Constraints on the Decay of ^180mTa

Ta-180m is a rare nuclear isomer whose decay has never been observed. Its remarkably long lifetime surpasses the half-lives of all other known beta 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 Ge-76 using an array of high-purity germanium detectors, to search for the decay of Ta-180m. More than 17 kg, the largest amount of tantalum metal ever used for such a search, was installed within the ultralow-background detector array. In this Letter, we present results from the first year of Ta data taking and provide an updated limit for the Ta-180m half-life on the different decay channels. With new limits up to 1.5x10(19) yr, we improved existing limits by 1-2 orders of magnitude which are the most sensitive searches for a single beta and electron capture decay ever achieved. Over all channels, the decay can be excluded for T-1/2<0.29x10(18) yr.