MINOS Document 4849-v1

A study of muon neutrino disappearance with the MINOS detectors and the NuMI neutrino beam

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John Marshall
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John Marshall
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14 Jul 2008, 13:24
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14 Jul 2008, 13:24
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16 Jan 2009, 16:58
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This thesis presents the results of an analysis of $\nu_{\mu}$ disappearance with the MINOS experiment, which studies the neutrino beam produced by the NuMI facility at Fermi National Accelerator Laboratory. The rates and energy spectra of charged current $\nu_{\mu}$ interactions are measured in two similar detectors, located at distances of 1\,km and 735\,km along the NuMI beamline. The Near Detector provides accurate measurements of the initial beam composition and energy, whilst the Far Detector is sensitive to the effects of neutrino oscillations. The analysis uses data collected between May 2005 and March 2007, corresponding to an exposure of $2.5\times10^{20}$ protons on target.

As part of the analysis, sophisticated software was developed to identify muon tracks in the detectors and to reconstruct muon kinematics. Events with reconstructed tracks were then analysed using a multivariate technique to efficiently isolate a pure sample of charged current $\nu_{\mu}$ events. An extrapolation method was also developed, which produces accurate predictions of the Far Detector neutrino energy spectrum, based on data collected at the Near Detector. Finally, several techniques to improve the sensitivity of an oscillation measurement were implemented, and a full study of the systematic uncertainties was performed.

Extrapolating from observations at the Near Detector, $733\pm29$ Far Detector events were expected in the absence of oscillations, but only 563 events were observed. This deficit in event rate corresponds to a significance of 4.3 standard deviations. The deficit is energy dependent and clear distortion of the Far Detector energy spectrum is observed. A maximum likelihood analysis, which fully accounts for systematic uncertainties, is used to determine the allowed regions for the oscillation parameters and identifies the best fit values as $\Delta m_{32}^{2}=2.29^{+0.14}_{-0.14}\times10^{-3}\text{eV}^{2}$ and $\sin^{2}2\theta_{23}>0.953$ (68\% confidence level). The models of neutrino decoherence and decay are disfavoured at the $5.0\sigma$ and $3.2\sigma$ levels respectively, whilst the no oscillation model is excluded at the $9.4\sigma$ level.

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