## First measurement of the νμ charged-current cross section without pions in the final state on a water target

We report the first differential measurement of the charged-current interaction cross section of νμ on water with no pions in the final state. This flux-averaged measurement has been made using the T2K experiment’s off-axis near detector, and is reported in doubly-differential bins of muon momentum and angle. The flux-averaged total cross section in a restricted region of phase space was found to be σ=(0.95±0.13)×10−38cm2 per n.

arXiv:1708.06771 [hep-ex].

## (English) Measurement of neutrino and antineutrino oscillations by the T2K experiment, including a new additional sample of electron neutrino interactions in the far detector

The T2K experiment reports an updated analysis of neutrino and antineutrino oscillations in appearance and disappearance channels. A sample of electron neutrino candidates at Super-Kamiokande in which a pion decay has been tagged is added to the four single-ring samples used in previous T2K oscillation analyses. Through combined analyses of these five samples, simultaneous measurements of four oscillation parameters, |Δm^{2}_{32}|, sin^{2}(θ_{23}), sin^{2}(θ_{13}) and δ_{CP}, and of the mass ordering are made. A set of studies of simulated data indicates that the sensitivity to the oscillation parameters is not limited by neutrino interaction model uncertainty. Multiple oscillation analyses are performed, and frequentist and Bayesian intervals are presented for combinations of the oscillation parameters with and without the inclusion of reactor constraints on sin^{2}(θ_{13}). When combined with reactor measurements, the hypothesis of CP conservation (δ_{CP} = 0 or π) is excluded at the 90% confidence level. The 90% confidence region for δ_{CP} is [–2.95,–0.44] ([–1.47,–1.27]) for normal (inverted) ordering. The central values and 68% confidence intervals for the other oscillation parameters for normal (inverted) ordering are |Δm^{2}_{32}| = 2.54 ± 0.08 (2.51 ± 0.08) × 10^{–3} eV^{2}/c^{4} and sin^{2}(θ_{23}) = 0.55_{–0.09}^{+0.05} (0.55_{–0.08}^{+0.05}), compatible with maximal mixing. In the Bayesian analysis, the data weakly prefer normal ordering (Bayes factor 3.7) and the upper octant for sin^{2}(θ_{23}) (Bayes factor 2.4).

ArXiv preprint: 1707.01048 [hep-ex]

## Search for Lorentz and CPT violation using sidereal time dependence of neutrino flavor transitions over a short baseline

A class of extensions of the Standard Model allows Lorentz and CPT violations, which can be identified by the observation of sidereal modulations in the neutrino interaction rate.

A search for such modulations was performed using the T2K on-axis near detector. Two complementary methods were used in this study, both of which resulted in no evidence of a signal. Limits on associated Lorentz and CPT violating terms from the Standard Model Extension have been derived taking into account their correlations in this model for the first time. These results imply such symmetry violations are suppressed by a factor of more than 10^{20} at the GeV scale.

ArXiv preprint: https://arxiv.org/abs/1703.01361

## Updated measurements of muon neutrino and antineutrino disappearance

English:

We report measurements by the T2K experiment of the parameters θ_{23} and ∆m^{2}_{32} governing the disappearance of muon neutrinos and antineutrinos in the three flavor neutrino oscillation model. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, the parameters are measured separately for neutrinos and antineutrinos. Using 7.482 × 10^{20} POT in neutrino running mode and 7.471 × 10^{20} POT in antineutrino mode, T2K obtains sin^{2}(θ_{23}) = 0.51^{+0.08}_{−0.07} and ∆m^{2}_{32} = 2.53^{+0.15}_{−0.13} × 10^{−3 }eV^{2}/c^{4} for neutrinos, and sin^{2}(θ̅_{23}) = 0.47^{+0.25}_{−0.07} and ∆m̅^{2}_{32} = 2.55^{+0.33}_{−0.27} × 10^{−3} eV^{2}/c^{4} for antineutrinos (assuming normal mass ordering). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed.

Top panels show the reconstructed energy distribution of the 135 far detector ν_{μ}-CCQE candidate events (left) and 66 ν̅_{μ}-CCQE candidate events (right), with predicted spectra for best fit and no oscillation cases. Bottom panels show the ratio to unoscillated predictions.

日本語:

We report measurements by the T2K experiment of the parameters θ_{23} and ∆m^{2}_{32} governing the disappearance of muon neutrinos and antineutrinos in the three flavor neutrino oscillation model. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, the parameters are measured separately for neutrinos and antineutrinos. Using 7.482 × 10^{20} POT in neutrino running mode and 7.471 × 10^{20} POT in antineutrino mode, T2K obtains sin^{2}(θ_{23}) = 0.51^{+0.08}_{−0.07} and ∆m^{2}_{32} = 2.53^{+0.15}_{−0.13} × 10^{−3 }eV^{2}/c^{4} for neutrinos, and sin^{2}(θ̅_{23}) = 0.47^{+0.25}_{−0.07} and ∆m̅^{2}_{32} = 2.55^{+0.33}_{−0.27} × 10^{−3} eV^{2}/c^{4} for antineutrinos (assuming normal mass ordering). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed.

Top panels show the reconstructed energy distribution of the 135 far detector ν_{μ}-CCQE candidate events (left) and 66 ν̅_{μ}-CCQE candidate events (right), with predicted spectra for best fit and no oscillation cases. Bottom panels show the ratio to unoscillated predictions.

## Measurement of Coherent π+ Production in Low Energy Neutrino-Carbon Scattering

We report the first measurement of the flux-averaged cross section for charged current coherent π+ production on carbon for neutrino energies less than 1.5 GeV, and with a restriction on the final state phase space volume in the T2K near detector, ND280. Comparisons are made with predictions from the Rein-Sehgal coherent production model and the model by Alvarez-Ruso et al., the latter representing the first implementation of an instance of the new class of microscopic coherent models in a neutrino interaction Monte Carlo event generator. We observe a clear event excess above background, disagreeing with the null results reported by K2K and SciBooNE in a similar neutrino energy region. The measured flux-averaged cross sections are below those predicted by both the Rein-Sehgal and Alvarez-Ruso et al. models.

## First combined analysis of neutrino and antineutrino oscillations at T2K

T2K reports its first results in the search for CP violation in neutrino oscillations using appearance and disappearance channels for neutrino- and antineutrino-mode beam. The data include all runs from Jan 2010 to May 2016 and comprise 7.482×10207.482×1020,protons on target in neutrino mode, which yielded in the far detector 32 e-like and 135 μμ-like events, and 7.471×10207.471×1020,protons on target in antineutrino mode which yielded 4 e-like and 66 μμ-like events. Reactor measurements of sin22θ13sin22θ13 have been used as an additional constraint. The one-dimensional confidence interval at 90% for δCPδCP spans the range (−3.13−3.13, −0.39−0.39) for normal mass ordering. The CP conservation hypothesis (δCP=0,πδCP=0,π) is excluded at 90% C.L.

## First Measurement of the Muon Neutrino Charged Current Single Pion Production Cross Section on Water with the T2K Near Detector

The T2K off-axis near detector, ND280, is used to make the first differential cross section measurements of muon neutrino charged current single positive pion production on a water target at energies ∼0.8~GeV. The differential measurements are presented as a function of muon and pion kinematics, in the restricted phase-space defined by p_{π+}>200MeV/c, p_{μ−} >200MeV/c, cosθ_{π+} >0.3 and cosθ_{μ−} >0.3. The total flux integrated ν_{μ} charged current single positive pion production cross section on water in the restricted phase-space is measured to be ⟨σ⟩_{ϕ}=4.25±0.48 (stat)±1.56 (syst)×10^{−40} cm^{2}/nucleon. The total cross section is consistent with the NEUT prediction (5.03×10^{−40} cm^{2}/nucleon) and 2σ lower than the GENIE prediction (7.68×10^{−40} cm^{2}/nucleon). The differential cross sections are in good agreement with the NEUT generator. The GENIE simulation reproduces well the shapes of the distributions, but over-estimates the overall cross section normalization.

## Measurement of Muon Antineutrino Oscillations with an Accelerator-Produced Off-Axis Beam

T2K reports its first measurements of the parameters governing the disappearance of ν̅_{μ} in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic ν̅_{μ} beam, produced with a peak energy of 0.6 GeV at J-PARC, is observed at the far detector Super-Kamiokande, 295 km away, where the ν̅_{μ} survival probability is expected to be minimal. Using a data set corresponding to 4.01×10^{20} protons on target, 34 fully contained μ-like events were observed. The best-fit oscillation parameters are sin2(θ̅_{23})=0.45 and |Δm̅_{23}^{2}|=2.51×10^{−3} eV^{2} with 68% confidence intervals of 0.38–0.64 and 2.26–2.80×10^{−3} eV^{2}, respectively. These results are in agreement with existing antineutrino parameter measurements and also with the ν_{μ} disappearance parameters measured by T2K.

## Measurement of double-differential muon neutrino charged-current interactions on C_{8}H_{8} without pions in the final state using the T2K off-axis beam

We report the measurement of muon neutrino charged-current interactions on carbon without pions in the final state at the T2K beam energy using 5.734×10^{20} protons on target. For the first time the measurement is reported as a flux-integrated, double-differential cross-section in muon kinematic variables (cosθμ, pμ), 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 σ=(0.417±0.047(syst)±0.005(stat))×10^{−38}cm^{2} nucleon^{−1} and the cross-section integrated in the region of phase space with largest efficiency and best signal-over-background ratio (cosθ_{μ}>0.6 and p_{μ}>200 MeV) is σ=(0.202±0.0359(syst)±0.0026(stat))×10^{−38}cm^{2} nucleon^{−1}.

## Upper bound on neutrino mass based on T2K neutrino timing measurements

The present work utilizes the T2K event timing measurements at the near and far detectors to study neutrino time of flight as a function of derived neutrino energy. Under the assumption of a relativistic relation between energy and time of flight, constraints on the neutrino rest mass can be derived. The sub-GeV neutrino beam in conjunction with timing precision of order tens of ns provide sensitivity to neutrino mass in the few MeV/c^{2} range. We study the distribution of relative arrival times of muon and electron neutrino candidate events at the T2K far detector as a function of neutrino energy. The 90% C.L. upper limit on the mixture of neutrino mass eigenstates represented in the data sample is found to be m_{ν}^{2}<5.6 MeV^{2}/c^{4}.