Search

The T2K off-axis near detector ND280 is used to make the first differential cross-section measurements of electron neutrino charged current interactions at energies $\sim 1\mathrm{GeV}$ as a function of electron momentum, electron scattering angle, and four-momentum transfer of the interaction. The total flux-averaged ${\nu }_e$ charged current cross section on carbon is measured to be $⟨\sigma {⟩}_{\varphi }=1.11\pm 0.10(\text{stat})\pm 0.18(\text{syst})\times 10^{-38}{\mathrm{cm}}^2/\text{nucleon}$. The differential and total cross-section measurements agree with the predictions of two leading neutrino interaction generators, NEUT and GENIE. The NEUT prediction is $1.23\times 10^{-38}{\mathrm{cm}}^2/\text{nucleon}$ and the GENIE prediction is $1.08\times 10^{-38}{\mathrm{cm}}^2/\text{nucleon}$. The total ${\nu }_e$ charged current cross-section result is also in agreement with data from the Gargamelle experiment.

We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to $1.43\times {10}^{20}$ protons on target, we observe 31 fully-contained single $\mu$-like ring events in Super-Kamiokande, compared with an expectation of $104\pm 14(\mathrm{syst})$ events without neutrino oscillations. The best-fit point for two-flavor ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations is ${sin}^2(2{\theta }_{23})=0.98$ and $|\Delta m_{32}^2|=2.65\times {10}^{-3}{\mathrm{eV}}^2$. The boundary of the 90% confidence region includes the points $({sin}^2(2{\theta }_{23}),|\Delta m_{32}^2|)=(1.0,3.1\times {10}^{-3}{\mathrm{eV}}^2)$, (0.84, $2.65\times {10}^{-3}{\mathrm{eV}}^2$) and (1.0, $2.2\times {10}^{-3}{\mathrm{eV}}^2$).

T2K has performed the first measurement of ${\nu }_{\mu }$ inclusive charged current interactions on carbon at neutrino energies of $\sim 1\mathrm{GeV}$ where the measurement is reported as a flux-averaged double differential cross section in muon momentum and angle. The flux is predicted by the beam Monte Carlo and external data, including the results from the NA61/SHINE experiment. The data used for this measurement were taken in 2010 and 2011, with a total of $10.8\times {10}^{19}$ protons-on-target. The analysis is performed on 4485 inclusive charged current interaction candidates selected in the most upstream fine-grained scintillator detector of the near detector. The flux-averaged total cross section is $⟨{\sigma }_{\mathrm{CC}}{⟩}_{\varphi }=(6.91\pm 0.13(\mathrm{stat})\pm 0.84(\mathrm{syst}))\times {10}^{-39}\frac{{\mathrm{cm}}^2}{\mathrm{nucleon}}$ for a mean neutrino energy of 0.85 GeV.

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.

We point out that some models with infinite additional dimension(s) of Randall-Sundrum type predict the disappearance of orthopositronium $(o-\mathrm{Ps})$ into additional dimension(s). The experimental signature of this effect is the $o-\mathrm{Ps}\to \mathrm{invisible}$ decay of orthopositronium which may occur at a rate within three orders of magnitude of the present experimental upper limit. This result enhances existing motivations for a more sensitive search for this decay mode and suggests additional directions for testing extra dimensions in nonaccelerator experiments.

A search for a new $Z^{\prime }$ gauge boson associated with (un)broken $B-L$ symmetry in the keV–GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with $3.22\times {10}^{11}$ electrons on target collected during 2016–2021 runs, no signal events were found. This allows us to derive new constraints on the $Z^{\prime }-e$ coupling strength, which, for the mass range $0.3\lesssim m_{Z^{\prime }}\lesssim 100\mathrm{MeV}$, are more stringent compared to those obtained from the neutrino-electron scattering data.

The T2K experiment has reported the first observation of the appearance of electron neutrinos in a muon neutrino beam. The main and irreducible background to the appearance signal comes from the presence in the neutrino beam of a small intrinsic component of electron neutrinos originating from muon and kaon decays. In T2K, this component is expected to represent 1.2% of the total neutrino flux. A measurement of this component using the near detector (ND280), located 280 m from the target, is presented. The charged current interactions of electron neutrinos are selected by combining the particle identification capabilities of both the time projection chambers and electromagnetic calorimeters of ND280. The measured ratio between the observed electron neutrino beam component and the prediction is $1.01\pm 0.10$ providing a direct confirmation of the neutrino fluxes and neutrino cross section modeling used for T2K neutrino oscillation analyses. Electron neutrinos coming from muons and kaons decay are also separately measured, resulting in a ratio with respect to the prediction of $0.68\pm 0.30$ and $1.10\pm 0.14$, respectively.

We report the first measurement of the neutrino-oxygen neutral-current quasielastic (NCQE) cross section. It is obtained by observing nuclear deexcitation $\gamma$ rays which follow neutrino-oxygen interactions at the Super-Kamiokande water Cherenkov detector. We use T2K data corresponding to $3.01\times 10^{20}$ protons on target. By selecting only events during the T2K beam window and with well-reconstructed vertices in the fiducial volume, the large background rate from natural radioactivity is dramatically reduced. We observe 43 events in the 4–30 MeV reconstructed energy window, compared with an expectation of 51.0, which includes an estimated 16.2 background events. The background is primarily nonquasielastic neutral-current interactions and has only 1.2 events from natural radioactivity. The flux-averaged NCQE cross section we measure is $1.55\times 10^{-38}{\mathrm{cm}}^2$ with a 68% confidence interval of $(1.22,2.20)\times 10^{-38}{\mathrm{cm}}^2$ at a median neutrino energy of 630 MeV, compared with the theoretical prediction of $2.01\times 10^{-38}{\mathrm{cm}}^2$.

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 the flux-averaged cross section for charged current coherent ${\pi }^{+}$ 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.

This paper reports a measurement by the T2K experiment of the ${\nu }_{\mu }$ charged current quasielastic (CCQE) cross section on a carbon target with the off-axis detector based on the observed distribution of muon momentum ($p_{\mu }$) and angle with respect to the incident neutrino beam (${\theta }_{\mu }$). The flux-integrated CCQE cross section was measured to be $⟨\sigma ⟩=(0.83\pm 0.12)\times {10}^{-38}{\mathrm{cm}}^2$. The energy dependence of the CCQE cross section is also reported. The axial mass, $M_A^{\mathrm{QE}}$, of the dipole axial form factor was extracted assuming the Smith-Moniz CCQE model with a relativistic Fermi gas nuclear model. Using the absolute (shape-only) $p_{\mu }\text{−}\cos {\theta }_{\mu }$ distribution, the effective $M_A^{\mathrm{QE}}$ parameter was measured to be $1.26_{-0.18}^{+0.21}\mathrm{GeV}/c^2$ ($1.43_{-0.22}^{+0.28}\mathrm{GeV}/c^2$).

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 improved results on a direct search for a new $X(16.7\mathrm{MeV})$ boson that could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the decays of the excited ${}^8{\mathrm{Be}}^{*}$ nuclei (“Berillium or X17 anomaly”) are reported. Interestingly, new recent results in the nuclear transitions of another nucleus, ${}^4\mathrm{He}$, seems to support this anomaly spurring the need for an independent measurement. If the $X$ boson exists, it could be produced in the bremsstrahlung reaction $e^{-}Z\to e^{-}ZX$ by a high energy beam of electrons incident on the active target in the NA64 experiment at the CERN Super Proton Synchrotron and observed through its subsequent decay into $e^{+}{e}^{-}$ pairs. No evidence for such decays was found from the combined analysis of the data samples with total statistics corresponding to $8.4\times {10}^{10}$ electrons on target collected in 2017 and 2018. This allows one to set new limits on the $X-e^{-}$ coupling in the range $1.2\times {10}^{-4}\lesssim {\epsilon }_e\lesssim 6.8\times {10}^{-4}$, excluding part of the parameter space favored by the X17 anomaly, and setting new bounds on the mixing strength of photons with dark photons ($A^{\prime }$) with a mass $\lesssim 24\mathrm{MeV}$. For the 2018 run, the setup was optimized to probe the region of parameter space characterized by a large coupling $\epsilon$. This allowed a significant improvement in sensitivity despite a relatively modest increase in statistics.

We carried out a model-independent search for light scalar ($s$) and pseudoscalar axionlike ($a$) particles that couple to two photons by using the high-energy CERN SPS H4 electron beam. The new particles, if they exist, could be produced through the Primakoff effect in interactions of hard bremsstrahlung photons generated by 100 GeV electrons in the NA64 active dump with virtual photons provided by the nuclei of the dump. The $a(s)$ would penetrate the downstream HCAL module, serving as a shield, and would be observed either through their $a(s)\to \gamma \gamma$ decay in the rest of the HCAL detector, or as events with a large missing energy if the $a(s)$ decays downstream of the HCAL. This method allows for the probing of the $a(s)$ parameter space, including those from generic axion models, inaccessible to previous experiments. No evidence of such processes has been found from the analysis of the data corresponding to $2.84\times {10}^{11}$ electrons on target, allowing us to set new limits on the $a(s)\gamma \gamma$-coupling strength for $a(s)$ masses below 55 MeV.

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 $\mathrm{\Delta }{m}_{32}^2=({+2.506}_{-0.052}^{+0.039})\times {10}^{-3}{\mathrm{eV}}^2$.

It has been recently suggested that the vacuum magnetic dichroism observed by the PVLAS experiment could be explained by pair production of a new light, $m_{\epsilon }\simeq 0.1\mathrm{eV}$, millicharged, $q_{\epsilon }\simeq 3\times {10}^{-6}e$, fermions ($\epsilon$). In addition, it has been pointed out that millicharged particles with $q_{\epsilon }\gtrsim {10}^{-9}e$ appear naturally in models based on the string theory. We show that low energy reactor neutrino experiments provide a sensitive probe of millicharged particles. Considering, as an example, recent results of the TEXONO experiment searching for the neutrino magnetic moment, a new upper bound $q_{\epsilon }\lesssim {10}^{-5}e$ for the mass region $m_{\epsilon }<1\mathrm{keV}$ is derived. These results enhance motivations for a more sensitive search for such particles in near future experiments. Furthermore, a direct experimental limit on the electric charge of the electron antineutrino $q_{{\overline{\nu }}_e}<3.7\times {10}^{-12}e$ is obtained.

The current most stringent constraints for the existence of sub-GeV dark matter coupling to Standard Model via a massive vector boson $A^{\prime }$ were set by the NA64 experiment for the mass region $m_{A^{\prime }}\lesssim 250\mathrm{MeV}$, by analyzing data from the interaction of $2.84\times {10}^{11}$ 100-GeV electrons with an active thick target and searching for missing-energy events. In this work, by including $A^{\prime }$ production via secondary positron annihilation with atomic electrons, we extend these limits in the 200–300 MeV region by almost an order of magnitude, touching for the first time the dark matter relic density constrained parameter combinations. Our new results demonstrate the power of the resonant annihilation process in missing energy dark-matter searches, paving the road to future dedicated $e^{+}$ beam efforts.