We present an improved search for neutrinoless double-beta (0νββ) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a ...significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νββ decay half-life of T_{1/2}^{0ν}>1.07×10^{26} yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.
Abstract
We report a search for electron antineutrinos at KamLAND with an 8.3−30.8 MeV energy range via the inverse beta decay. In 6.72 kton-yr of KamLAND data, we found 18 neutrino candidates and no ...significant excess over estimated backgrounds. From data interpretation, with the assumption of some supernova relic neutrino spectrum predictions, we give upper flux limits of 60−110 cm
−2
s
−1
(90% CL) in the analysis range and present a model-independent flux. These upper limits are the most stringent for 8−13 MeV region. We also improve on the upper probability limit of
8
B solar neutrinos converting into antineutrinos via the Resonant Spin Flavor Precession with the neutrino magnetic moment. Besides, we could set limits on the annihilation cross-section for light dark matter pairs to neutrino pairs.
We report on a precision measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A(PV) = -131 +/- 14(stat) +/- 10(syst) x 10(-9), leading to the ...determination of the weak mixing angle sin2(thetaW(eff) = 0.2397 +/- 0.0010(stat) +/- 0.0008(syst), evaluated at Q2 = 0.026 GeV2. Combining this result with the measurements of sin2(thetaW(eff) at the Z0 pole, the running of the weak mixing angle is observed with over 6sigma significance. The measurement sets constraints on new physics effects at the TeV scale.
In this paper, we present a precision measurement of the $^{136}$Xe two-neutrino $\beta\beta$ electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. ...An improved formalism for the two-neutrino $\beta\beta$ rate allows us to measure the ratio of the leading and subleading $2\nu\beta\beta$ nuclear matrix elements (NMEs), $\xi^{2\nu}_{31} = -0.26^{+0.31}_{-0.25}$. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the $\xi^{2\nu}_{31}$ range allowed by the QRPA is excluded by the present measurement at the 90%25 C.L. Our analysis reveals that predicted $\xi^{2\nu}_{31}$ values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Since these aspects are also at play in neutrinoless $\beta\beta$ decay, $\xi^{2\nu}_{31}$ provides new insights towards reliable neutrinoless $\beta\beta$ NMEs.
The KamLAND experiment started operation in the Spring of 2002 and is operational to this day. The experiment observes signals from electron antineutrinos from distant nuclear reactors. The program, ...spanning more than a decade, allowed the determination of LMA-MSW as the solution to the solar neutrino transformation results (under the assumption of CPT invariance) and the measurement of various neutrino oscillation parameters. In particular, the solar mass-splitting Δm212 was determined to high precision. Besides the study of neutrino oscillation, KamLAND started the investigation of geologically produced antineutrinos (geo-ν‾e). The collaboration also reported on a variety of other topics related to particle and astroparticle physics.
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