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 $R(\frac{\sigma (\overline{\nu })}{\sigma (\nu )})=0.373\pm 0.012(\mathrm{stat})\pm 0.015(\mathrm{syst})$.

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.

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.

The SciBooNE Collaboration reports a measurement of neutral current coherent ${\pi }^0$ production on carbon by a muon neutrino beam with average energy 0.8 GeV. The separation of coherent from inclusive ${\pi }^0$ production has been improved by detecting recoil protons from resonant ${\pi }^0$ production. We measure the ratio of the neutral current coherent ${\pi }^0$ production to total charged current cross sections to be $(1.16\pm 0.24)\times {10}^{-2}$. The ratio of charged current coherent ${\pi }^{+}$ to neutral current coherent ${\pi }^0$ production is calculated to be ${0.14}_{-0.28}^{+0.30}$, using our published charged current coherent pion measurement.

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.

Dark matter elastic scattering off nuclei can result in the excitation and ionization of the recoiling atom through the so-called Migdal effect. The energy deposition from the ionization electron adds to the energy deposited by the recoiling nuclear system and allows for the detection of interactions of sub-$\mathrm{GeV}/{\mathrm{c}}^2$ mass dark matter. We present new constraints for sub-$\mathrm{GeV}/{\mathrm{c}}^2$ dark matter using the dual-phase liquid argon time projection chamber of the DarkSide-50 experiment with an exposure of $(12306\pm 184)\mathrm{kg}\mathrm{d}$. The analysis is based on the ionization signal alone and significantly enhances the sensitivity of DarkSide-50, enabling sensitivity to dark matter with masses down to $40\mathrm{MeV}/c^2$. Furthermore, it sets the most stringent upper limit on the spin independent dark matter nucleon cross section for masses below $3.6\mathrm{GeV}/c^2$.

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 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.

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.

The DarkSide-50 direct-detection dark matter experiment is a dual-phase argon time projection chamber operating at Laboratori Nazionali del Gran Sasso. This paper reports on the blind analysis of a $(16660\pm 270)\mathrm{kg}\mathrm{d}$ exposure using a target of low-radioactivity argon extracted from underground sources. We find no events in the dark matter selection box and set a 90% C.L. upper limit on the dark matter–nucleon spin-independent cross section of $1.14\times {10}^{-44}{\mathrm{cm}}^2$ ($3.78\times {10}^{-44}{\mathrm{cm}}^2$, $3.43\times {10}^{-43}{\mathrm{cm}}^2$) for a WIMP mass of $100\mathrm{GeV}/c^2$ ($1\mathrm{TeV}/c^2$, $10\mathrm{TeV}/c^2$).

A liquid argon time projection chamber, constructed for the Argon Response to Ionization and Scintillation (ARIS) experiment, is exposed to the highly collimated and quasimonoenergetic LICORNE neutron beam at the Institut de Physique Nucléaire d’Orsay (IPNO) in order to study the scintillation response to nuclear and electronic recoils. An array of liquid scintillator detectors, arranged around the apparatus, tag scattered neutrons and select nuclear recoil energies in the [7, 120] keV energy range. The relative scintillation efficiency of nuclear recoils is measured to high precision at null field, and the ion-electron recombination probability is extracted for a range of applied electric fields. Single-scattered Compton electrons, produced by gammas emitted from the deexcitation of ${}^7{\mathrm{Li}}^{*}$ in coincidence with the beam pulse, along with calibration gamma sources, are used to extract the recombination probability as a function of energy and electron drift field. The ARIS results are compared with three recombination probability parametrizations (Thomas-Imel, Doke-Birks, and PARIS), allowing for the definition of a fully comprehensive model of the liquid argon response to nuclear and electronic recoils down to the few-keV range. The constraints provided by ARIS to the liquid argon response at low energy allow the reduction of systematics affecting the sensitivity of dark matter search experiments based on liquid argon.

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.

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: $\text{normal hierarchy}:{\sin }^2{\theta }_{23}=0.514_{-0.056}^{+0.055}$ and $\mathrm{\Delta }{m}_{32}^2=(2.51\pm 0.10)\times 10^{-3}{\mathrm{eV}}^2/c^4$ and $\text{inverted hierarchy}:{\sin }^2{\theta }_{23}=0.511\pm 0.055$ and $\mathrm{\Delta }{m}_{13}^2=(2.48\pm 0.10)\times 10^{-3}{\mathrm{eV}}^2/c^4$. The analysis accounts for multinucleon 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, $|\mathrm{\Delta }{m}^2|$, ${\sin }^2{\theta }_{23}$, ${\sin }^2{\theta }_{13}$, ${\delta }_{CP}$, 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 ${\delta }_{CP}=[0.15,0.83]\pi$ for normal hierarchy and ${\delta }_{CP}=[-0.08,1.09]\pi$ 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% one-dimensional credible intervals for the other oscillation parameters, when reactor data are included, are ${\sin }^2{\theta }_{23}=0.528_{-0.038}^{+0.055}$ and $|\mathrm{\Delta }{m}_{32}^2|=(2.51\pm 0.11)\times 10^{-3}{\mathrm{eV}}^2/c^4$.

We report a measurement of the ${\nu }_{\mu }$ inclusive charged current cross sections on iron and hydrocarbon in the Tokai-to-Kamioka (T2K) on-axis neutrino beam. The measured inclusive charged current cross sections on iron and hydrocarbon averaged over the T2K on-axis flux with a mean neutrino energy of 1.51 GeV are $(1.444\pm 0.002(\text{stat}{)}_{-0.157}^{+0.189}(\text{syst}))\times 10^{-38}{\mathrm{cm}}^2/\text{nucleon}$ and $(1.379\pm 0.009(\text{stat}{)}_{-0.147}^{+0.178}(\text{syst}))\times 10^{-38}{\mathrm{cm}}^2/\text{nucleon}$, respectively, and their cross-section ratio is $1.047\pm 0.007(\text{stat})\pm 0.035(\text{syst})$. These results agree well with the predictions of the neutrino interaction model, and thus we checked the correct treatment of the nuclear effect for iron and hydrocarbon targets in the model within the measurement precisions.

The Tokai-to-Kamioka (T2K) experiment studies neutrino oscillations using an off-axis muon neutrino beam with a peak energy of about 0.6 GeV that originates at the Japan Proton Accelerator Research Complex accelerator facility. Interactions of the neutrinos are observed at near detectors placed at 280 m from the production target and at the far detector—Super-Kamiokande—located 295 km away. The flux prediction is an essential part of the successful prediction of neutrino interaction rates at the T2K detectors and is an important input to T2K neutrino oscillation and cross section measurements. A FLUKA and GEANT3-based simulation models the physical processes involved in the neutrino production, from the interaction of primary beam protons in the T2K target, to the decay of hadrons and muons that produce neutrinos. The simulation uses proton beam monitor measurements as inputs. The modeling of hadronic interactions is reweighted using thin target hadron production data, including recent charged pion and kaon measurements from the NA61/SHINE experiment. For the first T2K analyses the uncertainties on the flux prediction are evaluated to be below 15% near the flux peak. The uncertainty on the ratio of the flux predictions at the far and near detectors is less than 2% near the flux peak.

We present a search for dark matter particles with sub-$\mathrm{GeV}/c^2$ masses whose interactions have final state electrons using the DarkSide-50 experiment’s $(12306\pm 184)\mathrm{kg}\mathrm{d}$ low-radioactivity liquid argon exposure. By analyzing the ionization signals, we exclude new parameter space for the dark matter-electron cross section ${\overline{\sigma }}_e$, the axioelectric coupling constant $g_{\mathrm{A}e}$, and the dark photon kinetic mixing parameter $\kappa$. We also set the first dark matter direct-detection constraints on the mixing angle ${|U_{e4}|}^2$ for $\mathrm{keV}/c^2$ sterile neutrinos.

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.

Dark matter may induce an event in an Earth-based detector, and its event rate is predicted to show an annual modulation as a result of the Earth’s orbital motion around the Sun. We searched for this modulation signature using the ionization signal of the DarkSide-50 liquid argon time projection chamber. No significant signature compatible with dark matter is observed in the electron recoil equivalent energy range above $40{\mathrm{eV}}_{\mathrm{ee}}$, the lowest threshold ever achieved in such a search.