Probing the Dark Sector with Nuclear Transition Photons.

Here we present world-leading sensitivity to light (<170 MeV) dark matter (DM) using beam-dump experiments. Dark sector particles produced during pion decay at accelerator beam dumps can be detected via scattering in neutrino detectors. The decay of nuclei excited by the inelastic scattering of DM is an unexploited channel which has significantly better sensitivity than similar searches using the elastic scattering channel. We show that this channel is a powerful probe of DM by demonstrating sensitivity to the thermal relic abundance benchmark in a scalar DM dark-photon portal model. This is achieved through the use of existing data, obtained by the KARMEN experiment over two decades ago, which allow us to set world-leading constraints on this model over a wide mass range. With experimental improvements planned for the future, this technique will be able to probe the thermal relic benchmark for fermionic DM across a wide mass range.

Inverse Primakoff Scattering as a Probe of Solar Axions at Liquid Xenon Direct Detection Experiments.

We show that XENON1T and future liquid xenon (LXe) direct detection experiments are sensitive to axions through the standard g_{aγ}aFF[over ˜] operators due to inverse-Primakoff scattering. This previously neglected channel significantly improves the sensitivity to the axion-photon coupling, with a reach extending to g_{aγ}∼10^{-10} GeV^{-1} for axion masses up to a keV, thereby extending into the region of heavier QCD axion models. This result modifies the couplings required to explain the XENON1T excess in terms of solar axions, opening a large region of g_{aγ}-m_{a} parameter space that is not ruled out by the CAST helioscope experiment and reducing the tension with the astrophysical constraints. We explore the sensitivity to solar axions for future generations of LXe detectors that can exceed future helioscope experiments, such as IAXO, for a large region of parameter space.

Explaining the XENON1T Excess with Luminous Dark Matter.

We show that the excess in electron recoil events seen by the XENON1T experiment can be explained by a relatively low-mass luminous dark matter candidate. The dark matter scatters inelastically in the detector (or the surrounding rock) to produce a heavier dark state with a ∼2-3 keV mass splitting. This heavier state then decays within the detector, producing a peak in the electron recoil spectrum that is a good fit to the observed excess. We comment on the ability of future direct detection experiments to differentiate this model from other "beyond the standard model" scenarios and from possible tritium backgrounds, including the use of diurnal modulation, multichannel signals, etc., as possible distinguishing features of this scenario.