B. D. Xu

Limits on Majoron-emitting double-β decays of 136Xe in the KamLAND-Zen experiment

We present limits on Majoron-emitting neutrinoless double-β decay modes based on an exposure of 112.3 days with 125 kg of 136Xe. In particular, a lower limit on the ordinary (spectral index n=1) Majoron-emitting decay half-life of 136Xe is obtained as T1/20νχ0>2.6×1024 yr at 90% C.L., a factor of five more stringent than previous limits. The corresponding upper limit on the effective Majoron-neutrino coupling, using a range of available nuclear matrix calculations, is 〈gee〉<(0.8-1.6)×10−5. This excludes a previously unconstrained region of parameter space and strongly limits the possible contribution of ordinary Majoron emission modes to 0νββ decay for neutrino masses in the inverted hierarchy scheme.

SEARCH FOR EXTRATERRESTRIAL ANTINEUTRINO SOURCES WITH THE KamLAND DETECTOR

We present the results of a search for extraterrestrial electron antineutrinos (ν_e’s)in the energy range 8.3MeV < E_(νe) < 31.8MeV using the KamLAND detector. In an exposure of 4.53 kton-year, we identify 25 candidate events. All of the candidate events can be attributed to background, most importantly neutral current atmospheric neutrino interactions, setting an upper limit on the probability of ^8B solar ν_es converting into ν_e’s at 5.3 × 10^(–5) (90% CL), if we assume an undistorted v_e shape. This limit corresponds to a solar v_e flux of 93 cm^(–2) s^(–1) or an event rate of 1.6 events (kton–year)^(–1) above the energy threshold (E_(ve) ≥ 8.3MeV). The present data also allows us to set more stringent limits on the diffuse supernova neutrino flux and on the annihilation rates for light dark matter particles.

Results from KamLAND-Zen

KamLAND-Zen reports on a preliminary search for neutrinoless double-beta decay with 136Xe based on 114.8 live-days after the purification of the xenon loaded liquid scintillator. In this data, the problematic 110mAg background peak identified in previous searches is reduced by more than a factor of 10. By combining the KamLAND-Zen pre- and post-purification data, we obtain a preliminary lower limit on the 0νββ decay half-life of T1/20ν>2.6×1025 yr at 90% C.L. The search sensitivity will be enhanced with additional low background data after the purification. Prospects for further improvements with future KamLAND-Zen upgrades are also presented.

Search for double-beta decay of

Abstract A search for double-beta decays of 136Xe to excited states of 136Ba has been performed with the first phase data set of the KamLAND-Zen experiment. The 0 1 + , 2 1 + and 2 2 + transitions of 0 ν β β decay were evaluated in an exposure of 89.5 kg ⋅ yr of 136Xe, while the same transitions of 2 ν β β decay were evaluated in an exposure of 61.8 kg ⋅ yr . No excess over background was found for all decay modes. The lower half-life limits of the 2 1 + state transitions of 0 ν β β and 2 ν β β decay were improved to T 1 / 2 0 ν ( 0 + → 2 1 + ) > 2.6 × 10 25 yr and T 1 / 2 2 ν ( 0 + → 2 1 + ) > 4.6 × 10 23 yr (90% C.L.), respectively. We report on the first experimental lower half-life limits for the transitions to the 0 1 + state of 136Xe for 0 ν β β and 2 ν β β decay. They are T 1 / 2 0 ν ( 0 + → 0 1 + ) > 2.4 × 10 25 yr and T 1 / 2 2 ν ( 0 + → 0 1 + ) > 8.3 × 10 23 yr (90% C.L.). The transitions to the 2 2 + states are also evaluated for the first time to be T 1 / 2 0 ν ( 0 + → 2 2 + ) > 2.6 × 10 25 yr and T 1 / 2 2 ν ( 0 + → 2 2 + ) > 9.0 × 10 23 yr (90% C.L.). These results are compared to recent theoretical predictions.

^{136}

We report a measurement of the neutrino-electron elastic scattering rate of 862 keV ^7Be solar neutrinos based on a 165.4 kt d exposure of KamLAND. The observed rate is 582±94(kt d)^(−1), which corresponds to an 862-keV Be7 solar neutrino flux of (3.26±0.52)×10^9cm^(−2)s^(−1), assuming a pure electron-flavor flux. Comparing this flux with the standard solar model prediction and further assuming three-flavor mixing, a ν_e survival probability of 0.66±0.15 is determined from the KamLAND data. Utilizing a global three-flavor oscillation analysis, we obtain a total ^7Be solar neutrino flux of (5.82±1.02)×10^9cm^(−2)s^(−1), which is consistent with the standard solar model predictions.

Xe to excited states of

In the late stages of nuclear burning for massive stars (

^{136}

M>8~M_{\sun}

Ba with the KamLAND-Zen experiment

), the production of neutrino-antineutrino pairs through various processes becomes the dominant stellar cooling mechanism. As the star evolves, the energy of these neutrinos increases and in the days preceding the supernova a significant fraction of emitted electron anti-neutrinos exceeds the energy threshold for inverse beta decay on free hydrogen. This is the golden channel for liquid scintillator detectors because the coincidence signature allows for significant reductions in background signals. We find that the kiloton-scale liquid scintillator detector KamLAND can detect these pre-supernova neutrinos from a star with a mass of

7Be Solar Neutrino Measurement with KamLAND

25~M_{\sun}

KamLAND Sensitivity to Neutrinos from Pre-Supernova Stars

at a distance less than 690~pc with 3