Search

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.

Weak-scale dark matter particles, in collisions with nuclei, can mediate transitions between different nuclear energy levels. In particular, owing to sizeable momentum exchange, dark matter particles can enable de-excitation of nuclear isomers that are extremely long lived with respect to regular radioactive decays. In this paper, we utilize data from a past experiment with $^{180}$Ta$\rm ^m$ to search for $\gamma$-lines that would accompany dark matter induced de-excitation of this isomer. Non-observation of such transitions above background yields the first direct constraint on the lifetime of $^{180}$Ta$\rm ^m$ against DM-initiated transitions: $T_{1/2}>1.3\times 10^{14}$~a at 90\% C.I. Using this result, we derive novel constraints on dark matter models with strongly interacting relics, and on models with inelastic dark matter particles. Existing constraints are strengthened by this independent new method. The obtained limits are also valid for the Standard Model $\gamma$-decay of $^{180}$Ta$\rm ^m$.

The GERDA experiment searches for the neutrinoless double beta ($0\nu\beta\beta$) decay in $^{76}$Ge in order to probe whether the neutrino is a Majorana particle and to shed light on the neutrino mass ordering. For investigating such a rare decay it is necessary to minimize the background of the experiment. In Phase II of the GERDA experiment the scintillation light of liquid argon (LAr) is used as an additional background veto. In order to estimate the efficiency of such a LAr veto it has to be known how far the scintillation light, which peaks at 128 nm, can travel within the LAr. A dedicated setup was built to measure the attenuation length of the scintillation light in the LAr in-situ within the cryostat of GERDA. The setup is composed of a stainless steel housing with a photomultiplier tube (PMT) at one side and a moveable $^{90}$Sr source at the other side to measure the light intensity at different distances between source and PMT. Furthermore, a sophisticated simulation was developed in order to determine the solid angle correction as well as the background for this measurement. The analysis results in an absorption length of $15.8 \pm 0.7$ (stat) ${}^{+1.5}_{-3.2}$ (syst) cm under the assumption of a scattering length of 70 cm at 128 nm. The obtained value of the absorption length is specific for the LAr in GERDA at the time of the measurement., Comment: 10 pages, 12 figures

$^{\rm {180m}}$Ta is the rarest naturally occurring quasi-stable isotope and the longest lived metastable state which is known. Its possible decay via the $\beta^-$ or the electron capture channel has never been observed. This article presents a search for the decay of $^{\rm {180m}}$Ta with an ultra low background Sandwich HPGe gamma spectrometry setup in the HADES underground laboratory. No signal is observed and improved lower partial half-life limits are set with a Bayesian analysis to $5.8\cdot10^{16}$ yr for the $\beta^-$ channel and $2.0\cdot10^{17}$ yr for the electron capture channel (90% credibility). The total half-life of $^{\rm {180m}}$Ta is longer than $4.5\cdot10^{16}$ yr. This is more than a factor of two improvement compared to previous searches.

The GERDA experiment searches for neutrinoless double beta ($0\nu\beta\beta$) decay in $^{76}$Ge using an array of high purity germanium (HPGe) detectors immersed in liquid argon (LAr). Phase II of the experiment uses 30 new broad energy germanium (BEGe) detectors with superior pulse shape discrimination capabilities compared to the previously used semi-coaxial detector design. By far the largest background component for BEGe detectors in GERDA are n$^+$-surface events from $^{42}$K $\beta$ decays which are intrinsic in LAr. The $\beta$ particles with up to 3.5 MeV can traverse the 0.5 to 0.9 mm thick electrode and deposit energy within the region of interest for the $0\nu\beta\beta$ decay. However, those events have particular pulse shape features allowing for a strong discrimination. The understanding and simulation of this background, showing a reduction by up to a factor 145 with pulse shape discrimination alone, is presented in this work., Comment: Proceeding TAUP 2015

A search for excited state transitions of double beta decay in Pd110 and double electron capture in Pd102 has been performed in the HADES underground laboratory with two low background HPGe detectors in sandwich configuration. After an exposure of 35.92 kg*d, no signal was found. The frequentist spectral analysis resulted in lower half-life limits of 1.98e20 yr and 1.72e20 yr (95% CL) for the first 0+ and 2+ excited state in Pd110 respectively. This is an improvement by more than a factor of 3 with respect to previous measurements. In Pd102, the lower half-life limit could be improved to 5.95e18 yr (95% CL) for the first 0+ excited state. Furthermore, first experimental lower half-live limits are found for all possible excited states in the Pd110 and Pd102 systems., Comment: 22 pages, 9 figures, submitted to PRC

A search for double beta decays of the palladium isotopes 110Pd and 102Pd into excited states of their daughters was performed and first half-life limits for the 2{\nu}{\beta}{\beta} and 0{\nu}{\beta}{\beta} decays into first excited 0+ and 2+ states of 5.89e19 yr and 4.40e19 yr (95% CL) for the 110Pd decay were obtained. The half-life limits for the corresponding double electron capture transition of 102Pd are 7.64e18 yr and 2.68e18 yr (95% CL) respectively. These are the first measurements for 102Pd., Comment: Updated to published version. Refined analysis and minor text changes. Half-life limits changed