Low Energy Investigations at Kamioka Observatory

TL;DR

This paper reports on low energy rare event search activities at Kamioka Observatory, including upgrades to Super-Kamiokande, dark matter detection experiments, and results from low-threshold analyses that constrain dark matter parameter space.

Contribution

It presents recent upgrades and results from multiple low energy rare event experiments, improving background reduction and sensitivity in dark matter and neutrino measurements.

Findings

Super-Kamiokande achieved lower background levels and measured solar neutrino parameters.

NEWAGE developed a lower pressure operation mode with reduced energy threshold.

XMASS set new limits on dark matter interactions, excluding some claimed signals.

Abstract

At Kamioka Observatory many activities for low energy rare event search are ongoing. Super-Kamiokande(SK), the largest water Cherenkov neutrino detector, currently continues data taking as the fourth phase of the experiment (SK-IV). In SK-IV, we have upgraded the water purification system and tuned water flow in the SK tank. Consequently the background level was lowered significantly. This allowed SK-IV to derive solar neutrino results down to 3.5MeV energy region. With these data, neutrino oscillation parameters are updated from global fit; $\Delta m^2_{12}=7.44^{+0.2}_{-0.19}\times10^{-5} {\rm eV}^2$, $\sin^2\theta_{12}=0.304\pm0.013$, $\sin^2\theta_{13}=0.030^{+0.017}_{-0.015}$. NEWAGE, the directional sensitive dark matter search experiment, is currently operated as "NEWAGE-0.3a" which is a $0.20\times0.25\times0.31$ m$^3$ micro-TPC filled with CF4 gas at 152 Torr. Recently we have developed "NEWAGE-0.3b". It was succeeded to lower the operation pressure down to 76 Torr and the threshold down to 50 keV (F recoils). XMASS experiment is looking for scintillation signals from dark matter interaction in 1 ton of liquid xenon. It was designed utilizing its self-shielding capability with fiducial volume confinement. However, we could lower the analysis threshold down to 0.3 keVee using whole volume of the detector. In February 2012, low threshold and very large exposure data (5591 kg$\cdot$days) were collected. With these data, we have excluded some part of the parameter spaces claimed by DAMA/LIBRA and CoGeNT experiments.