Recent measurements at the T2K experiment indicate that CP violation in neutrino mixing may be observed in the future by long-baseline neutrino oscillation experiments. We explore the physics program ...of an extension to the currently approved T2K running of \(7.8\times 10^{21}\) protons-on-target to \(20\times 10^{21}\) protons-on-target,aiming at initial observation of CP violation with 3\(\,\sigma\) or higher significance for the case of maximum CP violation. With accelerator and beam line upgrades, as well as analysis improvements, this program would occur before the next generation of long-baseline neutrino oscillation experiments that are expected to start operation in 2026.
We report the measurement of muon neutrino charged-current interactions on carbon without pions in the final state at the T2K beam energy using 5.734\(\times10^{20}\) protons on target. For the first ...time the measurement is reported as a flux-integrated, double-differential cross-section in muon kinematic variables (\(\cos\theta_\mu\), \(p_\mu\)), without correcting for events where a pion is produced and then absorbed by final state interactions. Two analyses are performed with different selections, background evaluations and cross-section extraction methods to demonstrate the robustness of the results against biases due to model-dependent assumptions. The measurements compare favorably with recent models which include nucleon-nucleon correlations but, given the present precision, the measurement does not solve the degeneracy between different models. The data also agree with Monte Carlo simulations which use effective parameters that are tuned to external data to describe the nuclear effects. The total cross-section in the full phase space is \(\sigma = (0.417 \pm 0.047 \text{(syst)} \pm 0.005 \text{(stat)})\times 10^{-38} \text{cm}^2\) \(\text{nucleon}^{-1}\) and the cross-section integrated in the region of phase space with largest efficiency and best signal-over-background ratio (\(\cos\theta_\mu>0.6\) and \(p_\mu > 200\) MeV) is \(\sigma = (0.202 \pm 0.0359 \text{(syst)} \pm 0.0026 \text{(stat)}) \times 10^{-38} \text{cm}^2\) \(\text{nucleon}^{-1}\).
The Charged-Current Quasi-Elastic (CCQE) interaction, \(\nu_{l} + n \rightarrow l^{-} + p\), is the dominant CC process at \(E_\nu \sim 1\) GeV and contributes to the signal in accelerator-based ...long-baseline neutrino oscillation experiments operating at intermediate neutrino energies. This paper reports a measurement by the T2K experiment of the \(\nu_{\mu}\) CCQE cross section on a carbon target with the off-axis detector based on the observed distribution of muon momentum (\(p_\mu\)) and angle with respect to the incident neutrino beam (\(\theta_\mu\)). The flux-integrated CCQE cross section was measured to be \((0.83 \pm 0.12) \times 10^{-38}\textrm{ cm}^{2}\) in good agreement with NEUT MC value of \({0.88 \times 10^{-38}} \textrm{ cm}^{2}\). The energy dependence of the CCQE cross section is also reported. The axial mass, \(M_A^{QE}\), of the dipole axial form factor was extracted assuming the Smith-Moniz CCQE model with a relativistic Fermi gas nuclear model. Using the absolute (shape-only) \(p_{\mu}cos\theta_\mu\) distribution, the effective \(M_A^{QE}\) parameter was measured to be \({1.26^{+0.21}_{-0.18} \textrm{ GeV}/c^{2}}\) (\({1.43^{+0.28}_{-0.22} \textrm{ GeV}/c^{2}}\)).
We report a measurement of the \(\nu_\mu\) charged current quasi-elastic cross-sections on carbon in the T2K on-axis neutrino beam. The measured charged current quasi-elastic cross-sections on carbon ...at mean neutrino energies of 1.94 GeV and 0.93 GeV are \((11.95\pm 0.19(stat.)_{-1.47}^{+1.82} (syst.))\times 10^{-39}\mathrm{cm}^2/\mathrm{neutron}\) and \((10.64\pm 0.37(stat.)_{-1.65}^{+2.03} (syst.))\times 10^{-39}\mathrm{cm}^2/\mathrm{neutron}\), respectively. These results agree well with the predictions of neutrino interaction models. In addition, we investigated the effects of the nuclear model and the multi-nucleon interaction.
The T2K experiment has performed a search for \(\nu_e\) disappearance due to sterile neutrinos using \(5.9 \times 10^{20}\) protons on target for a baseline of \(280 m\) in a neutrino beam peaked at ...about \(500 MeV\). A sample of \nu_e CC interactions in the off-axis near detector has been selected with a purity of 63\% and an efficiency of 26\%. The p-value for the null hypothesis is 0.085 and the excluded region at 95\% CL is approximately \(sin^2 2 \theta_{ee} > 0.3\) for \(\Delta m^2_{eff} > 7 eV^2 / c^4\).
We report the first measurement of the neutrino-oxygen neutral-current quasielastic (NCQE) cross section. It is obtained by observing nuclear deexcitation \(\gamma\)-rays which follow neutrino-oxygen ...interactions at the Super-Kamiokande water Cherenkov detector. We use T2K data corresponding to \(3.01 \times 10^{20}\) protons on target. By selecting only events during the T2K beam window and with well-reconstructed vertices in the fiducial volume, the large background rate from natural radioactivity is dramatically reduced. We observe 43 events in the \(4-30\) MeV reconstructed energy window, compared with an expectation of 51.0, which includes an estimated 16.2 background events. The background is primarily nonquasielastic neutral-current interactions and has only 1.2 events from natural radioactivity. The flux-averaged NCQE cross section we measure is \(1.55 \times 10^{-38}\) cm\(^2\) with a 68\% confidence interval of \((1.22, 2.20) \times 10^{-38}\) cm\(^2\) at a median neutrino energy of 630 MeV, compared with the theoretical prediction of \(2.01 \times 10^{-38}\) cm\(^2\).
We report a measurement of the \(\nu_\mu\) inclusive charged current cross sections on iron and hydrocarbon in the T2K on-axis neutrino beam. The measured inclusive charged current cross sections on ...iron and hydrocarbon averaged over the T2K on-axis flux with a mean neutrino energy of 1.51 GeV are \((1.444\pm0.002(stat.)_{-0.157}^{+0.189}(syst.))\times 10^{-38}\mathrm{cm}^2/\mathrm{nucleon}\), and \((1.379\pm0.009(stat.)_{-0.147}^{+0.178}(syst.))\times 10^{-38}\mathrm{cm}^2/\mathrm{nucleon}\), respectively, and their cross section ratio is \(1.047\pm0.007(stat.)\pm0.035(syst.)\). These results agree well with the predictions of the neutrino interaction model, and thus we checked the correct treatment of the nuclear effect for iron and hydrocarbon targets in the model within the measurement precisions.
