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

\(^{180m}\)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 \mTa 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 \(\beta^-\): \(T_{1/2,~\mathrm{EC}} > 1.6 \times 10^{18}\) yr and \(T_{1/2,~\beta^-} > 1.1\times 10^{18}\) yr (\(90\)\% C.\,I.), respectively. We also set the limit on the \(\gamma\) de-excitation / IC channel: \(T_{1/2,~\mathrm{IC}} > 4.1 \times 10^{15}\) yr (\(90\)\% C.\,I.). These are, as of now, the most stringent bounds on the decay of \(^{180m}\)Ta worldwide. Finally, we test the hypothetical scenarios of de-excitation of \(^{180m}\)Ta by cosmological Dark Matter and constrain new parameter space for strongly-interacting dark-matter particle with mass up to \(10^5\) GeV.