Deep-underground search for the decay of 180mTa with an ultra-low-background HPGe detector.

180 m Ta is the longest-lived metastable state presently known. Its decay has not been observed yet. In this work, we report a new result on the decay of 180 m Ta obtained with a 2015.12-g tantalum sample measured for 527.7 d with an ultra-low background HPGe detector in the STELLA laboratory of the Laboratori Nazionali del Gran Sasso (LNGS), in Italy. Before the measurement, the sample has been stored deep-underground for ten years, resulting in subdominant background contributions from cosmogenically activated 182 Ta . We observe no signal in the regions of interest and set half-life limits on the process for the two channels EC and β - : T 1 / 2 , EC > 1.6 × 10 18 year and T 1 / 2 , β - > 1.1 × 10 18 year (90% C. I.), respectively. We also set the limit on the γ de-excitation / IC channel: T 1 / 2 , IC > 4.1 × 10 15 year (90% C. I.). These are, as of now, the most stringent bounds on the decay of 180 m Ta worldwide. Finally, we test the hypothetical scenarios of de-excitation of 180 m Ta by cosmological Dark Matter and constrain new parameter space for strongly-interacting dark-matter particle with mass up to 10 5 GeV. [ABSTRACT FROM AUTHOR]