Beam extrapolation and photosensor testing for the T2K experiment

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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.

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