Search for Light Dark Matter Interactions Enhanced by the Migdal Effect or Bremsstrahlung in XENON1T

Direct dark matter detection experiments based on a liquid xenon target are leading the search for darkmatter particles with masses above∼5GeV=c2, but have limited sensitivity to lighter masses because ofthe small momentum transfer in dark matter-nucleus elastic scattering. However, there is an irreduciblecontribution from inelastic processes accompanying the elastic scattering, which leads to the excitation andionization of the recoiling atom (the Migdal effect) or the emission of a bremsstrahlung photon. In thisLetter, we report on a probe of low-mass dark matter with masses down to about85MeV=c2by looking forelectronic recoils induced by the Migdal effect and bremsstrahlung using data from the XENON1Texperiment. Besides the approach of detecting both scintillation and ionization signals, we exploit anapproach that uses ionization signals only, which allows for a lower detection threshold. This analysissignificantly enhances the sensitivity of XENON1T to light dark matter previously beyond its reach