MINOS Document 8704-v1

Measuring nu_mu Disappearance with the MINOS Experiment

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Jess S Mitchell
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Jess S Mitchell
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24 Nov 2011, 10:19
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24 Nov 2011, 10:19
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07 May 2014, 05:40
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The MINOS Experiment consists of two steel-scintillator calorimeters, sampling the long baseline NuMI muon neutrino beam. It was designed to
make a precise measurement of the `atmospheric' neutrino mixing parameters, $\Delta m_{\mathrm{atm.}}^2$ and
$\sin^{2}\left(2\theta_{\mathrm{atm.}}\right)$. The Near Detector measures the initial spectrum of the neutrino beam 1\,km from
the production target, and the Far Detector, at a distance of 735\,km, measures the impact of oscillations in the neutrino energy spectrum.
Work performed to validate the quality of the data collected by the Near Detector is presented as part of this thesis.

This thesis primarily details the results of a $\nu_{\mu}$ disappearance analysis, and presents a new sophisticated fitting software framework,
which employs a maximum likelihood method to extract the best fit oscillation parameters. The software is entirely decoupled from the extrapolation
procedure between the detectors, and is capable of fitting multiple event samples (defined by the selections applied) in parallel, and any
combination of energy dependent and independent sources of systematic error.

Two techniques to improve the sensitivity of the oscillation measurement were also developed. The inclusion of information on the energy resolution
of the neutrino events results in a significant improvement in the allowed region for the oscillation parameters. The degree to which \sintheta\,$=1.0$
could be disfavoured with the exposure of the current dataset if the true mixing angle was non-maximal, was also investigated, with an improved
neutrino energy reconstruction for very low energy events.

The best fit oscillation parameters, obtained by the fitting software and incorporating resolution information were:
$|\Delta m^{2}| = 2.32^{+0.12}_{-0.08} \times 10^{-3}\,\mathrm{e}\mathrm{V}^{2}$ and $\sin^{2}\left(2\theta\right) > 0.90\:\mathrm{(90\%\:C.L.)}$.
The analysis provides the current world best measurement of the atmospheric neutrino mass splitting $\Delta m^{2}$. The alternative models of
neutrino decay and decoherence are disfavoured by $7.8\sigma$ and $9.7\sigma$ respectively.

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