Search for deexcitations of 178m2Hf isomer induced by inelastic Dark Matter candidates in ultra-low-background conditions

The $\gamma$-ray radiation from a ^178m2Hf source with an activity of $\approx 63$ Bq was measured for the first time with two ultra-low-background HPGe $\gamma$ spectrometers over cumulative 1170 h deep underground at the Gran Sasso National Laboratory (LNGS) of the INFN. The aim of the measurements was the search for deexcitations of the ^178m2Hf isomer induced by inelastic Dark Matter candidates. The process would lead to $\gamma$-ray emission from the excited levels of the ¹⁷⁸Hf that are not populated in the spontaneous decay of the ^178m2Hf. In this way improved limits on the half-life of eighteen inelastic-dark-matter-induced transitions, at the level of $lim{T}_{1/2}\sim 4\times 10^5\text{–}6\times 10^6$ yr are derived. The parameters’ space of such inelastic candidates is also derived for some given model assumptions. Elements towards progress in sensitivity are briefly mentioned.