Showing papers by "Tsuyoshi Nakaya published in 2014"

Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

Abstract: The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of 7.3 sigma when compared to 4.92 +/- 0.55 expected background events. In the Pontecorvo-Maki-Nakagawa-Sakata mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles theta(12), theta(23), theta(13), a mass difference vertical bar Delta m(32)(2)vertical bar and a CP violating phase delta(CP). In this neutrino oscillation scenario, assuming vertical bar Delta m(32)(2)vertical bar = 2.4 x 10(-3) eV(2), sin theta(2)(23) = 0.5, and vertical bar Delta m(32)(2)vertical bar > 0 (vertical bar Delta m(32)(2)vertical bar <0), a best- fit value of sin2 theta(2)(13) = 0.140(- 0.032)(+0.038) (0.170(-0.037)(+0.045)) is obtained at delta(CP) = 0. When combining the result with the current best knowledge of oscillation parameters including the world average value of theta(13) from reactor experiments, some values of delta(CP) are disfavored at the 90% C. L.

Precise Measurement of the Neutrino Mixing Parameter \theta_{23} from Muon Neutrino Disappearance in an Off-axis Beam

Abstract: New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter theta_{23}. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to 6.57 x 10^{20} protons on target, T2K has fit the energy-dependent nu_mu oscillation probability to determine oscillation parameters. Marginalizing over the values of other oscillation parameters yields sin^2 (theta_{23}) = 0.514 +0.055/-0.056 (0.511 +- 0.055), assuming normal (inverted) mass hierarchy. The best-fit mass-squared splitting for normal hierarchy is Delta m^2_{32} = (2.51 +- 0.10) x 10^{-3} eV^2/c^4 (inverted hierarchy: Delta m^2_{13} = (2.48 +- 0.10) x 10^{-3} eV^2/c^4). Adding a model of multinucleon interactions that affect neutrino energy reconstruction is found to produce only small biases in neutrino oscillation parameter extraction at current levels of statistical uncertainty.

Precise measurement of the neutrino mixing parameter θ23 from muon neutrino disappearance in an off-axis beam

Abstract: New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter theta_{23}. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to 6.57 x 10^{20} protons on target, T2K has fit the energy-dependent nu_mu oscillation probability to determine oscillation parameters. Marginalizing over the values of other oscillation parameters yields sin^2 (theta_{23}) = 0.514 +0.055/-0.056 (0.511 +- 0.055), assuming normal (inverted) mass hierarchy. The best-fit mass-squared splitting for normal hierarchy is Delta m^2_{32} = (2.51 +- 0.10) x 10^{-3} eV^2/c^4 (inverted hierarchy: Delta m^2_{13} = (2.48 +- 0.10) x 10^{-3} eV^2/c^4). Adding a model of multinucleon interactions that affect neutrino energy reconstruction is found to produce only small biases in neutrino oscillation parameter extraction at current levels of statistical uncertainty.

A Long Baseline Neutrino Oscillation Experiment Using J-PARC Neutrino Beam and Hyper-Kamiokande

Abstract: Hyper-Kamiokande will be a next generation underground water Cherenkov detector with a total (fiducial) mass of 0.99 (0.56) million metric tons, approximately 20 (25) times larger than that of Super-Kamiokande. One of the main goals of Hyper-Kamiokande is the study of $CP$ asymmetry in the lepton sector using accelerator neutrino and anti-neutrino beams. In this document, the physics potential of a long baseline neutrino experiment using the Hyper-Kamiokande detector and a neutrino beam from the J-PARC proton synchrotron is presented. The analysis has been updated from the previous Letter of Intent [K. Abe et al., arXiv:1109.3262 [hep-ex]], based on the experience gained from the ongoing T2K experiment. With a total exposure of 7.5 MW $\times$ 10$^7$ sec integrated proton beam power (corresponding to $1.56\times10^{22}$ protons on target with a 30 GeV proton beam) to a $2.5$-degree off-axis neutrino beam produced by the J-PARC proton synchrotron, it is expected that the $CP$ phase $\delta_{CP}$ can be determined to better than 19 degrees for all possible values of $\delta_{CP}$, and $CP$ violation can be established with a statistical significance of more than $3\,\sigma$ ($5\,\sigma$) for $76%$ ($58%$) of the $\delta_{CP}$ parameter space.

Calibration of the Super-Kamiokande detector

Abstract: Procedures and results on hardware-level detector calibration in Super-Kamiokande (SK) are presented in this paper. In particular, we report improvements made in our calibration methods for the experimental phase IV in which new readout electronics have been operating since 2008. The topics are separated into two parts. The first part describes the determination of constants needed to interpret the digitized output of our electronics so that we can obtain physical numbers such as photon counts and their arrival times for each photomultiplier tube (PMT). In this context, we developed an in situ procedure to determine high-voltage settings for PMTs in large detectors like SK, as well as a new method for measuring PMT quantum efficiency and gain in such a detector. The second part describes modeling of the detector in Monte Carlo simulations, including, in particular, the optical properties of the water target and their variability over time. Detailed studies on water quality are also presented. As a result of this work, we have achieved a precision sufficient for physics analyses over a wide energy range (from a few MeV to above 1 TeV). For example, charge determination was at the level of 1%, and the timing resolution was 2.1 ns at the one-photoelectron charge level and 0.5 ns at the 100-photoelectron charge level.

Search for proton decay via p→νK + using 260 kiloton.year data of Super-Kamiokande

Abstract: We have searched for proton decay via p→νK+ using Super-Kamiokande data from April 1996 to February 2013, 260 kiloton•year exposure in total. No evidence for this proton decay mode is found. A lower limit of the proton lifetime is set to τ/B(p→νK+)>5.9×1033 years at 90% confidence level.

First indication of terrestrial matter effects on solar neutrino oscillation

Abstract: We report an indication that the elastic scattering rate of solar B 8 neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through Earth during nighttime. We determine the day-night asymmetry, defined as the difference of the average day rate and average night rate divided by the average of those two rates, to be [ - 3.2 ± 1.1 ( stat ) ± 0.5 ( syst ) ] % , which deviates from zero by 2.7 σ . Since the elastic scattering process is mostly sensitive to electron-flavored solar neutrinos, a nonzero day-night asymmetry implies that the flavor oscillations of solar neutrinos are affected by the presence of matter within the neutrinos’ flight path. Super-Kamiokande’s day-night asymmetry is consistent with neutrino oscillations for 4 × 1 0 - 5 eV 2 ≤ Δ m 21 2 ≤ 7 × 1 0 - 5 eV 2 and large mixing values of θ 12 , at the 68% C.L.

Measurement of the inclusive electron neutrino charged current cross section on carbon with the T2K near detector.

Abstract: 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 ∼1 GeV as a function of electron momentum, electron scattering angle, and four-momentum transfer of the interaction. The total flux-averaged νe charged current cross section on carbon is measured to be ⟨σ⟩ϕ=1.11±0.10(stat)±0.18(syst)×10−38 cm2/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 1.23×10−38 cm2/nucleon and the GENIE prediction is 1.08×10−38 cm2/nucleon. The total νe charged current cross-section result is also in agreement with data from the Gargamelle experiment.

Measurement of the inclusive νμ charged current cross section on iron and hydrocarbon in the T2K on-axis neutrino beam

Abstract: 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 GeVare (1.444 +/- 0.002(stat)(-0.157)(+0.189)- (syst)) x 10(-38) cm(2)/ nucleon and (1.379 +/- 0.009(stat)(-0.147)(+0.178) (syst)) x 10(-38) 38 cm(2)/ nucleon, respectively, and their cross-section ratio is 1.047 +/- 0.007(stat) +/- 0.035(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.

Search for nucleon decay via n → ν ¯ π0 and p → ν ¯ π+ in super-kamiokande

Abstract: 2Although there is strong theoretical support that na-ture can be described by a grand uni ed theory (GUT) [1,2], there is currently no direct experimental evidence.One of the most powerful ways to test grand uni cationis to look for proton (or bound neutron) decay. MostGUTs have an unstable proton; in the absence of anobservation, setting experimental limits on the protonlifetime can provide useful constraints on the nature ofgrand uni ed theories. Observation, on the other hand,would be tantalizing evidence of new physics beyond theStandard Model.One of the more simple but interesting candidates forgrand uni cation is SO(10), where the Standard Model’sSU(3), SU(2), and U(1) are contained within the largergauge group. The class of models based on SO(10) uni- cation generally make predictions for neutrino massesand mixing that are broadly in accord with all knownneutrino mixing data [3, 4]. The minimal supersym-metric SO(10) model with a 126 Higgs eld describedin Ref. [3] is the particular motivation for the analysispresented here. In addition to predicting neutrino massand mixing in agreement with observations, it leaves R-parity unbroken, which guarantees the existence of stabledark matter. For some region of its allowed parameterspace, this model predicts that the dominant nucleon de-cay modes will be p!ˇ

Search for dinucleon decay into kaons in Super-Kamiokande.

Abstract: A search for the dinucleon decay $pp\ensuremath{\rightarrow}{K}^{+}{K}^{+}$ has been performed using $91.6\text{ }\text{ }\mathrm{kton}\ifmmode\cdot\else\textperiodcentered\fi{}\text{yr}$ data from Super-Kamiokande-I. This decay provides a sensitive probe of the $R$-parity-violating parameter ${{\ensuremath{\lambda}}_{112}}^{\ensuremath{'}\ensuremath{'}}$. A boosted decision tree analysis found no signal candidates in the data. The expected background was $0.28\ifmmode\pm\else\textpm\fi{}0.19$ atmospheric neutrino induced events and the estimated signal detection efficiency was $12.6%\ifmmode\pm\else\textpm\fi{}3.2%$. A lower limit of $1.7\ifmmode\times\else\texttimes\fi{}{10}^{32}\text{ }\text{ }\text{years}$ has been placed on the partial lifetime of the decay $^{16}\mathrm{O}\ensuremath{\rightarrow}^{14}\mathrm{C}{K}^{+}{K}^{+}$ at 90% C.L. A corresponding upper limit of $7.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ has been placed on the parameter ${{\ensuremath{\lambda}}_{112}}^{\ensuremath{'}\ensuremath{'}}$.

Measurement of the intrinsic electron neutrino component in the T2K neutrino beam with the ND280 detector

Abstract: 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 +/- 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 +/- 0.30 and 1.10 +/- 0.14, respectively.

Neutrino Oscillation Physics Potential of the T2K Experiment

Abstract: The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle $\theta_{13}$ have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal $\sin^22\theta_{23}$, the octant of $\theta_{23}$, and the mass hierarchy, in addition to the measurements of $\delta_{CP}$, $\sin^2\theta_{23}$, and $\Delta m^2_{32}$, for various combinations of $ u$-mode and \(\bar{ u}\)-mode data-taking. With an exposure of $7.8\times10^{21}$~protons-on-target, T2K can achieve 1-$\sigma$ resolution of 0.050(0.054) on $\sin^2\theta_{23}$ and $0.040(0.045)\times10^{-3}~\rm{eV}^2$ on $\Delta m^2_{32}$ for 100\%(50\%) neutrino beam mode running assuming $\sin^2\theta_{23}=0.5$ and $\Delta m^2_{32} = 2.4\times10^{-3}$ eV$^2$. T2K will have sensitivity to the CP-violating phase $\delta_{\rm{CP}}$ at 90\% C.L. or better over a significant range. For example, if $\sin^22\theta_{23}$ is maximal (i.e $\theta_{23}$=$45^\circ$) the range is $-115^\circ<\delta_{\rm{CP}}<-60^\circ$ for normal hierarchy and $+50^\circ<\delta_{\rm{CP}}<+130^\circ$ for inverted hierarchy. When T2K data is combined with data from the NO$ u$A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero $\delta_{CP}$ is substantially increased compared to if each experiment is analyzed alone.

Letter of Intent to Construct a nuPRISM Detector in the J-PARC Neutrino Beamline

Abstract: As long-baseline neutrino experiments enter the precision era, the difficulties associated with understanding neutrino interaction cross sections on atomic nuclei are expected to limit experimental sensitivities to oscillation parameters. In particular, the ability to relate experimental observables to neutrino energy in previous experiments has relied solely on theoretical models of neutrino-nucleus interactions, which currently suffer from very large theoretical uncertainties. By observing charged current $ u_\mu$ interactions over a continuous range of off-axis angles from 1 to 4 degrees, the nuPRISM water Cherenkov detector can provide a direct measurement of the far detector lepton kinematics for any given set of oscillation parameters, which largely removes neutrino interaction modeling uncertainties from T2K oscillation measurements. This naturally provides a direct constraint on the relationship between lepton kinematics and neutrino energy. In addition, nuPRISM is a sensitive probe of sterile neutrino oscillations with multiple energy spectra, which provides unique constraints on possible background-related explanations of the MiniBooNE anomaly. Finally, high-precision measurements of neutrino cross sections on water are possible, including $ u_e$ measurements and the first ever measurements of neutral current interactions as a function of neutrino energy. The nuPRISM detector also benefits the proposed Hyper-Kamiokande project. A demonstration that neutrino interaction uncertainties can be controlled will be important to understanding the physics reach of Hyper-K. In addition, nuPRISM will provide an easily accessible prototype detector for many of the new hardware components currently under consideration for Hyper-K. The following document presents the configuration, physics impact, and preliminary cost estimates for a nuPRISM detector in the J-PARC neutrino beamline.

Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation γ rays

Abstract: We report the first measurement of the neutrino-oxygen neutral-current quasielastic (NCQE) cross section. It is obtained by observing nuclear deexcitation γ rays which follow neutrino-oxygen interactions at the Super-Kamiokande water Cherenkov detector. We use T2K data corresponding to 3.01×1020 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 1.55×10−38 cm2 with a 68% confidence interval of (1.22,2.20)×10−38 cm2 at a median neutrino energy of 630 MeV, compared with the theoretical prediction of 2.01×10−38 cm2.

Search for trilepton nucleon decay via p→e+νν and p→μ+νν in the Super-Kamiokande experiment.

Abstract: The trilepton nucleon decay modes p→e+νν and p→μ+νν violate |Δ(B-L)| by two units. Using data from a 273.4 kt yr exposure of Super-Kamiokande a search for these decays yields a fit consistent with no signal. Accordingly, lower limits on the partial lifetimes of τp→e+νν>1.7×10(32) years and τp→μ+νν>2.2×10(32) years at a 90% confidence level are obtained. These limits can constrain Grand Unified Theories which allow for such processes.

Measurement of the muon beam direction and muon flux for the T2K neutrino experiment

Abstract: The Tokai-to-Kamioka (T2K) neutrino experiment measures neutrino oscillations by using an almost pure muon neutrino beam produced at the J-PARC accelerator facility. The T2K muon monitor was installed to measure the direction and stability of the muon beam which is produced together with the muon neutrino beam. The systematic error in the muon beam direction measurement was estimated, using data and MC simulation, to be 0.28 mrad. During beam operation, the proton beam has been controlled using measurements from the muon monitor and the direction of the neutrino beam has been tuned to within 0.3 mrad with respect to the designed beam-axis. In order to understand the muon beam properties,measurement of the absolute muon yield at the muon monitor was conducted with an emulsion detector. The number of muon tracks was measured to be $(4.06\pm0.05)\times10^4$ cm$^{-2}$ normalized with $4\times10^{11}$ protons on target with 250 kA horn operation. The result is in agreement with the prediction which is corrected based on hadron production data.

Recent results from the T2K experiment

Abstract: The Tokai to Kamioka (T2K) experiment studies neutrino oscillations using a beam of muon neutrinos produced by an accelerator. The neutrinos travel from J-PARC on the east coast of Japan and are detected 295 kilometers further away in the Super-Kamiokande detector. A complex of near detectors located 280 meters away from the neutrino production target is used to better characterize the neutrino beam and reduce systematic uncertainties. The experiment aims at measuring electronic neutrino appearance ( ν μ → ν e oscillation) to measure the neutrino mixing angle θ 13 , and muon neutrino disappearance to measure the neutrino mixing angle θ 23 and mass splitting | Δ m 32 2 |. We report here electron neutrino appearance results using three years of data, recorded until the 2012 summer, as well as muon neutrino disappearance results based on the data coming from the first two years of the experiment.

New large aperture, hybrid photo-detector and photo multiplier tube for a gigantic water Cherenkov ring imaging detector

Abstract: We are developing a 20-in. aperture high quantum efficiency photo-multiplier tube (PMT) and a hybrid photo-detector (HPD) for Hyper-Kamiokande which is a next generation underground large water Cherenkov detector. We have measured prototypes of 20-in. PMT with a high quantum efficiency photocathode, 30% at 400 nm, and 8-in. HPDs with a normal quantum efficiency photocathode, 22% at 400 nm, in a 200-ton water tank and checked their performance. The PMTs have a 2.7 ns (sigma) timing resolution and 43% (sigma) charge resolution for single photo-electron. Compared to PMTs, HPDs show a better performance with a 1.7 ns timing resolution and 32.

Developement of hybrid photo-detectors for the Hyper-Kamiokande experiment

Abstract: We are developing a hybrid photo detector (HPD) for the Hyper-Kamiokande Project. Eight-inch HPDs were prepared to evaluate their performance. Based on the results from these measurements, HPDs achieve a better performance such as single photon separation than conventional PMTs. A verification study lasting a few years in a water tank is planned in 2013 to check their feasibility.