be attributed to terrestrial RFI and no candidates events were suspected as potential lunar Čerenkov pulses. A new method to calibrate the dispersive effect of the ionosphere on lunar Čerenkov pulses is also presented. This method exploits radial symmetries of the lunar polarisation distribution to make Faraday rotation measurements in the visibility domain of synthesis array data. The Faraday rotation measurements are combined with geomagnetic field models to estimate the ionospheric Total Electron Content (TEC). An accurate knowledge of the ionospheric TEC can be used to perform pulse dedispersion and recover maximum Čerenkov pulse amplitude before detection. This method of ionospheric calibration is particularly attractive for the lunar Čerenkov technique as it can be used in real time to give values of the ionospheric TEC which are line-of-sight to the Moon