Dark Matter Search with the Nuclear Isomer Ta-180m

Abstract

There is compelling cosmological and astrophysical evidence of dark matter comprising 27% of the energy budget of the Universe. However, dark matter has never been observed in direct detection experiments. The long-time favorite model of Weakly Interacting Massive Particles saw a large experimental effort with steady progress over recent decades. Since also these large-scale searches remain unsuccessful to date, it is compelling to look at more exotic dark matter models which can be constrained with new approaches and much less scientific resources. Using nuclear isomers is one of these approaches. $^{180m}$Ta is the rarest known isotope with the longest-lived meta-stable state whose partial half-life limits are on the order of 10$^{14}$-10$^{16}$ yr. We investigate how strongly interacting dark matter and inelastic dark dark matter collides with $^{180m}$Ta, leading to its de-excitation. The energy stored in the meta-stable state is released in the transition, which becomes the signature for thermalized dark matter in a well-shielded underground experiment. We report on a direct detection experiment searching for these dark-matter-induced decay signatures which has further constrained the open parameter space. We also propose an indirect geochemical experiment to search for decay products of $^{180m}$Ta in tantalum minerals accumulated over 1 billion years.