Updates from the COSINE experiment

Abstract

The COSINE project aims to independently test the DAMA experiment's long-standing claim of detecting dark matter interactions using the same NaI(Tl) scintillating crystal technology. In its first phase, COSINE-100 collected over six years of data at the Yangyang Underground Laboratory and achieved a three-sigma exclusion of the DAMA modulation signal under standard dark matter assumptions. The experiment is now being upgraded to the COSINE-100 Upgrade, deploying higher-light-yield NaI(Tl) detectors at the new, deeper Yemilab facility to further enhance sensitivity. Here, I will discuss the model-dependent WIMP searches performed with COSINE-100 and provide an update on the ongoing upgrade effort, highlighting its future prospects for crystal-based dark matter direct detection in Korea.

Figures (7)

• Figure 1: COSINE shielding overview. From outside inward, muon panels (3 cm thick, light blue), a lead brick castle (20 cm thick on all sides, grey), a copper box (3 cm thick), an acrylic box (1 cm thick) and eight encapsulated crystal detectors immersed in the liquid scintillator ($>$40 cm from crystal assembly to wall on all sides) are shown. Also indicated are the locations of the calibration holes, the size of the PMTs, and labeling scheme for the different sides.
• Figure 2: COSINE-100 exposure summary. Over the total run period of 6.39 years, the detector operated for 6.12 years, with 5.84 years of data passing all WIMP-search quality requirements. Periodic calibrations account for the remaining downtime. The data set is analyzed in four subsets (SET1–SET4), as indicated.
• Figure 3: Energy threshold improvement in the SET4 analysis. The SET4 analysis reduces the energy threshold down to 0.52 keV, nearly doubling the effective low-energy exposure. The figure shows the energy spectrum along with the selection efficiency, as a function of the photoelectron-equivalent energy.
• Figure 4: Pulse-shape discrimination in COSINE-100. The plot shows the two-dimensional PSD parameter space for neutron-induced (signal-like) and gamma-induced (background-like) events. The vertical axis corresponds to the CNN-based PSD output, while the horizontal axis shows the BDT-based PSD output. Events in the lower-left region are more gamma-like, whereas those in the upper-right region are more neutron-like.
• Figure 5: COSINE-100 Upgrade crystal detectors. The upper panel compares the light yields of eight crystals measured in three configurations: the original COSINE-100 at Y2L, the COSINE-100 Upgrade in the ground laboratory, and the COSINE-100 Upgrade at Yemilab. More than a 50% increase in light yield is observed after the upgrade. The lower panel shows the energy resolution of the 59.5 keV $^{241}$Am gamma peak, which also demonstrates significant improvement in the upgraded detectors.