Search

We report a measurement of cross section $\sigma ({\nu }_{\mu }+\text{nucleus}\to {\mu }^{-}+X)$ and the first measurements of the cross section $\sigma ({\overline{\nu }}_{\mu }+\text{nucleus}\to {\mu }^{+}+X)$ and their ratio $R(\frac{\sigma (\overline{\nu })}{\sigma (\nu )})$ at (anti) neutrino energies below 1.5 GeV. We determine the single momentum bin cross section measurements, averaged over the T2K $\overline{\nu }/\nu$-flux, for the detector target material (mainly carbon, oxygen, hydrogen and copper) with phase space restricted laboratory frame kinematics of ${\theta }_{\mu }<32°$ and $p_{\mu }>500\mathrm{MeV}/c$. The results are $\sigma (\overline{\nu })=(0.900\pm 0.029(\mathrm{stat})\pm 0.088(\mathrm{syst}))\times {10}^{-39}$ and $\sigma (\nu )=(2.41\pm 0.022(\mathrm{stat})\pm 0.231(\mathrm{syst}))\times {10}^{-39}$ in units of ${\mathrm{cm}}^2/\mathrm{nucleon}$ and $R(\frac{\sigma (\overline{\nu })}{\sigma (\nu )})=0.373\pm 0.012(\mathrm{stat})\pm 0.015(\mathrm{syst})$.

We report a measurement of the flux-integrated cross section for inclusive muon neutrino charged-current interactions on carbon. The double-differential measurements are given as a function of the muon momentum and angle. Relative to our previous publication on this topic, these results have an increased angular acceptance and higher statistics. The data sample presented here corresponds to $5.7\times {10}^{20}$ protons on target. The total flux-integrated cross section is measured to be $(6.950\pm 0.662)\times {10}^{-39}{\mathrm{cm}}^2{\text{nucleon}}^{-1}$ and is consistent with our simulation.

Neutrino- and antineutrino-oxygen neutral-current quasielasticlike interactions are measured at Super-Kamiokande using nuclear deexcitation $\gamma$ rays to identify signal-like interactions in data from a $14.94(16.35)\times {10}^{20}$ protons-on-target exposure of the T2K neutrino (antineutrino) beam. The measured flux-averaged cross sections on oxygen nuclei are $⟨{\sigma }_{\nu \text{−}\mathrm{NCQE}}⟩=1.70\pm 0.17(\mathrm{stat}.{)}_{-0.38}^{+0.51}(\mathrm{syst}.)\times {10}^{-38}{\mathrm{cm}}^2/\text{oxygen}$ with a flux-averaged energy of 0.82 GeV and $⟨{\sigma }_{\overline{\nu }\text{−}\mathrm{NCQE}}⟩=0.98\pm 0.16(\mathrm{stat}.{)}_{-0.19}^{+0.26}(\mathrm{syst}.)\times {10}^{-38}{\mathrm{cm}}^2/\text{oxygen}$ with a flux-averaged energy of 0.68 GeV, for neutrinos and antineutrinos, respectively. These results are the most precise to date, and the antineutrino result is the first cross section measurement of this channel. They are compared with various theoretical predictions. The impact on evaluation of backgrounds to searches for supernova relic neutrinos at present and future water Cherenkov detectors is also discussed.

The single ${\pi }^0$ production rate in neutral current neutrino interactions on water in a neutrino beam with a peak neutrino energy of 0.6 GeV has been measured using the PØD, one of the subdetectors of the T2K near detector. The production rate was measured for data taking periods when the PØD contained water ($2.64\times {10}^{20}$ protons-on-target) and also periods without water ($3.49\times {10}^{20}$ protons-on-target). A measurement of the neutral current single ${\pi }^0$ production rate on water is made using appropriate subtraction of the production rate with water in from the rate with water out of the target region. The subtraction analysis yields $106\pm 41\pm 69$ signal events where the uncertainties are statistical (stat.) and systematic (sys.) respectively. This is consistent with the prediction of 157 events from the nominal simulation. The measured to expected ratio is $0.68\pm 0.26(\mathrm{stat})\pm 0.44(\mathrm{sys})\pm 0.12(\mathrm{flux})$. The nominal simulation uses a flux integrated cross section of $7.63\times {10}^{-39}{\mathrm{cm}}^2$ per nucleon with an average neutrino interaction energy of 1.3 GeV.

We report measurements by the T2K experiment of the parameters ${\theta }_{23}$ and $\mathrm{\Delta }{m}_{32}^2$, which govern the disappearance of muon neutrinos and antineutrinos in the three-flavor PMNS neutrino oscillation model at T2K’s neutrino energy and propagation distance. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, muon-like events from each beam mode are used to measure these parameters separately for neutrino and antineutrino oscillations. Data taken from $1.49\times {10}^{21}$ protons on target (POT) in neutrino mode and $1.64\times {10}^{21}$ POT in antineutrino mode are used. The best-fit values obtained by T2K were ${\sin }^2({\theta }_{23})={0.51}_{-0.07}^{+0.06}({0.43}_{-0.05}^{+0.21})$ and $\mathrm{\Delta }{m}_{32}^2={2.47}_{-0.09}^{+0.08}({2.50}_{-0.13}^{+0.18})\times {10}^{-3}{\mathrm{eV}}^2/c^4$ for neutrinos (antineutrinos). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed. An analysis using an effective two-flavor neutrino oscillation model where the sine of the mixing angle is allowed to take nonphysical values larger than 1 is also performed to check the consistency of our data with the three-flavor model. Our data were found to be consistent with a physical value for the mixing angle.

This paper reports measurements of final-state proton multiplicity, muon and proton kinematics, and their correlations in charged-current pionless neutrino interactions, measured by the T2K ND280 near detector in its plastic scintillator (${\mathrm{C}}_8{\mathrm{H}}_8$) target. The data were taken between years 2010 and 2013, corresponding to approximately $6\times {10}^{20}$ protons on target. Thanks to their exploration of the proton kinematics and of imbalances between the proton and muon kinematics, the results offer a novel probe of the nuclear-medium effects most pertinent to the (sub-)GeV neutrino-nucleus interactions that are used in accelerator-based long-baseline neutrino oscillation measurements. These results are compared to many neutrino-nucleus interaction models which all fail to describe at least part of the observed phase space. In case of events without a proton above a detection threshold in the final state, a fully consistent implementation of the local Fermi gas model with multinucleon interactions gives the best description of the data. In the case of at least one proton in the final state, the spectral function model agrees well with the data, most notably when measuring the kinematic imbalance between the muon and the proton in the plane transverse to the incoming neutrino. Within the models considered, only the existence of multinucleon interactions are able to describe the extracted cross section within regions of high transverse kinematic imbalance. The effect of final-state interactions is also discussed.

The T2K experiment measures muon neutrino disappearance and electron neutrino appearance in accelerator-produced neutrino and antineutrino beams. With an exposure of $14.7(7.6)\times {10}^{20}$ protons on target in the neutrino (antineutrino) mode, 89 ${\nu }_e$ candidates and seven anti-${\nu }_e$ candidates are observed, while 67.5 and 9.0 are expected for ${\delta }_{CP}=0$ and normal mass ordering. The obtained $2\sigma$ confidence interval for the $CP$-violating phase, ${\delta }_{CP}$, does not include the $CP$-conserving cases (${\delta }_{CP}=0$, $\pi$). The best-fit values of other parameters are ${\sin }^2{\theta }_{23}=0.526_{-0.036}^{+0.032}$ and $\mathrm{\Delta }{m}_{32}^2={2.463}_{-0.070}^{+0.071}\times {10}^{-3}{\mathrm{eV}}^2/c^4$.

We report the first measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasielasticlike interactions at the gadolinium-loaded Super-Kamiokande detector using the T2K neutrino beam, which has a peak energy of about 0.6 GeV. A total of 30 neutral-current quasielasticlike event candidates were selected from T2K data corresponding to an exposure of $1.76\times {10}^{20}$ protons on target. The $\gamma$ ray signals resulting from neutron captures were identified using a neural network. The flux-averaged mean neutron capture multiplicity was measured to be ${1.37\pm 0.33(\mathrm{stat}.)}_{-0.27}^{+0.17}(\mathrm{syst}.)$, which is compatible within 2.3 sigma than predictions obtained using our nominal simulation. We discuss potential sources of systematic uncertainty in the prediction and demonstrate that a significant portion of this discrepancy arises from the modeling of hadron-nucleus interactions in the detector medium.

The Super-Kamiokande and T2K Collaborations present a joint measurement of neutrino oscillation parameters from their atmospheric and beam neutrino data. It uses a common interaction model for events overlapping in neutrino energy and correlated detector systematic uncertainties between the two datasets, which are found to be compatible. Using 3244.4 days of atmospheric data and a beam exposure of $19.7(16.3)\times {10}^{20}$ protons on target in (anti)neutrino mode, the analysis finds a $1.9\sigma$ exclusion of $CP$ conservation (defined as $J_{CP}=0$) and a $1.2\sigma$ exclusion of the inverted mass ordering.

We have made improved measurements of three-flavor neutrino mixing with $19.7(16.3)\times {10}^{20}$ protons on target in (anti-)neutrino-enhanced beam modes. A new sample of muon-neutrino events with tagged pions has been added at the far detector, as well as new proton and photon-tagged samples at the near detector. Significant improvements have been made to the flux and neutrino interaction modeling. T2K data continue to prefer the normal mass ordering and upper octant of ${\sin }^2{\theta }_{23}$ with a near-maximal value of the charge-parity violating phase with best-fit values in the normal ordering of ${\delta }_{\mathrm{CP}}=-{2.18}_{-0.47}^{+1.22}$, ${\sin }^2{\theta }_{23}={0.559}_{-0.078}^{+0.018}$ and $\mathrm{\Delta }{m}_{32}^2=({+2.506}_{-0.052}^{+0.039})\times {10}^{-3}{\mathrm{eV}}^2$.

This article presents a study of single ${\pi }^{+}$ production in neutrino neutral-current interactions ($\mathrm{NC}1{\pi }^{+}$) using the FGD1 hydrocarbon target of the ND280 detector of the T2K experiment. We report the largest sample of such events selected by any experiment, providing the first new data for this channel in over four decades and the first using a sub-GeV neutrino flux. The signal selection strategy and its performance are detailed together with validations of a robust cross section extraction methodology. The measured flux-averaged integrated cross-section is $\sigma =(6.07\pm 1.22)\times {10}^{-41}{\mathrm{cm}}^2/\mathrm{nucleon}$, $1.3\sigma$ above the NEUT v5.4.0 expectation.

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 by 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.

We perform a search for light sterile neutrinos using the data from the T2K far detector at a baseline of 295 km, with an exposure of $14.7(7.6)\times {10}^{20}$ protons on target in neutrino (antineutrino) mode. A selection of neutral-current interaction samples is also used to enhance the sensitivity to sterile mixing. No evidence of sterile neutrino mixing in the $3+1$ model was found from a simultaneous fit to the charged-current muon, electron and neutral-current neutrino samples. We set the most stringent limit on the sterile oscillation amplitude ${\sin }^2{\theta }_{24}$ for the sterile neutrino mass splitting $\mathrm{\Delta }{m}_{41}^2<3\times {10}^{-3}{\mathrm{eV}}^2/c^4$.

T2K reports its first results in the search for $CP$ violation in neutrino oscillations using appearance and disappearance channels for neutrino- and antineutrino-mode beams. The data include all runs from January 2010 to May 2016 and comprise $7.482\times 10^{20}$ protons on target in neutrino mode, which yielded in the far detector 32 $e$-like and $135\mu$-like events, and $7.471\times 10^{20}$ protons on target in antineutrino mode, which yielded 4 $e$-like and $66\mu$-like events. Reactor measurements of ${\sin }^{2}2{\theta }_{13}$ have been used as an additional constraint. The one-dimensional confidence interval at 90% for the phase ${\delta }_{CP}$ spans the range ($-3.13$, $-0.39$) for normal mass ordering. The $CP$ conservation hypothesis (${\delta }_{CP}=0$, $\pi$) is excluded at 90% C.L.

The T2K Collaboration presents the first measurement of electron neutrino-induced charged-current pion production on a predominantly carbon target in a restricted kinematical phase space. This is performed using data from the 2.5° off-axis near detector, ND280. The differential cross sections with respect to the outgoing electron and pion kinematics, in addition to the total flux-integrated cross section, are obtained. Comparisons between the measured and predicted cross-section results using the neut, genie, and nuwro Monte Carlo event generators are presented. The measured total flux-integrated cross section is $[2.52\pm 0.52(\mathrm{stat})\pm 0.30(\mathrm{syst})]\times {10}^{-39}{\mathrm{cm}}^2{\text{nucleon}}^{-1}$, which is lower than the event generator predictions.

This paper reports the first differential measurement of the charged-current ${\overline{\nu }}_{\mu }$ interaction cross section on water with no pions in the final state. The unfolded flux-averaged measurement using the T2K off-axis near detector is given in double-differential bins of ${\mu }^{+}$ momentum and angle. The integrated cross section in a restricted phase space is $\sigma =(1.11\pm 0.18)\times {10}^{-38}{\mathrm{cm}}^2$ per water molecule. Comparisons with several nuclear models are also presented.

We report a flux-integrated cross section measurement of muon neutrino interactions on water and hydrocarbon via charged current reactions without charged pions in the final state with the WAGASCI-BabyMIND detector, which was installed in the T2K near detector hall in 2018. The detector is located 1.5° off-axis and is exposed to a more energetic neutrino flux than ND280, another T2K near detector, which is located at a different off-axis position. The total flux-integrated cross section is measured to be $1.26\pm 0.18(\text{stat}+\text{syst})\times {10}^{-39}{\mathrm{cm}}^2/\mathrm{nucleon}$ on CH and $1.44\pm 0.21(\text{stat}+\text{syst})\times {10}^{-39}{\mathrm{cm}}^2/\mathrm{nucleon}$ on ${\mathrm{H}}_2\mathrm{O}$. These results are compared to model predictions provided by the neut v5.3.2 and genie v2.8.0 Monte Carlo generators and the measurements are compatible with these models. Differential cross sections in muon momentum and cosine of the muon scattering angle are also reported. This is the first such measurement reported with the WAGASCI-BabyMIND detector and utilizes the 2020 and 2021 datasets.

This paper reports the first differential measurement of the charged-current interaction cross section of ${\nu }_{\mu }$ 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 $\sigma =(0.95\pm 0.08(\text{stat})\pm 0.06(\text{det syst})\pm 0.04(\text{model syst})\pm 0.08(\text{flux}))\times {10}^{-38}{\mathrm{cm}}^2/\mathrm{n}$.

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, $|\mathrm{\Delta }{m}_{32}^2|$, ${\sin }^2{\theta }_{23}$, ${\sin }^2{\theta }_{13}$, and ${\delta }_{\mathrm{CP}}$ 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 ${\sin }^2{\theta }_{13}$. When combined with reactor measurements, the hypothesis of $CP$ conservation (${\delta }_{\mathrm{CP}}=0$ or $\pi$) is excluded at 90% confidence level. The 90% confidence region for ${\delta }_{\mathrm{CP}}$ 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 $\mathrm{\Delta }{m}_{32}^2=2.54\pm 0.08(2.51\pm 0.08)\times {10}^{-3}{\mathrm{eV}}^2/c^4$ and ${\sin }^2{\theta }_{23}=0.55_{-0.09}^{+0.05}$ ($0.55_{-0.08}^{+0.05}$), compatible with maximal mixing. In the Bayesian analysis, the data weakly prefer normal ordering (Bayes factor 3.7) and the upper octant for ${\sin }^2{\theta }_{23}$ (Bayes factor 2.4).

This paper reports on the search for heavy neutrinos with masses in the range $140<M_N<493\mathrm{MeV}/{\mathrm{c}}^2$ using the off-axis near detector ND280 of the T2K experiment. These particles can be produced from kaon decays in the standard neutrino beam and then subsequently decay in ND280. The decay modes under consideration are $N\to {\ell }_{\alpha }^{\pm }{\pi }^{\mp }$ and $N\to {\ell }_{\alpha }^{+}{\ell }_{\beta }^{-}\stackrel{(-)}{\nu }(\alpha ,\beta =e,\mu )$. A search for such events has been made using the Time Projection Chambers of ND280, where the background has been reduced to less than two events in the current dataset in all channels. No excess has been observed in the signal region. A combined Bayesian statistical approach has been applied to extract upper limits on the mixing elements of heavy neutrinos to electron-, muon- and tau- flavored currents ($U_e^2$, $U_{\mu }^2$, $U_{\tau }^2$) as a function of the heavy neutrino mass, e.g., $U_e^2<{10}^{-9}$ at 90% C.L. for a mass of $390\mathrm{MeV}/{\mathrm{c}}^2$. These constraints are competitive with previous experiments.