Constraints on the decay of ${}^{180m}$Ta

Abstract

${}^{180m}$Ta is a rare nuclear isomer whose decay has never been observed. Its remarkably long lifetime surpasses the half-lives of all other known ${\hat{I}}^2$ and electron capture decays due to the large K-spin differences and small energy differences between the isomeric and lower energy states. Detecting its decay presents a significant experimental challenge but could shed light on neutrino-induced nucleosynthesis mechanisms, the nature of dark matter and K-spin violation. For this study, we repurposed the MAJORANA DEMONSTRATOR, an experimental search for the neutrinoless double-beta decay of ${}^{76}$Ge using an array of high-purity germanium detectors, to search for the decay of ${}^{180m}$Ta. More than 17 kilograms, the largest amount of tantalum metal ever used for such a search was installed within the ultra-low background detector array. In this paper we present results from the first year of Ta data taking and provide an updated limit for the ${}^{180m}$Ta half-life on the different decay channels. With new limits up to 1.5 x $10^{19}$ years, we improved existing limits by one to two orders of magnitude. This result is the most sensitive search for a single ${\hat{I}}^2$ and electron capture decay ever achieved.