Beam extrapolation and photosensor testing for the T2K experiment

Abstract

Our understanding of the physics of neutrino oscillations has evolved rapidly over the past decade or so, with results from the SNO, Super-K, MINOS and CHOOZ experiments, among others, producing results favouring a three-neutrino mixing model, and significantly constraining the parameter space for this mixing. There are still several important questions to be answered however: we do not know whether Θ13 is non-zero, or whether (sin2 2Θ23) is maximal; also, we do not know the sign of the large mass splitting ΔM2, or whether CP-violation occurs in the lepton sector. The latter is possibly the most exciting of all - leptonic CP- violation is a requirement for leptogenesis, and could therefore indicate a solution to the matter-antimatter asymmetry problem in cosmology. The T2K long-baseline neutrino experiment is one of a new generation of neutrino projects, which will make more precise measurements of Θ13 and Θ23 than has been achieved by previous experiments. It uses the Super-K water Čerenkov detector at Kamioka as a far detector, and also has a suite of new near detectors. These are largely scintillator-based, but use a novel photosensor, the silicon photomultiplier (SiPM), for light readout. T2K has been leading the effort to understand and model these new sensors, and the present work will describe the current state-of-the-art in device characterisation, and also the effort to ensure the quality of the devices installed in the calorimeter of the ND280 near detector. An important part of a long-baseline analysis is the extrapolation of the neutrino flux measured at the near detector to predict that at the far detector. Methods to do this have been developed by previous experiments; however T2K uses an innovative configuration whereby the main detectors are displaced from the neutrino beam centre, removing much of the high-energy tail in the neutrino flux to reduce background from non-quasielastic events. This thesis evaluates the effectiveness of two extrapolation techniques, used by previous experiments, for the T2K configuration