$^{160}$Gd is a candidate for double beta decay with relatively high natural abundance (21.9%). However, its low Q-value (1.73 MeV) makes the observation of even the two-neutrino double beta decay (2$\nu$2$\beta$) extremely challenging. The most sensitive search used a 2 inch Gd$_2$SiO$^5$ (GSO) scintillator. However, they was not able to observe 2$\nu$2$\beta$ due to significant background from intrinsic radioactive impurities which are uranium and thorium decay chains. Consequently, they established a lower limit of 1.9$\times$10$^{19}$ years on the 2$\nu$2$\beta$ half-life ($T^{2\nu}_{1/2}$). Theoretical prediction suggests $T^{2\nu}_{1/2}$ of approximately 7.4$\times$10$^{20}$ years.
The PIKACHU experiment was designed to observe $^{160}$Gd 2$\nu$2$\beta$ by employing large Gd$_3$Ga$_3$Al$_2$O$_{12}$ (GAGG) single crystals. GAGG offers several advantages over GSO: higher light yield, possibility of pulse shape discrimination, and a higher $^{160}$Gd content by increasing in size. We planned two phases: Phase 1 aims to search 2$\nu$2$\beta$ in $^{160}$Gd with the best sensitivity in the world, and then, Phase 2 is intended to achieve a sensitivity approximately an order of magnitude better than previous study, with the goal of observing the decay.
In this conference, I introduced the concept of the PIKACHU experiment, present on the development of high-purity GAGG crystals, and report the current status of data acquisition and analysis for Phase 1.