A precision measurement of nu _mu disappearance in the T2K experiment

Thesis

T2K is a long-baseline accelerator neutrino oscillation experiment using the high-intensity ν_μ beam produced at J-PARC. Sitting 295 km away, the giant Super-Kamiokande detector, a 50 kt water tank instrumented with 11,129 photosensitive detectors, sees a narrow band beam peaked at 600 MeV. The baseline to energy ratio is finely tuned for studying neutrino oscillations at the atmospheric neutrino squared-mass splitting. The beam is also sampled 280m downstream of the neutrino production target by a series of finely segmented solid scintillator and time projection chamber detectors. Observing changes in the neutrino beam between the two detectors allows oscillation parameters to be accurately extracted.

A ν_μ-disappearance analysis was performed on the combined T2K Run 1+2+3+4 dataset, corresponding to integrated J-PARC neutrino beam exposure of 6.57x10²⁰ POT, in a framework of three active neutrino flavour oscillations including matter effects in constant-density matter. The observed reconstructed energy spectrum of 1 μ-like ring events was fitted, and separate fits were made for the normal and the inverted mass hierarchies. In these fits, sin² θ₂₃ and either |Δm²₃₂| (normal mass hierarchy) or |Δm²₃₁| (inverted mass hierarchy) were allowed to float. The oscillation parameters sin² θ₁₃, sin² θ₁₂, Δm² ₂₁, δ_CP , and all 41 systematic parameters considered in this analysis were also allowed to float in the fit.

This analysis predicts 445.98±23.46(syst) 1 μ-like ring events in SK in the absence of any oscillation, but only 120 were observed. The observed deficit has a strong energy dependence; the ratio of observed to expected, under the no-oscillation hypothesis, is ∼26% < 0.5 GeV, ∼10% between 0.5 and 1 GeV and ∼72% > 1 GeV.

The 68% confidence intervals on the oscillation parameters can be summarised as sin² θ₂₃ = 0.514 ^+0:049 _-0:050 |Δm²₃₂| = 2.51±0.10 eV²⁄c⁴ for the normal mass hierarchy, and sin² θ₂₃ = 0.511 ^+0:050 _-0:049 |Δm²₃₁| = 2.48±0.10 eV²⁄c⁴ for the inverted mass hierarchy. This is the most precise measurement of sin² θ₂₃ to date.

Authors: Dealtry, T

• Publication date: 2014
• DOI: 10.5287/ora-bxaq8rnkd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: particle physics; neutrino oscillation physics; neutrino physics; T2K
• Subjects: Physics