Deep-underground search for the decay of $$^{180\textrm{m}}\text {Ta}$$ 180 m Ta with an ultra-low-background HPGe detector

Abstract $$^{180\textrm{m}}\text {Ta}$$ 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 $$^{180\textrm{m}}\text {Ta}$$ 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 $$^{182}\text {Ta}$$ 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 $$\beta ^-$$ β - : $$T_{1/2,~\textrm{EC}}> 1.6 \times 10^{18}$$ T 1 / 2 , EC > 1.6 × 10 18 year and $$T_{1/2,~\beta ^-}> 1.1\times 10^{18}$$ T 1 / 2 , β - > 1.1 × 10 18 year (90% C. I.), respectively. We also set the limit on the $$\gamma $$ γ de-excitation / IC channel: $$T_{1/2,~\textrm{IC}}> 4.1 \times 10^{15}$$ 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 $$^{180\textrm{m}}\text {Ta}$$ 180 m Ta worldwide. Finally, we test the hypothetical scenarios of de-excitation of $$^{180\textrm{m}}\text {Ta}$$ 180 m Ta by cosmological Dark Matter and constrain new parameter space for strongly-interacting dark-matter particle with mass up to $$10^5$$ 10 5 GeV.