Publications

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

September 13, 2017

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, |Δm232|, sin223), sin213) and δCP, 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 sin213).  When combined with reactor measurements, the hypothesis of CP conservation (δCP = 0 or π) is excluded at the 90% confidence level.  The 90% confidence region for δCP 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 |Δm232| = 2.54 ± 0.08 (2.51 ± 0.08) × 10–3 eV2/c4 and sin223) = 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 sin223) (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

June 28, 2017

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 1020 at the GeV scale.

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

Updated measurements of muon neutrino and antineutrino disappearance

April 20, 2017

English:
We report measurements by the T2K experiment of the parameters θ23 and ∆m232 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 × 1020 POT in neutrino running mode and 7.471 × 1020 POT in antineutrino mode, T2K obtains sin223) = 0.51+0.08−0.07 and ∆m232 = 2.53+0.15−0.13 × 10−3 eV2/c4 for neutrinos, and sin2(θ̅23) = 0.47+0.25−0.07  and ∆m̅232 = 2.55+0.33−0.27 × 10−3 eV2/c4 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.

 

t2k-2016-numu-numubar

Top panels show the reconstructed energy distribution of the 135 far detector νμ-CCQE candidate events (left) and 66 ν̅μ-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 θ23 and ∆m232 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 × 1020 POT in neutrino running mode and 7.471 × 1020 POT in antineutrino mode, T2K obtains sin223) = 0.51+0.08−0.07 and ∆m232 = 2.53+0.15−0.13 × 10−3 eV2/c4 for neutrinos, and sin2(θ̅23) = 0.47+0.25−0.07  and ∆m̅232 = 2.55+0.33−0.27 × 10−3 eV2/c4 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.

 

t2k-2016-numu-numubar

Top panels show the reconstructed energy distribution of the 135 far detector νμ-CCQE candidate events (left) and 66 ν̅μ-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

February 17, 2017

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

February 11, 2017

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 sin22θ13sin2⁡2θ13 have been used as an additional constraint. The one-dimensional confidence interval at 90% for δCPδCP spans the range (3.13−3.130.39−0.39) for normal mass ordering. The CP conservation hypothesis (δCP=0,πδCP=0,π) is excluded at 90% C.L.

t2k-2016_delta_cp

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

nd280data-numu-cc1pi-xs-on-h2o-2015
May 26, 2016

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 pπ+>200MeV/c, pμ− >200MeV/c, cosθπ+ >0.3 and cosθμ− >0.3. The total flux integrated νμ 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 cm2/nucleon. The total cross section is consistent with the NEUT prediction (5.03×10−40 cm2/nucleon) and 2σ lower than the GENIE prediction (7.68×10−40 cm2/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

2016_numubar_contour
May 9, 2016

T2K reports its first measurements of the parameters governing the disappearance of ν̅μ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic ν̅μ 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 ν̅μ survival probability is expected to be minimal. Using a data set corresponding to 4.01×1020 protons on target, 34 fully contained μ-like events were observed. The best-fit oscillation parameters are sin2(θ̅23)=0.45 and |Δm̅232|=2.51×10−3  eV2 with 68% confidence intervals of 0.38–0.64 and 2.26–2.80×10−3  eV2, respectively. These results are in agreement with existing antineutrino parameter measurements and also with the νμ disappearance parameters measured by T2K.

Measurement of double-differential muon neutrino charged-current interactions on C8H8 without pions in the final state using the T2K off-axis beam

2015_ND280CC0Pi_Results
February 16, 2016

We report the measurement of muon neutrino charged-current interactions on carbon without pions in the final state at the T2K beam energy using 5.734×1020 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))×10−38cm2 nucleon−1 and the cross-section integrated in the region of phase space with largest efficiency and best signal-over-background ratio (cosθμ>0.6 and pμ>200 MeV) is σ=(0.202±0.0359(syst)±0.0026(stat))×10−38cm2 nucleon−1.

Upper bound on neutrino mass based on T2K neutrino timing measurements

2016_TOF_EnuDerived
February 4, 2016

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/c2 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 mν2<5.6  MeV2/c4.

Measurement of the muon neutrino inclusive charged-current cross section in the energy range of 1-3 GeV with the T2K INGRID detector

xsec
September 24, 2015

We report a measurement of the νμ-nucleus inclusive charged current cross section (=σcc) on iron using data from exposed to the J-PARC neutrino beam. The detector consists of 14 modules in total, which are spread over a range of off-axis angles from 0° to 1.1°. The variation in the neutrino energy spectrum as a function of the off-axis angle, combined with event topology information, is used to calculate this cross section as a function of neutrino energy. The cross section is measured to be σcc(1.1 GeV)=1.10±0.15 (10−38cm2/nucleon), σcc(2.0 GeV)=2.07±0.27 (10−38cm2/nucleon), and σcc(3.3 GeV)=2.29±0.45 (10−38cm2/nucleon), at energies of 1.1, 2.0, and 3.3 GeV, respectively. These results are consistent with the cross section calculated by the neutrino interaction generators currently used by T2K. More importantly, the method described here opens up a new way to determine the energy dependence of neutrino-nucleus cross sections.