AXEL: High-pressure Xe gas TPC for BG-free [formula omitted] decay search.

AXEL is a high-pressure xenon gas time projection chamber for neutrinoless double-beta decay (0 ν 2 β) search. The AXEL has a unique readout system called ELCC which has a cellular structure and photosensors to detect electroluminescence light produced by ionization electrons. We demonstrated the performance of the ELCC with a small prototype detector (AXEL-HP10L). The obtained energy resolution corresponds to 0. 82 − 1. 74 % (FWHM) at the 0 ν 2 β Q-value. We are constructing a new prototype (AXEL-HP180L) in order to study the energy resolution at the Q-value of 0 ν 2 β with a new design of ELCC with unit structure, newly developed electronics board, field-shaping electrodes, and Cockcroft–Walton-type high voltage power supply. For a future 1-ton scale large AXEL detector, we are developing new background-reduction techniques; topology identification with machine learning, positive-ion detection, and active-shield options. [ABSTRACT FROM AUTHOR]