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.

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

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

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

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

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

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.

Measurement of the Electron Neutrino Charged-current Interaction Rate on Water with the T2K ND280 pi-zero Detector

March 31, 2015

The first direct observation of the appearance of electron neutrinos in a muon neutrino beam through neutrino oscillation was recently reported by the T2K experiment. The main background in this observation was the presence of the electron neutrino component of the beam, which accounts for 1.2% of the beam below the 1.2 GeV threshold. This paper presents a measurement of the charged current interaction rate of the electron neutrino beam component using the large fiducial mass of the T2K π0 detector. The measured ratio of the observed beam interaction rate to the predicted rate in the detector with water targets filled is 0.89 ±0.08 (stat.) ±0.11 (sys.), and with the water targets emptied is 0.90 ±0.09 (stat.) ±0.13 (sys.). The ratio obtained for the interactions only on-water only an event subtraction method is 0.87 ±0.33 (stat.) ±0.21 (sys.). These are pioneering measurements of the νe interaction rate, particularly on water, which will become crucial in future CP-violation searches in the lepton sector.

Measurement of the νμ charged current quasi-elastic cross-section on carbon with the T2K on-axis neutrino beam

March 27, 2015

We report a measurement of the νμ 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±0.19(stat.)+1.82−1.47(syst.))×10−39cm2/neutron and (10.64±0.37(stat.)+2.03−1.65(syst.))×10−39cm2/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.

Measurements of neutrino oscillation in appearance and disappearance channels by the T2K experiment with 6.6E20 protons on target

February 6, 2015

We report on measurements of neutrino oscillation using data from the T2K long-baseline neutrino experiment collected between 2010 and 2013. In an analysis of muon neutrino disappearance alone, we find the following estimates and 68% confidence intervals for the two possible mass hierarchies:
Normal Hierarchy: sin2θ23=0.514+0.055−0.056 and Δm232=(2.51±0.10)×10−3 eV2/c4
Inverted Hierarchy: sin2θ23=0.511±0.055 and Δm232=(2.48±0.10)×10−3 eV2/c4
The analysis accounts for multi-nucleon mechanisms in neutrino interactions which were found to introduce negligible bias.
We describe our first analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance to estimate four oscillation parameters and the mass hierarchy. Frequentist and Bayesian intervals are presented for combinations of these parameters, with and without including recent reactor measurements. At 90% confidence level and including reactor measurements, we exclude the region:
δCP=[0.15,0.83]π for normal hierarchy and δCP=[−0.08,1.09]π for inverted hierarchy.
The T2K and reactor data weakly favor the normal hierarchy with a Bayes Factor of 2.2. The most probable values and 68% 1D credible intervals for the other oscillation parameters, when reactor data are included, are:
sin2θ23=0.528+0.055−0.038 and |Δm232|=(2.51±0.11)×10−3 eV2/c4.