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$^{180m}$Ta is the longest-lived metastable state presently known. Its decayhas 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 anultra-low background HPGe detector in the STELLA laboratory of the LaboratoriNazionali del Gran Sasso (LNGS), in Italy. Before the measurement, the samplehas been stored deep-underground for ten years, resulting in subdominantbackground contributions from cosmogenically activated $^{182}$Ta. We observeno signal in the regions of interest and set half-life limits on the processfor the two channels EC and $\beta^-$: $T_{1/2,~\mathrm{EC}} > 1.6 \times10^{18}$ 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, asof now, the most stringent bounds on the decay of $^{180m}$Ta worldwide.Finally, we test the hypothetical scenarios of de-excitation of $^{180m}$Ta bycosmological Dark Matter and constrain new parameter space forstrongly-interacting dark-matter particle with mass up to $10^5$ GeV.

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