Experimental search for the Migdal Effect in a compact liquid xenon TPC

Direct dark matter searches have reported dramatically increased sensitivity to sub-GeV parameter space by taking into account the "Migdal Effect": a predicted inelastic process in which a particle scattering with a nucleus occasionally ejects a bound electron from the recoiling atom. The resulting atomic relaxation can emit energy at the ~keV level even for very low-energy nuclear recoils, enabling the detection of nuclear scattering events with recoil energies well below the detectors' ionization or scintillation thresholds. However, the Migdal Effect has never been experimentally observed in a scattering process, and should be confirmed and characterized before a potential dark matter signal in this channel can be reliably discovered. In this talk, we report on a dedicated experimental campaign to search for the Migdal Effect using neutron scattering in a small liquid xenon detector at Lawrence Livermore National Laboratory. Scattered neutrons (14.1 MeV incident energy) are detected by a ring of liquid scintillator detectors placed at an angle of 15 degrees from the neutron beam axis, resulting in a 300,000-event sample of 7-9 keV nuclear recoils in the liquid xenon. We make use of the two-dimensional ionization vs. scintillation phase space to search for nuclear recoil events with an electronic recoil component consistent with atomic excitation from the Migdal Effect.