We have successfully built and operated a source deployment system for the KamLAND detector. This system was used to position radioactive sources throughout the delicate 1-kton liquid scintillator ...volume, while meeting stringent material cleanliness, material compatibility, and safety requirements. The calibration data obtained with this device were used to fully characterize detector position and energy reconstruction biases. As a result, the uncertainty in the size of the detector fiducial volume was reduced by a factor of two. Prior to calibration with this system, the fiducial volume was the largest source of systematic uncertainty in measuring the number of anti-neutrinos detected by KamLAND. This paper describes the design, operation and performance of this unique calibration system.
A new ultra-low background high-purity germanium (HPGe) detector has been installed at the Kamioka underground experimental site. The background count rate in the energy range from 40 keV to 2700 keV ...is about 25% lower than that of the first HPGe detector installed in 2016, which has the same detector specification and similar shielding geometry. This paper describes the shielding configuration, including the cleaning of the material surface, the comparison of calibration data and simulation, the time variation of the background spectra, the sample measurement procedure, and some results of the radioactivity in the selected samples.
Many astrophysical models predict a diffuse flux of high-energy neutrinos from active galactic nuclei and other extragalactic sources. At muon energies above 1 TeV, the upward-going muon flux induced ...by neutrinos from active galactic nuclei is expected to exceed the flux due to atmospheric neutrinos. We have performed a search for this astrophysical neutrino flux by looking for upward-going muons in the highest energy data sample from the Super-Kamiokande detector using 1679.6 live days of data. We found one extremely high energy upward-going muon event, compared with an expected atmospheric neutrino background of 0.46 c 0.23 events. Using this result, we set an upper limit on the diffuse flux of upward-going muons due to neutrinos from astrophysical sources in the muon energy range 3.16-100 TeV.
We present a search for neutrinoless double-beta (\(0\nu\beta\beta\)) decay of \(^{136}\)Xe using the full KamLAND-Zen 800 dataset with 745 kg of enriched xenon, corresponding to an exposure of ...\(2.097\) ton yr of \(^{136}\)Xe. This updated search benefits from a more than twofold increase in exposure, recovery of photo-sensor gain, and reduced background from muon-induced spallation of xenon. Combining with the search in the previous KamLAND-Zen phase, we obtain a lower limit for the \(0\nu\beta\beta\) decay half-life of \(T_{1/2}^{0\nu} > 3.8 \times 10^{26}\) yr at 90% C.L., a factor of 1.7 improvement over the previous limit. The corresponding upper limits on the effective Majorana neutrino mass are in the range 28-122 meV using phenomenological nuclear matrix element calculations.
Particle dark matter could belong to a multiplet that includes an electrically charged state. WIMP dark matter (\(\chi^{0}\)) accompanied by a negatively charged excited state (\(\chi^{-}\)) with a ...small mass difference (e.g. \(<\) 20 MeV) can form a bound-state with a nucleus such as xenon. This bound-state formation is rare and the released energy is \(\mathcal{O}(1-10\)) MeV depending on the nucleus, making large liquid scintillator detectors suitable for detection. We searched for bound-state formation events with xenon in two experimental phases of the KamLAND-Zen experiment, a xenon-doped liquid scintillator detector. No statistically significant events were observed. For a benchmark parameter set of WIMP mass \(m_{\chi^{0}} = 1\) TeV and mass difference \(\Delta m = 17\) MeV, we set the most stringent upper limits on the recombination cross section times velocity \(\langle\sigma v\rangle\) and the decay-width of \(\chi^{-}\) to \(9.2 \times 10^{-30}\) \({\rm cm^3/s}\) and \(8.7 \times 10^{-14}\) GeV, respectively at 90% confidence level.
We present results from the first phase of the KamLAND-Zen double-beta decay experiment, corresponding to an exposure of 89.5 kg yr of (136)Xe. We obtain a lower limit for the neutrinoless ...double-beta decay half-life of T(1/2)(0ν)>1.9×10(25) yr at 90% C.L. The combined results from KamLAND-Zen and EXO-200 give T(1/2)(0ν)>3.4×10(25) yr at 90% C.L., which corresponds to a Majorana neutrino mass limit of <m(ββ)> <(120-250) meV based on a representative range of available matrix element calculations. Using those calculations, this result excludes the Majorana neutrino mass range expected from the neutrinoless double-beta decay detection claim in (76)Ge, reported by a part of the Heidelberg-Moscow Collaboration, at more than 97.5% C.L.
We report a measurement of the strange axial coupling constant \(g_A^s\) using atmospheric neutrino data at KamLAND. This constant is a component of the axial form factor of the neutral-current ...quasielastic (NCQE) interaction. The value of \(g_A^s\) significantly changes the ratio of proton and neutron NCQE cross sections. KamLAND is suitable for measuring NCQE interactions as it can detect nucleon recoils with low-energy thresholds and measure neutron multiplicity with high efficiency. KamLAND data, including the information on neutron multiplicity associated with the NCQE interactions, makes it possible to measure \(g_A^s\) with a suppressed dependence on the axial mass \(M_A\), which has not yet been determined. For a comprehensive prediction of the neutron emission associated with neutrino interactions, we establish a simulation of particle emission via nuclear deexcitation of \(^{12}\)C, a process not considered in existing neutrino Monte Carlo event generators. Energy spectrum fitting for each neutron multiplicity gives \(g_A^s =-0.14^{+0.25}_{-0.26}\), which is the most stringent limit obtained using NCQE interactions without \(M_A\) constraints. The two-body current contribution considered in this analysis relies on a theoretically effective model and electron scattering experiments and requires future verification by direct measurements and future model improvement.
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