Searches for new physics below twice the electron mass with GERDA

A search for full energy depositions from bosonic keV-scale dark matter candidates of masses between 65 and 1021 keV has been performed with data collected during Phase II of the GERmanium Detector Array (Gerda) experiment. Our analysis includes direct dark matter absorption as well as dark Compton scattering. With a total exposure of 105.5 kg years, no evidence for a signal above the background has been observed. The resulting exclusion limits deduced with either Bayesian or Frequentist statistics are the most stringent direct constraints in the major part of the 140–1021 keV mass range. As an example, at a mass of 150 keV the dimensionless coupling of dark photons and axion-like particles to electrons has been constrained to $alpha '/\alpha <8.7~\times 10^{-24}$ and $g_\textrm{ae}<3.3~\times 10^{-12}$ at 90% credible interval (CI), respectively. Additionally, a search for peak-like signals from beyond the Standard Model decays of nucleons and electrons is performed. We find for the inclusive decay of a single neutron in $^{76}$Ge a lower lifetime limit of $\tau _\textrm{n} > 1.5 \times 10^{24}$ years and for a proton $\tau _\textrm{p} > 1.3 \times 10^{24}$ years at 90% CI. For the electron decay $e^\text {-} \rightarrow \nu _\textrm{e} \gamma $ a lower limit of $\tau _\textrm{e} > 5.4\times 10^{25}$ years at 90% CI has been determined.