arXiv subjects

S. Hirota

Publications and source records attributed to S. Hirota.

At least 37 records · Page 2Linked to original sources

arXiv2016

Measurements of the atmospheric neutrino flux by Super-Kamiokande: energy spectra, geomagnetic effects, and solar modulation

A comprehensive study on the atmospheric neutrino flux in the energy region from sub-GeV up to several TeV using the Super-Kamiokande water Cherenkov detector is presented in this paper. The energy and azimuthal spectra of the atmospheric $ν_e+{\barν}_e$ and $ν_μ+{\barν}_μ$ fluxes are measured. The energy spectra are obtained using an iterative unfolding method by combining various event topologies with differing energy responses. The azimuthal spectra depending on energy and zenith angle, and their modulation by geomagnetic effects, are also studied. A predicted east-west asymmetry is observed in both the $ν_e$ and $ν_μ$ samples at 8.0 σ and 6.0 σ significance, respectively, and an indication that the asymmetry dipole angle changes depending on the zenith angle was seen at the 2.2 σ level. The measured energy and azimuthal spectra are consistent with the current flux models within the estimated systematic uncertainties. A study of the long-term correlation between the atmospheric neutrino flux and the solar magnetic activity cycle is also performed, and a weak indication of a correlation was seen at the 1.1 σ level, using SK I-IV data spanning a 20 year period. For particularly strong solar activity periods known as Forbush decreases, no theoretical prediction is available, but a deviation below the typical neutrino event rate is seen at the 2.4 σ level.

hep-ex↗

arXiv2016

Search for Neutrinos in Super-Kamiokande associated with Gravitational Wave Events GW150914 and GW151226

We report the results from a search in Super-Kamiokande for neutrino signals coincident with the first detected gravitational wave events, GW150914 and GW151226, using a neutrino energy range from 3.5 MeV to 100 PeV. We searched for coincident neutrino events within a time window of $\pm$500 seconds around the gravitational wave detection time. Four neutrino candidates are found for GW150914 and no candidates are found for GW151226. The remaining neutrino candidates are consistent with the expected background events. We calculated the 90\% confidence level upper limits on the combined neutrino fluence for both gravitational wave events, which depends on event energy and topologies. Considering the upward going muon data set (1.6 GeV - 100 PeV) the neutrino fluence limit for each gravitational wave event is 14 - 37 (19 - 50) cm$^{-2}$ for muon neutrinos (muon antineutrinos), depending on the zenith angle of the event. In the other data sets, the combined fluence limits for both gravitational wave events range from 2.4$\times 10^{4}$ to 7.0$\times 10^{9}$ cm$^{-2}$.

astro-ph.HE↗

arXiv2016

Sensitivity of the T2K accelerator-based neutrino experiment with an Extended run to $20\times10^{21}$ POT

Recent measurements at the T2K experiment indicate that CP violation in neutrino mixing may be observed in the future by long-baseline neutrino oscillation experiments. We explore the physics program of an extension to the currently approved T2K running of $7.8\times 10^{21}$ protons-on-target to $20\times 10^{21}$ protons-on-target,aiming at initial observation of CP violation with 3$\,σ$ or higher significance for the case of maximum CP violation. With accelerator and beam line upgrades, as well as analysis improvements, this program would occur before the next generation of long-baseline neutrino oscillation experiments that are expected to start operation in 2026.

hep-ex↗

arXiv2016

Measurement of Muon Antineutrino Oscillations with an Accelerator-Produced Off-Axis Beam

T2K reports its first measurements of the parameters governing the disappearance of $\barν_μ$ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic $\barν_μ$ beam, produced with a peak energy of 0.6 GeV at J-PARC, is observed at the far detector Super-Kamiokande, 295 km away, where the $\barν_μ$ survival probability is expected to be minimal. Using a dataset corresponding to $4.01 \times 10^{20}$ protons on target, $34$ fully contained $μ$-like events were observed. The best-fit oscillation parameters are $\sin^2 (\barθ_{23}) = 0.45$ and $|Δ\bar{m}^2_{32}| = 2.51 \times 10^{-3}$ eV$^2$ with 68% confidence intervals of 0.38 - 0.64 and 2.26 - 2.80 $\times 10^{-3}$ eV$^2$ respectively. These results are in agreement with existing antineutrino parameter measurements and also with the $ν_μ$ disappearance parameters measured by T2K.

hep-ex↗

arXiv2016

Real-Time Supernova Neutrino Burst Monitor at Super-Kamiokande

We present a real-time supernova neutrino burst monitor at Super-Kamiokande (SK). Detecting supernova explosions by neutrinos in real time is crucial for giving a clear picture of the explosion mechanism. Since the neutrinos are expected to come earlier than light, a fast broadcasting of the detection may give astronomers a chance to make electromagnetic radiation observations of the explosions right at the onset. The role of the monitor includes a fast announcement of the neutrino burst detection to the world and a determination of the supernova direction. We present the online neutrino burst detection system and studies of the direction determination accuracy based on simulations at SK.

astro-ph.HE↗

arXiv2016

Measurement of double-differential muon neutrino charged-current interactions on C$_8$H$_8$ without pions in the final state using the T2K off-axis beam

We report the measurement of muon neutrino charged-current interactions on carbon without pions in the final state at the T2K beam energy using 5.734$\times10^{20}$ protons on target. For the first time the measurement is reported as a flux-integrated, double-differential cross-section in muon kinematic variables ($\cosθ_μ$, $p_μ$), without correcting for events where a pion is produced and then absorbed by final state interactions. Two analyses are performed with different selections, background evaluations and cross-section extraction methods to demonstrate the robustness of the results against biases due to model-dependent assumptions. The measurements compare favorably with recent models which include nucleon-nucleon correlations but, given the present precision, the measurement does not solve the degeneracy between different models. The data also agree with Monte Carlo simulations which use effective parameters that are tuned to external data to describe the nuclear effects. The total cross-section in the full phase space is $σ= (0.417 \pm 0.047 \text{(syst)} \pm 0.005 \text{(stat)})\times 10^{-38} \text{cm}^2$ $\text{nucleon}^{-1}$ and the cross-section integrated in the region of phase space with largest efficiency and best signal-over-background ratio ($\cosθ_μ>0.6$ and $p_μ> 200$ MeV) is $σ= (0.202 \pm 0.0359 \text{(syst)} \pm 0.0026 \text{(stat)}) \times 10^{-38} \text{cm}^2$ $\text{nucleon}^{-1}$.

hep-ex↗

arXiv2015

Measurement of the muon neutrino inclusive charged-current cross section in the energy range of 1-3 GeV with the T2K INGRID detector

We report a measurement of the $ν_μ$-nucleus inclusive charged current cross section (=$σ^{cc}$) on iron using data from exposed to the J-PARC neutrino beam. The detector consists of 14 modules in total, which are spread over a range of off-axis angles from 0$^\circ$ to 1.1$^\circ$. The variation in the neutrino energy spectrum as a function of the off-axis angle, combined with event topology information, is used to calculate this cross section as a function of neutrino energy. The cross section is measured to be $σ^{cc}(1.1\text{ GeV}) = 1.10 \pm 0.15$ $(10^{-38}\text{cm}^2/\text{nucleon})$, $σ^{cc}(2.0\text{ GeV}) = 2.07 \pm 0.27$ $(10^{-38}\text{cm}^2/\text{nucleon})$, and $σ^{cc}(3.3\text{ GeV}) = 2.29 \pm 0.45$ $(10^{-38}\text{cm}^2/\text{nucleon})$, at energies of 1.1, 2.0, and 3.3 GeV, respectively. These results are consistent with the cross section calculated by the neutrino interaction generators currently used by T2K. More importantly, the method described here opens up a new way to determine the energy dependence of neutrino-nucleus cross sections.

hep-ex↗

arXiv2015

Search for Nucleon and Dinucleon Decays with an Invisible Particle and a Charged Lepton in the Final State at the Super-Kamiokande Experiment

Search results for nucleon decays $p \rightarrow e^+X$, $p \rightarrow μ^+X$, $n \rightarrow νγ$ (where $X$ is an invisible, massless particle) as well as dinucleon decays $np \rightarrow e^+ν$, $np \rightarrow μ^+ν$ and $np \rightarrow τ^+ν$ in the Super-Kamiokande experiment are presented. Using single-ring data from an exposure of 273.4 kton $\cdot$ years, a search for these decays yields a result consistent with no signal. Accordingly, lower limits on the partial lifetimes of $τ_{p \rightarrow e^+X} > 7.9 \times 10^{32}$ years, $τ_{p \rightarrow μ^+X} > 4.1 \times 10^{32}$ years, $τ_{n \rightarrow νγ} > 5.5 \times 10^{32}$ years, $τ_{np \rightarrow e^+ν} > 2.6 \times 10^{32}$ years, $τ_{np \rightarrow μ^+ν} > 2.2 \times 10^{32}$ years and $τ_{np \rightarrow τ^+ν} > 2.9 \times 10^{31}$ years at a $90 \% $ confidence level are obtained. Some of these searches are novel.

hep-ex↗

arXiv2015

Measurement of the Electron Neutrino Charged-current Interaction Rate on Water with the T2K ND280 pi-zero Detector

This paper presents a measurement of the charged current interaction rate of the electron neutrino beam component of the beam above $1.5$~GeV using the large fiducial mass of the T2K $π^0$ detector. The predominant poriton of the $ν_e$ flux ($\sim$85 %) at these energies comes from kaon decays. The measured ratio of the observed beam interaction rate to the predicted rate in the detector with water targets filled is 0.89 $\pm$ 0.08 (stat.) $\pm$ 0.11 (sys.), and with the water targets emptied is 0.90 $\pm$ 0.09 (stat.) $\pm$ 0.13 (sys.). The ratio obtained for the interactions on water only from an event subtraction method is 0.87 $\pm$ 0.33 (stat.) $\pm$ 0.21 (sys.). This is the first measurement of the interaction rate of electron neutrinos on water, which is particularly of interest to experiments with water Cherenkov detectors.

hep-ex↗

arXiv2015

Search for dinucleon decay into pions at Super-Kamiokande

A search for dinucleon decay into pions with the Super-Kamiokande detector has been performed with an exposure of 282.1 kiloton-years. Dinucleon decay is a process that violates baryon number by two units. We present the first search for dinucleon decay to pions in a large water Cherenkov detector. The modes $^{16}$O$(pp) \rightarrow$ $^{14}$C$π^{+}π^{+}$, $^{16}$O$(pn) \rightarrow$ $^{14}$N$π^{+}π^{0}$, and $^{16}$O$(nn) \rightarrow$ $^{14}$O$π^{0}π^{0}$ are investigated. No significant excess in the Super-Kamiokande data has been found, so a lower limit on the lifetime of the process per oxygen nucleus is determined. These limits are: $τ_{pp\rightarrowπ^{+}π^{+}} > 7.22 \times 10^{31}$ years, $τ_{pn\rightarrowπ^{+}π^{0}} > 1.70 \times 10^{32}$ years, and $τ_{nn\rightarrowπ^{0}π^{0}} > 4.04 \times 10^{32}$ years. The lower limits on each mode are about two orders of magnitude better than previous limits from searches for dinucleon decay in iron.

hep-ex↗

arXiv2015

Physics Potential of a Long Baseline Neutrino Oscillation Experiment Using J-PARC Neutrino Beam and Hyper-Kamiokande

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 paper, 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 uses the framework and systematic uncertainties derived 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, it is expected that the leptonic $CP$ phase $δ_{CP}$ can be determined to better than 19 degrees for all possible values of $δ_{CP}$, and $CP$ violation can be established with a statistical significance of more than $3\,σ$ ($5\,σ$) for $76\%$ ($58\%$) of the $δ_{CP}$ parameter space. Using both $ν_e$ appearance and $ν_μ$ disappearance data, the expected 1$σ$ uncertainty of $\sin^2θ_{23}$ is 0.015(0.006) for $\sin^2θ_{23}=0.5(0.45)$.

hep-ex↗

arXiv2015

Limits on sterile neutrino mixing using atmospheric neutrinos in Super-Kamiokande

We present limits on sterile neutrino mixing using 4,438 live-days of atmospheric neutrino data from the Super-Kamiokande experiment. We search for fast oscillations driven by an eV$^2$-scale mass splitting and for oscillations into sterile neutrinos instead of tau neutrinos at the atmospheric mass splitting. When performing both these searches we assume that the sterile mass splitting is large, allowing $\sin^2(Δm^2 L/4E)$ to be approximated as $0.5$, and we assume that there is no mixing between electron neutrinos and sterile neutrinos ($|U_{e4}|^2 = 0$). No evidence of sterile oscillations is seen and we limit $|U_{\mu4}|^2$ to less than 0.041 and $|U_{\tau4}|^2$ to less than 0.18 for $Δm^2 > 0.8$ eV$^2$ at the 90% C.L. in a 3+1 framework. The approximations that can be made with atmospheric neutrinos allow these limits to be easily applied to 3+N models, and we provide our results in a generic format to allow comparisons with other sterile neutrino models.

hep-ex↗

arXiv2015

Test of Lorentz invariance with atmospheric neutrinos

A search for neutrino oscillations induced by Lorentz violation has been performed using 4,438 live-days of Super-Kamiokande atmospheric neutrino data. The Lorentz violation is included in addition to standard three-flavor oscillations using the non-perturbative Standard Model Extension (SME), allowing the use of the full range of neutrino path lengths, ranging from 15 to 12,800 km, and energies ranging from 100 MeV to more than 100 TeV in the search. No evidence of Lorentz violation was observed, so limits are set on the renormalizable isotropic SME coefficients in the $eμ$, $μτ$, and $eτ$ sectors, improving the existing limits by up to seven orders of magnitude and setting limits for the first time in the neutrino $μτ$ sector of the SME.

hep-ex↗

arXiv2015

Search for neutrinos from annihilation of captured low-mass dark matter particles in the Sun by Super-Kamiokande

Super-Kamiokande (SK) can search for weakly interacting massive particles (WIMPs) by detecting neutrinos produced from WIMP annihilations occurring inside the Sun. In this analysis, we include neutrino events with interaction vertices in the detector in addition to upward-going muons produced in the surrounding rock. Compared to the previous result, which used the upward-going muons only, the signal acceptances for light (few-GeV/$c^2$ $\sim$ 200-GeV/$c^2$) WIMPs are significantly increased. We fit 3903 days of SK data to search for the contribution of neutrinos from WIMP annihilation in the Sun. We found no significant excess over expected atmospheric-neutrino background and the result is interpreted in terms of upper limits on WIMP-nucleon elastic scattering cross sections under different assumptions about the annihilation channel. We set the current best limits on the spin-dependent (SD) WIMP-proton cross section for WIMP masses below 200 GeV/$c^2$ (at 10 GeV/$c^2$, 1.49$\times 10^{-39}$ cm$^2$ for $χχ\rightarrow b \bar{b}$ and 1.31$\times 10^{-40}$ cm$^2$ for $χχ\rightarrowτ^+τ^-$ annihilation channels), also ruling out some fraction of WIMP candidates with spin-independent (SI) coupling in the few-GeV/$c^2$ mass range.

hep-ex↗

arXiv2015

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

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 $δ_{CP}$ can be determined to better than 19 degrees for all possible values of $δ_{CP}$, and $CP$ violation can be established with a statistical significance of more than $3\,σ$ ($5\,σ$) for $76%$ ($58%$) of the $δ_{CP}$ parameter space.

physics.ins-det↗

arXiv2014

Search for short baseline $ν_e$ disappearance with the T2K near detector

The T2K experiment has performed a search for $ν_e$ disappearance due to sterile neutrinos using $5.9 \times 10^{20}$ protons on target for a baseline of $280 m$ in a neutrino beam peaked at about $500 MeV$. A sample of ν_e CC interactions in the off-axis near detector has been selected with a purity of 63\% and an efficiency of 26\%. The p-value for the null hypothesis is 0.085 and the excluded region at 95\% CL is approximately $sin^2 2 θ_{ee} > 0.3$ for $Δm^2_{eff} > 7 eV^2 / c^4$.

hep-ex↗

arXiv2014

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

We report a measurement of the $ν_μ$ inclusive charged current cross sections on iron and hydrocarbon in the 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 $(1.444\pm0.002(stat.)_{-0.157}^{+0.189}(syst.))\times 10^{-38}\mathrm{cm}^2/\mathrm{nucleon}$, and $(1.379\pm0.009(stat.)_{-0.147}^{+0.178}(syst.))\times 10^{-38}\mathrm{cm}^2/\mathrm{nucleon}$, respectively, and their cross section ratio is $1.047\pm0.007(stat.)\pm0.035(syst.)$. 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.

hep-ex↗

arXiv2014

Search for Trilepton Nucleon Decay via $p \rightarrow e^+ νν$ and $p \rightarrow μ^+ νν$ in the Super-Kamiokande Experiment

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

hep-ex↗