Search

We present two results of a search for MeV-scale neutrino and anti-neutrino events correlated with gravitational wave events/candidates and large solar flares with KamLAND. The KamLAND detector is a large-volume neutrino detector using liquid scintillator, which is located at 1 km underground under the top of Mt. Ikenoyama in Kamioka, Japan. KamLAND has multiple reaction channels to detect neutrinos. Electron antineutrino can be detected via inverse-beta decay with 1.8 MeV neutrino energy threshold. All flavors of neutrinos can be detected via neutrino-electron scattering without neutrino energy threshold. KamLAND has continued the neutrino observation since 2002 March. We use the data set of 60 gravitational waves provided by the LIGO/Virgo collaboration during their second and third observing runs and search for coincident electron antineutrino events in KamLAND. We find no significant coincident signals within a ±500 s timing window from each gravitational wave and present 90% C.L. upper limits on the electron antineutrino fluence between 108–1013 cm−2 for neutrino energies of 1.8–111 MeV. For a solar-flare neutrino search at KamLAND, we determine the timing window using the solar X-ray data set provided by the GOES satellite series from 2002 to 2019 and search for the excess of coincident event rate on the all-flavor neutrinos. We find no significant event rate excess in the flare time windows and get 90% C.L. upper limits on the fluence of neutrinos of all flavors (electron anti-neutrinos) between 1010–1013 cm−2 (108–1013 cm−2) for neutrino energies in the energy range of 0.4–35 MeV.

KamLAND-Zen is a neutrinoless double beta decay $(0\nu\beta\beta)$ search experiment using $^{136}$Xe. Taking advantages of the low-background environment of KamLAND, we realize the most sensitive $0\nu\beta\beta$ search. While $0\nu\beta\beta$ is a pure $\beta$ event, the backgrounds such as $^{214}$Bi and spallation products emit $\gamma$-rays. Therefore, particle identification(PID) is effective to improve the sensitivity. In this study, we develop a PID method with a neural network focusing on difference of scintillation timing property between $\beta$ and $\gamma$. It rejects gamma backgrounds based on hit-timing spectrum of PMTs. We applied the method to hit-timing spectrum of MC and data of KamLAND-Zen400 Phase1, where $^{110m}$Ag (gamma event) was the most dominant background. We found that this method could reject 60% of gamma backgrounds and had a potential of ~10% improvement of its limit.

KamLAND2-Zen is the future plan for the existing KamLND-Zen experiment, which will search for neutrinoless double-beta decay. In this experiment, muon spallation products are expected to be the main background. Spallation backgrounds can be tagged by delayed coincidence with neutrons emitted in spallation. Therefore, it is important to detect neutrons with high efficiency. However, enhancement of light yield will increase after-pulses and fake triggers due to them resulting in DAQ deadtime and low detection efficiency of neutrons. To solve this problem, a new trigger scheme which uses simultaneous hit of neighboring PMTs was developed. According to a simulation performed with real PMT waveforms, this scheme is expected to improve neutron detection efficiency from 54% to 89%. Consequently 99% of 10C background events can be tagged.