Upgrading the COSINE-100 experiment for enhanced sensitivity to low-mass dark matter detection

The DAMA/LIBRA experiment has reported an annual modulation signal in NaI(Tl) detectors, which has been interpreted as a possible indication of dark matter interactions. However, this claim remains controversial, as several experiments have tested the modulation signal using NaI(Tl) detectors. Among them, the COSINE-100 experiment, specifically designed to test DAMA/LIBRA’s claim, observed no significant signal, revealing a more than 3σ discrepancy with DAMA/LIBRA’s results. Here we present COSINE-100U, an upgraded version of the experiment, which aims to expand the search for dark matter interactions by improving light collection efficiency and reducing background noise. The detector, consisting of eight NaI(Tl) crystals with a total mass of 99.1 kg, has been relocated to Yemilab, a new underground facility in Korea, and features direct PMT-coupling technology to enhance sensitivity. These upgrades significantly improve the experiment’s ability to probe low-mass dark matter candidates, contributing to the ongoing global effort to clarify the nature of dark matter. The COSINE-100U experiment upgrades the COSINE-100 detector to improve the search for dark matter interactions using ultra-low background NaI(Tl) crystals. By enhancing light collection efficiency and reducing background noise through a new encapsulation design, the experiment achieves higher sensitivity, advancing the investigation of the DAMA/LIBRA annual modulation signal.