First Measurement of Neutrino and Antineutrino Oscillation at T2K

Abstract

The T2K experiment is a long-baseline neutrino oscillation experiment, in which a muon neutrino beam is directed over a 295 km baseline from the J-PARC facility to the Super-Kamiokande detector. This allows neutrino oscillation to be studied in two channels: disappearance of ν μ and appearance of ν e . The T2K beam can be run either in neutrino or antineutrino configuration, for a neutrino beam which is predominantly composed of ν μ or anti ¯ν μ respectively. This thesis presents the first oscillation analysis to combine neutrino-mode and antineutrino-mode data sets, giving the first ever sensitivity to the CP-violating phase δ CP from T2K data alone, as well as the most precise T2K measurement of the other neutrino oscillation parameters. The analysis uses a Markov Chain Monte Carlo method to construct a sample from the Bayesian posterior distribution, from which the oscillation parameters of interest and their uncertainties are estimated. Data samples from the T2K near detector, ND280, are fit simultaneously with data from the far detector in order to reduce the uncertainty in the far-detector prediction. When fitting the T2K data alone, the best-fit oscillation parameter values are sin 2 θ 23 = 0.519 +0.031 -0.059 , sin 2 θ 13 = 0.0257 +0.0106 -0.0440 , Δm 2 32 = 2.54 +0.12 -0.10 × 10 -3 eV 2 ,and δ CP =-1.91 rad. The 90p credible interval for δ CP excludes values around π/2: δ CP ∉ [0.38, 2.60] rad. When fitting the T2K data with a constraint on sin 2 θ 13 from measurements by reactor experiments, the best-fit oscillation parameter values are sin 2 θ 23 = 0.519 +0.061 -0.029 , sin 2 θ 13 = +0.0014 -0.0014 , Dm 2 32 = 2.54 +0.12 -0.12 × 10 -3 eV 2 , and δ CP =-1.72 rad. The 90p credible interval for δ CP contains values δ CP ∈ [-3.10,-0.17] rad, excluding the CP-conserving values 0 and ±π at 90p probability.