Search Results (60 total)

We report the development of a proton identification method for the Super-Kamiokande (SK) detector. This new tool is applied to the search for events with a single proton track, a high purity neutral current sample of interest for sterile neutrino searches. After selection using a neural network, we observe 38 events in the combined SK-I and SK-II data corresponding to 22 85.1 days of exposure, with an estimated signal-to-background ratio of 1.6 to 1. Proton identification was also applied to a direct search for charged-current quasielastic (CCQE) events, obtaining a high precision sample of fully kinematically reconstructed atmospheric neutrinos, which has not been previously reported in water Cherenkov detectors. The CCQE fraction of this sample is 55%, and its neutrino (as opposed to antineutrino) fraction is 91.7±3%. We selected 78μ-like and 47 e-like events in the SK-I and SK-II data set. With this data, a clear zenith angle distortion of the neutrino direction itself is reported in a sub-GeV sample of μ neutrinos where the lepton angular correlation to the incoming neutrino is weak. Our fit to ν_μ→ν_τ oscillations using the neutrino L / E distribution of the CCQE sample alone yields a wide acceptance region compatible with our previous results and excludes the no-oscillation hypothesis at 3-sigma.

We have searched for proton decays via p→e⁺π⁰ and p→μ⁺π⁰ using data from a 91.7  kt·yr exposure of Super-Kamiokande-I and a 49.2  kt·yr exposure of Super-Kamiokande-II. No candidate events were observed with expected backgrounds induced by atmospheric neutrinos of 0.3 events for each decay mode. From these results, we set lower limits on the partial lifetime of 8.2×10³³ and 6.6×10³³ years at 90% confidence level for p→e⁺π⁰ and p→μ⁺π⁰ modes, respectively.

Single charged pion production in charged-current muon neutrino interactions with carbon is studied using data collected in the K2K long-baseline neutrino experiment. The mean energy of the incident muon neutrinos is 1.3 GeV. The data used in this analysis are mainly from a fully active scintillator detector, SciBar. The cross section for single π⁺ production in the resonance region (W<2  GeV/c²) relative to the charged-current quasielastic cross section is found to be 0.734_-0.153^+0.140. The energy-dependent cross section ratio is also measured. The results are consistent with a previous experiment and the prediction of our model.

The results of the second phase of the Super-Kamiokande solar neutrino measurement are presented and compared to the first phase. The solar neutrino flux spectrum and time variation as well as oscillation results are statistically consistent with the first phase and do not show spectral distortion. The time-dependent flux measurement of the combined first and second phases coincides with the full period of solar cycle 23 and shows no correlation with solar activity. The measured ⁸B total flux is (2.38±0.05(stat.)_-0.15^+0.16(sys.))×10⁶  cm^-2 s^-1 and the day-night difference is found to be (-6.3±4.2(stat.)±3.7(sys.))%. There is no evidence of systematic tendencies between the first and second phases.

Experiment E949 at Brookhaven National Laboratory studied the rare decay K⁺→π⁺νν̅ and other processes with an exposure of 1.77×10¹² K⁺’s. The data were analyzed using a blind analysis technique yielding one candidate event with an estimated background of 0.30±0.03 events. Combining this result with the observation of two candidate events by the predecessor experiment E787 gave the branching ratio B(K⁺→π⁺νν̅ )=(1.47_-0.89^+1.30)×10^-10, consistent with the standard model prediction of (0.74±0.20)×10^-10. This is a more detailed report of results previously published [V. V. Anisimovsky , Phys. Rev. Lett. 93, 031801 (2004)].

We consider ν_μ→ν_τ oscillations in the context of the mass varying neutrino (MaVaN) model, where the neutrino mass can vary depending on the electron density along the flight path of the neutrino. Our analysis assumes a mechanism with dependence only upon the electron density, hence ordinary matter density, of the medium through which the neutrino travels. Fully-contained, partially-contained and upward-going muon atmospheric neutrino data from the Super-Kamiokande detector, taken from the entire SK-I period of 1489 live days, are compared to MaVaN model predictions. We find that, for the case of 2-flavor oscillations, and for the specific models tested, oscillation independent of electron density is favored over density dependence. Assuming maximal mixing, the best-fit case and the density-independent case do not differ significantly.

The atmospheric neutrino background for proton decay via p→e⁺π⁰ in ring imaging water Cherenkov detectors is studied with an artificial accelerator neutrino beam for the first time. In total, 3.14×10⁵ neutrino events corresponding to about 10 megaton-years of atmospheric neutrino interactions were collected by a 1000 ton water Cherenkov detector (KT). The KT charged-current single π⁰ production data are well reproduced by simulation programs of neutrino and secondary hadronic interactions used in the Super-Kamiokande (SK) proton decay search. The obtained p→e⁺π⁰ background rate by the KT data for SK from the atmospheric neutrinos whose energies are below 3 GeV is 1.63_-0.33^+0.42(stat)_-0.51^+0.45(syst)(megaton-year)^-1. This result is also relevant to possible future, megaton-scale water Cherenkov detectors.

A search for the appearance of tau neutrinos from ν_μ↔ν_τ oscillations in the atmospheric neutrinos has been performed using 1489.2 days of atmospheric neutrino data from the Super-Kamiokande-I experiment. A best fit tau neutrino appearance signal of 138±48(stat)_-32⁺¹⁵(syst) events is obtained with an expectation of 78±26(syst). The hypothesis of no tau neutrino appearance is disfavored by 2.4 sigma.

We present measurements of ν_μ disappearance in K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment. One-hundred and twelve beam-originated neutrino events are observed in the fiducial volume of Super-Kamiokande with an expectation of 158.1_-8.6^+9.2 events without oscillation. A distortion of the energy spectrum is also seen in 58 single-ring muonlike events with reconstructed energies. The probability that the observations are explained by the expectation for no neutrino oscillation is 0.0015% (4.3σ). In a two-flavor oscillation scenario, the allowed Δm² region at sin⁡²2θ=1 is between 1.9 and 3.5×10^-3  eV² at the 90% C.L. with a best-fit value of 2.8×10^-3  eV².

The weak nucleon axial-vector form factor for quasielastic interactions is determined using neutrino interaction data from the K2K Scintillating Fiber detector in the neutrino beam at KEK. More than 12 000 events are analyzed, of which half are charged-current quasielastic interactions ν_μn→μ^-p occurring primarily in oxygen nuclei. We use a relativistic Fermi gas model for oxygen and assume the form factor is approximately a dipole with one parameter, the axial-vector mass M_A, and fit to the shape of the distribution of the square of the momentum transfer from the nucleon to the nucleus. Our best fit result for M_A=1.20±0.12  GeV. Furthermore, this analysis includes updated vector form factors from recent electron scattering experiments and a discussion of the effects of the nucleon momentum on the shape of the fitted distributions.

We report on the results of a three-flavor oscillation analysis using Super-Kamiokande I atmospheric neutrino data, with the assumption of one mass scale dominance (Δm₁₂²=0). No significant flux change due to matter effect, which occurs when neutrinos propagate inside the Earth for θ₁₃≠0, has been seen either in a multi-GeV ν_e-rich sample or in a ν_μ-rich sample. Both normal and inverted mass hierarchy hypotheses are tested and both are consistent with observation. Using Super-Kamiokande data only, 2-dimensional 90% confidence allowed regions are obtained: mixing angles are constrained to sin⁡²θ₁₃<0.14 and 0.37<sin⁡²θ₂₃<0.65 for the normal mass hierarchy. Weaker constraints, sin⁡²θ₁₃<0.27 and 0.37<sin⁡²θ₂₃<0.69, are obtained for the inverted mass hierarchy case.

The details of Super-Kamiokande-I’s solar neutrino analysis are given. Solar neutrino measurement in Super-Kamiokande is a high statistics collection of ⁸B solar neutrinos via neutrino-electron scattering. The analysis method and results of the 1496 day data sample are presented. The final oscillation results for the data are presented also.

We performed an improved search for ν_μ→ν_e oscillation with the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment, using the full data sample of 9.2×10¹⁹ protons on target. No evidence for a ν_e appearance signal was found, and we set bounds on the ν_μ→ν_e oscillation parameters. At Δm²=2.8×10^-3  eV², the best-fit value of the K2K ν_μ disappearance analysis, we set an upper limit of sin⁡²2θ_μe<0.13 at a 90% confidence level.

At the 12-GeV proton synchrotron of KEK, an experiment (KEK-PS-E246) was performed to measure the transverse muon polarization (P_T) in K⁺→π⁰μ⁺ν decays as a direct search of violation of time reversal invariance (T-violation). P_T is a very sensitive probe of CP violation beyond the standard model. A stopped positive kaon beam was used. An elaborate high-acceptance detector, in conjunction with a 12-sector superconducting toroidal spectrometer, was utilized. The stopped kaon method provided several advantages in performing a high-precision experiment. The decay was identified by detecting a μ⁺ with the spectrometer and a π⁰ with a CsI(Tl) calorimeter surrounding the target as two photons as well as one photon with high energy. For the P_T measurement, π⁰ events in the forward (fwd) and backward (bwd) regions in the calorimeter were relevant. Systematic errors were greatly suppressed by exploiting the rotational and fwd-bwd symmetry of the system. Polarization measurement was done in terms of asymmetry measurement of decay positrons of stopped muons in the polarimeter. A longitudinal field method was applied for the P_T polarization component. In the data analysis a novel technique of the two-analysis method was employed. Two independent analyses were performed following their own policy and criteria and good events were selected. Then, the two analyses were combined. A data quality check was carefully done by looking at the null asymmetry A₀, the sensitivity function A_N, the kinematical attenuation coefficient, and the decay-plane distributions. The asymmetry measurement for finite-size muon stoppers was performed “differentially” using the position information by the chamber in front of the polarimeter. From the measurements during 1996–2000, we accumulated 11.8×10⁶ good events after the two-analysis combination, and deduced P_T=-0.0017±0.0023(stat)±0.0011(syst), corresponding to the T-violating parameter Imξ=-0.0053±0.0071(stat)±0.0036(syst). From these results the upper limits of |P_T|<0.0050 (90% C.L.) and |Imξ|<0.016 (90% C.L.) were deduced. Systematic errors were carefully studied and estimated. Almost all of the errors were canceled by the 12-sector summation and/or fwd-bwd subtraction, and the total systematic error was concluded to be as a half of the statistical error. The present result set constraints on model parameters of several theoretical models. For the three-Higgs-doublet model, e.g., a limit |Im(α₁γ₁^*)|<544(M_H1/GeV)² was obtained. Implications for several other models are discussed.

We report the result from a search for charged-current coherent pion production induced by muon neutrinos with a mean energy of 1.3 GeV. The data are collected with a fully active scintillator detector in the K2K long-baseline neutrino oscillation experiment. No evidence for coherent pion production is observed, and an upper limit of 0.60×10^-2 is set on the cross section ratio of coherent pion production to the total charged-current interaction at 90% confidence level. This is the first experimental limit for coherent charged pion production in the energy region of a few GeV.

We report the results for nucleon decay searches via modes favored by supersymmetric grand unified models in Super-Kamiokande. Using 1489 days of full Super-Kamiokande-I data, we searched for p→ν̅ K⁺, n→ν̅ K⁰, p→μ⁺K⁰, and p→e⁺K⁰ modes. We found no evidence for nucleon decay in any of these modes. We set lower limits of partial nucleon lifetime 2.3×10³³, 1.3×10³², 1.3×10³³, and 1.0×10³³  years at 90% confidence level for p→ν̅ K⁺, n→ν̅ K⁰, p→μ⁺K⁰, and p→e⁺K⁰ modes, respectively. These results give a strong constraint on supersymmetric grand unification models.

We present a combined analysis of fully-contained, partially-contained and upward-going muon atmospheric neutrino data from a 1489 d exposure of the Super-Kamiokande detector. The data samples span roughly five decades in neutrino energy, from 100 MeV to 10 TeV. A detailed Monte Carlo comparison is described and presented. The data is fit to the Monte Carlo expectation, and is found to be consistent with neutrino oscillations of ν_μ↔ν_τ with sin⁡²2θ>0.92 and 1.5×10^-3<Δm²<3.4×10^-3  eV² at 90% confidence level.

We present results for ν_μ oscillation in the KEK to Kamioka (K2K) long-baseline neutrino oscillation experiment. K2K uses an accelerator-produced ν_μ beam with a mean energy of 1.3 GeV directed at the Super-Kamiokande detector. We observed the energy-dependent disappearance of ν_μ, which we presume have oscillated to ν_τ. The probability that we would observe these results if there is no neutrino oscillation is 0.0050% (4.0σ).

In models where the gravitino is the lightest supersymmetric particle, the next-to-lightest supersymmetric particle (NLSP) is long-lived. We consider an important charged NLSP candidate, the scalar tau τ˜. Slow charged NLSPs may be produced at future colliders and they may be stopped in a massive stopper which simultaneously serves as a detector for the NLSP and its decay products. We found the number of events at a 1 kton to O(10) kton detector could be significant enough to study the NLSP decays with lifetime shorter than 10¹⁰sec⁡ at the LHC. The performance of existing 1 kton detectors may be good enough to do such studies at the LHC, if they can be placed close to the ATLAS/CMS detectors. At a future e^-e^- collider, scalar electrons e˜^-'s are copiously produced. Slow NLSPs may be produced from the e˜^- decay. The number of stopped NLSPs at a future linear collider could be large enough to study rare decay modes of the NLSP.

A search for T-violating transverse muon polarization (P_T) in the K⁺→π⁰μ⁺ν decay was performed using kaon decays at rest. A new improved value P_T=-0.0017±0.0023(stat)±0.0011(syst) was obtained giving an upper limit |P_T|<0.0050. The T-violation parameter was determined to be Imξ=-0.0053±0.0071(stat)±0.0036(syst) giving an upper limit |Imξ|<0.016.

Muon neutrino disappearance probability as a function of neutrino flight length L over neutrino energy E was studied. A dip in the L/E distribution was observed in the data, as predicted from the sinusoidal flavor transition probability of neutrino oscillation. The observed L/E distribution constrained ν_μ↔ν_τ neutrino oscillation parameters; 1.9×10^-3<Δm²<3.0×10^-3  eV² and sin⁡²2θ>0.90 at 90% confidence level.

We report the results of a search for the decay K⁺→π⁺νν̅ in the kinematic region with π⁺ momentum 140<P<195 MeV/c using the data collected by the E787 experiment at BNL. No events were observed. When combined with our previous search in this region, one candidate event with an expected background of 1.22±0.24 events results in a 90% C.L. upper limit of 2.2×10^-9 on the branching ratio of K⁺→π⁺νν̅ . We also report improved limits on the rates of K⁺→π⁺X⁰ and K⁺→π⁺X¹X² where X⁰,X¹,X² are hypothetical, massless, long-lived neutral particles.

A search for a nonzero neutrino magnetic moment has been conducted using 1496 live days of solar neutrino data from Super-Kamiokande-I. Specifically, we searched for distortions to the energy spectrum of recoil electrons arising from magnetic scattering due to a nonzero neutrino magnetic moment. In the absence of a clear signal, we found μ_ν≤(3.6×10^-10)μ_B at 90% C.L. by fitting to the Super-Kamiokande day-night spectra. The fitting took into account the effect of neutrino oscillation on the shapes of energy spectra. With additional information from other solar neutrino and KamLAND experiments constraining the oscillation region, a limit of μ_ν≤(1.1×10^-10)μ_B at 90% C.L. was obtained.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We have performed a search for the angular-momentum forbidden decay K⁺→π⁺γ with the E787 detector at BNL. No events were observed in the π⁺ kinematic region around 227 MeV/c. An upper limit on the branching ratio for the decay is determined to be 3.6×10^-7 at 90% confidence level.