Search

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.

The Super-Kamiokande detector has measured solar neutrinos for more than 25 years. The sensitivity for solar neutrino measurement is limited by the uncertainties of energy scale and background modeling. Decays of unstable isotopes with relatively long half-lives through nuclear muon capture, such as ${}^{16}\mathrm{N}$, ${}^{15}\mathrm{C}$, ${}^{12}\mathrm{B}$, and ${}^{13}\mathrm{B}$, are detected as background events for solar neutrino observations. In this study, we developed a method to form a pair of stopping muon and decay candidate events and evaluated the production rates of such unstable isotopes. We then measured their branching ratios considering both their production rates and the estimated number of nuclear muon capture processes as $\mathrm{Br}\left({}^{16}\mathrm{N}\right)=(9.0\pm 0.1)\%$, $\mathrm{Br}\left({}^{15}\mathrm{C}\right)=(0.6\pm 0.1)\%$, $\mathrm{Br}\left({}^{12}\mathrm{B}\right)=(0.98\pm 0.18)\%$, $\mathrm{Br}\left({}^{13}\mathrm{B}\right)=(0.14\pm 0.12)\%$, respectively. The result for ${}^{16}\mathrm{N}$ has world-leading precision at present and the results for ${}^{15}\mathrm{C}$, ${}^{12}\mathrm{B}$, and ${}^{13}\mathrm{B}$ are the first branching ratio measurements for those isotopes.

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.

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.

We present measurements of total neutron production from atmospheric neutrino interactions in water, analyzed as a function of electron-equivalent visible energy over a range of 30 MeV to 10 GeV. These results are based on 4,270 days of data collected by Super-Kamiokande, including 564 days with 0.011 wt% gadolinium added to enhance neutron detection. Neutron signal selection is based on a neural network trained on simulation, with its performance validated using an Am/Be neutron point source. The measurements are compared to predictions from neutrino event generators combined with various hadron-nucleus interaction models, which include an intranuclear cascade model and a nuclear deexcitation model. We observe significant variations in the predictions depending on the choice of hadron-nucleus interaction model. We discuss key factors that contribute to describing our data, such as in-medium effects in the intranuclear cascade and the accuracy of statistical evaporation modeling.

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.

Among three leptonic mixing angles, ${\theta }_{23}$, which characterizes the fractional contribution of two flavor eigenstates ${\nu }_{\mu }$ and ${\nu }_{\tau }$ to the third mass eigenstate ${\nu }_3$, is known to be the largest but least precisely measured. The work investigates the possible reach of ${\theta }_{23}$ precision with two forthcoming gigantic accelerator-based long-baseline neutrino experiments, namely, Hyper-Kamiokande (T2HK) and the Deep Underground Neutrino Experiment (DUNE), as well as a possible joint analyses of future neutrino facilities. Our simulation yields that each experiment will definitely establish the octant of ${\theta }_{23}$ for all values within a $1\sigma$ parameter interval, while considering the current limitation. However, if the actual value is $0.48\le {\sin }^2{\theta }_{23}\le 0.54$, it becomes challenging for these two experiments to reject the maximal (${\theta }_{23}=\pi /4$) hypothesis and conclude its octant. This octant-blind region can be further explored with the proposed facilities European Spallation Source Neutrino Super Beam and Neutrino Factory. The accurate determination of the mixing angle ${\theta }_{23}$, as well as the accuracy of ${\delta }_{CP}$, is crucial for examining a certain category of discrete non-Abelian leptonic flavor models. Specifically, if $CP$ is conserved in the leptonic sector, the combined analysis of T2HK and DUNE will rule out the majority of these models. However, if the $CP$ is maximally violated, a higher precision of ${\delta }_{CP}$ is necessary to test these flavor models.

Super-Kamiokande (SK) has observed ${}^8\mathrm{B}$ solar neutrino elastic scattering at recoil electron kinetic energies ($E_{\mathrm{kin}}$) as low as 3.49 MeV to study neutrino flavor conversion within the Sun. At SK-observable energies, these conversions are dominated by the Mikheyev-Smirnov-Wolfenstein effect. An upturn in the electron neutrino survival probability in which vacuum neutrino oscillations become dominant is predicted to occur at lower energies, but radioactive background increases exponentially with decreasing energy. New machine learning approaches provide substantial background reduction below 3.49 MeV such that statistical extraction of solar neutrino interactions becomes feasible. This article presents an analysis of the solar neutrino interaction rate at $E_{\mathrm{kin}}<3.49\mathrm{MeV}$ with the full SK-IV period, using data from a wideband intelligent trigger when available and with a boosted decision tree for event selection. A solar neutrino signal is observed between $2.99\mathrm{MeV}<E_{\mathrm{kin}}<3.49\mathrm{MeV}$ with $2.76\sigma$ significance and a data to unoscillated Monte Carlo ratio of ${0.307}_{-0.111}^{+0.112}$. These additional low-energy data have a negligible effect on the $1\sigma$ intervals of the fits to the solar neutrino energy spectrum but have a noticeable effect on the best fit when using the exponential parametrization.

Electron antineutrino appearance is measured by the T2K experiment in an accelerator-produced antineutrino beam, using additional neutrino beam operation to constrain parameters of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix. T2K observes 15 candidate electron antineutrino events with a background expectation of 9.3 events. Including information from the kinematic distribution of observed events, the hypothesis of no electron antineutrino appearance is disfavored with a significance of $2.40\sigma$ and no discrepancy between data and PMNS predictions is found. A complementary analysis that introduces an additional free parameter which allows non-PMNS values of electron neutrino and antineutrino appearance also finds no discrepancy between data and PMNS predictions.

This paper reports the first T2K measurement of the transverse kinematic imbalance in the single-${\pi }^{+}$ production channel of neutrino interactions. We measure the differential cross sections in the muon-neutrino charged-current interaction on hydrocarbon with a single ${\pi }^{+}$ and at least one proton in the final state, at the ND280 off axis near detector of the T2K experiment. The extracted cross sections are compared to the predictions from different neutrino-nucleus interaction event generators. Overall, the results show a preference for models that have a more realistic treatment of nuclear medium effects including the initial nuclear state and final-state interactions.

The T2K experiment reports updated measurements of neutrino and antineutrino oscillations using both appearance and disappearance channels. This result comes from an exposure of $14.9(16.4)\times 10^{20}$ protons on target in neutrino (antineutrino) mode. Significant improvements have been made to the neutrino interaction model and far detector reconstruction. An extensive set of simulated data studies have also been performed to quantify the effect interaction model uncertainties have on the T2K oscillation parameter sensitivity. T2K performs multiple oscillation analyses that present both frequentist and Bayesian intervals for the Pontecorvo-Maki-Nakagawa-Sakata parameters. For fits including a constraint on ${\sin }^2{\theta }_{13}$ from reactor data and assuming normal mass ordering T2K measures ${\sin }^2{\theta }_{23}={0.53}_{-0.04}^{+0.03}$ and $\mathrm{\Delta }{m}_{32}^2=(2.45\pm 0.07)\times {10}^{-3}{\mathrm{eV}}^2{\mathrm{c}}^{-4}$. The Bayesian analyses show a weak preference for normal mass ordering (89% posterior probability) and the upper ${\sin }^2{\theta }_{23}$ octant (80% posterior probability), with a uniform prior probability assumed in both cases. The T2K data exclude CP conservation in neutrino oscillations at the $2\sigma$ level.

This paper reports the first measurement of muon neutrino charged-current interactions without pions in the final state using multiple detectors with correlated energy spectra at T2K. The data was collected on hydrocarbon targets using the off-axis T2K near detector (ND280) and the on-axis T2K near detector (INGRID) with neutrino energy spectra peaked at 0.6 GeV and 1.1 GeV, respectively. The correlated neutrino flux presents an opportunity to reduce the impact of the flux uncertainty and to study the energy dependence of neutrino interactions. The extracted double-differential cross sections are compared to several Monte Carlo neutrino-nucleus interaction event generators showing the agreement between both detectors individually and with the correlated result.

We present the results of searches for nucleon decays via $p\to \nu {\pi }^{+}$ and $n\to \nu {\pi }^0$ using a $0.484\mathrm{Mt}·\mathrm{yr}$ exposure of Super-Kamiokande I-V data covering the entire pure water phase of the experiment. Various improvements on the previous 2014 nucleon decay search [Phys. Rev. Lett. 113, 121802 (2014)], which used an exposure of $0.173\mathrm{Mt}·\mathrm{yr}$, are incorporated. The physics models related to pion production and nuclear interaction are refined with external data, and a more comprehensive set of systematic uncertainties, now including those associated with the atmospheric neutrino flux and pion production channels, is considered. Also, the fiducial volume has been expanded by 21%. No significant indication of a nucleon decay signal is found beyond the expected background. Lower bounds on the nucleon partial lifetimes are determined to be $3.5\times {10}^{32}\mathrm{yr}$ for $p\to \nu {\pi }^{+}$ and $1.4\times {10}^{33}\mathrm{yr}$ for $n\to \nu {\pi }^0$ at 90% confidence level.

In recent neutrino detectors, neutrons produced in neutrino reactions play an important role. Muon capture on oxygen nuclei is one of the processes that produce neutrons in water Cherenkov detectors. We measured neutron multiplicity in the process using cosmic ray muons that stop in the gadolinium-loaded Super-Kamiokande detector. For this measurement, neutron detection efficiency is obtained with the muon capture events followed by gamma rays to be ${50.2}_{-2.1}^{+2.0}\%$. By fitting the observed multiplicity considering the detection efficiency, we measure neutron multiplicity in muon capture as $P(0)=24\pm 3\%$, $P(1)=70_{-2}^{+3}\%$, $P(2)=6.1\pm 0.5\%$, $P(3)=0.38\pm 0.09\%$. This is the first measurement of the multiplicity of neutrons associated with muon capture on oxygen without neutron energy threshold.

Neutrino oscillation experiments have measured precisely at few percent levels the mass-squared differences ($\mathrm{\Delta }{m}_{21}^2$, $\mathrm{\Delta }{m}_{31}^2$) of three neutrino mass eigenstates, and the three leptonic mixing angles (${\theta }_{12}$, ${\theta }_{13}$, ${\theta }_{23}$) by utilizing both neutrino and antineutrino oscillations. The possible $CPT$ violation may manifest itself in the difference of neutrino and antineutrino oscillation parameters, making these experiments promising tools for testing $CPT$ invariance at unprecedented precision. We investigate empirically the sensitivity of the $CPT$ test via the difference in mass-squared splittings ($\mathrm{\Delta }{m}_{31}^2-\mathrm{\Delta }{\overline{m}}_{31}^2$) and in leptonic mixing angles (${\sin }^2{\theta }_{23}-{\sin }^2{\overline{\theta }}_{23}$) with the synergy of T2K-II, $\mathrm{NO}\nu \mathrm{A}$ extension, and JUNO experiments. If the $CPT$ symmetry is found to be conserved, the joint analysis of the three experiments will be able to establish limits of $|\mathrm{\Delta }{m}_{31}^2-\mathrm{\Delta }{\overline{m}}_{31}^2|<5.3\times {10}^{-3}{\mathrm{eV}}^2$ and $|{\sin }^2{\theta }_{23}-{\sin }^2{\overline{\theta }}_{23}|<0.10$ at $3\sigma$ confidence level (CL) on the possible $CPT$ violation, extending substantially the current bound of these parameters. We find that with ($\mathrm{\Delta }{m}_{31}^2-\mathrm{\Delta }{\overline{m}}_{31}^2$), the dependence of the statistical significance on the relevant parameters to exclude the $CPT$ conservation is marginal, and that, if the difference in the best-fit values of $\mathrm{\Delta }{m}_{31}^2$ and $\mathrm{\Delta }{\overline{m}}_{31}^2$ measured by $\mathrm{MINOS}(+)$ and $\mathrm{NO}\nu \mathrm{A}$ persists as the true, the combined analysis will rule out the $CPT$ conservation at $4\sigma$ CL. With the (${\sin }^2{\theta }_{23}-{\sin }^2{\overline{\theta }}_{23}$), the statistical significance to exclude $CPT$ invariance depends strongly on the true value of ${\theta }_{23}({\overline{\theta }}_{23})$ mixing angle. In the case of maximal mixing of ${\theta }_{23}$, as indicated by the current T2K and $\mathrm{NO}\nu \mathrm{A}$ measurements, the $CPT$ conservation will be excluded at $3\sigma$ CL or higher if the difference in the best-fit values of ${\theta }_{23}$ and ${\overline{\theta }}_{23}$ remains as the true.

Muon neutrino and antineutrino disappearance probabilities are identical in the standard three-flavor neutrino oscillation framework, but $CPT$ violation and nonstandard interactions can violate this symmetry. In this work we report the measurements of ${\sin }^2{\theta }_{23}$ and $\mathrm{\Delta }{m}_{32}^2$ independently for neutrinos and antineutrinos. The aforementioned symmetry violation would manifest as an inconsistency in the neutrino and antineutrino oscillation parameters. The analysis discussed here uses a total of $1.97\times {10}^{21}$ and $1.63\times {10}^{21}$ protons on target taken with a neutrino and antineutrino beam respectively, and benefits from improved flux and cross section models, new near-detector samples and more than double the data reducing the overall uncertainty of the result. No significant deviation is observed, consistent with the standard neutrino oscillation picture.

We searched for bound neutron decay via $n\to \overline{\nu }+K^0$ predicted by the grand unified theories in $0.401\mathrm{Mton}·\mathrm{years}$ exposure of all pure water phases in the Super-Kamiokande detector. About 4.4 times more data than in the previous search have been analyzed by a new method including a spectrum fit to kaon invariant mass distributions. No significant data excess has been observed in the signal regions. As a result of this analysis, we set a lower limit of $7.8\times {10}^{32}\mathrm{years}$ on the neutron lifetime at a 90% confidence level.

This paper presents the first combined measurement of the double-differential muon neutrino and antineutrino charged-current cross sections with no pions in the final state on hydrocarbon at the off-axis near detector of the T2K experiment. The data analyzed in this work comprise $5.8\times {10}^{20}$ and $6.3\times {10}^{20}$ protons on target in neutrino and antineutrino mode respectively, at a beam energy peak of 0.6 GeV. Using the two measured cross sections, the sum, difference, and asymmetry were calculated with the aim of better understanding the nuclear effects involved in such interactions. The extracted measurements have been compared with the prediction from different Monte Carlo generators and theoretical models showing that the difference between the two cross sections have interesting sensitivity to nuclear effects.

We report the measurements of the single and double differential cross section of muon neutrino charged-current interactions on carbon with a single positively charged pion in the final state at the T2K off-axis near detector using $5.56\times {10}^{20}$ protons on target. The analysis uses data control samples for the background subtraction and the cross section signal, defined as a single negatively charged muon and a single positively charged pion exiting from the target nucleus, is extracted using an unfolding method. The model-dependent cross section, integrated over the T2K off-axis neutrino beam spectrum peaking at 0.6 GeV, is measured to be $\sigma =(11.76\pm 0.44(\mathrm{stat})\pm 2.39(\mathrm{syst}))\times {10}^{-40}{\mathrm{cm}}^2{\text{nucleon}}^{-1}$. Various differential cross sections are measured, including the first measurement of the Adler angles for single charged pion production in neutrino interactions with heavy nuclei target.

Leptonic CP violation search, neutrino mass hierarchy determination, and the precision measurements of oscillation parameters for a unitary test of the leptonic mixing matrix are among the major targets of the ongoing and future neutrino oscillation experiments. The work explores the physics reach for these targets by around 2027, when the third generation of the neutrino experiments starts operation, with a combined sensitivity of three experiments: T2K-II, $\mathrm{NO}\nu \mathrm{A}$ extension, and JUNO. It is shown that a joint analysis of these three experiments can conclusively determine the neutrino mass hierarchy. Also, at certain values of true ${\delta }_{CP}$, it provides closely around a $5\sigma$ confidence level (C.L.) to exclude $CP$-conserving values and more than a 50% fractional region of true ${\delta }_{CP}$ values can be explored with a statistic significance of at least a $3\sigma$ C.L. Besides, the joint analysis can provide unprecedented precision measurements of the atmospheric neutrino oscillation parameters and a great offer to solve the ${\theta }_{23}$ octant degeneracy in the case of nonmaximal mixing.