When a particle elastically scatters off a xenon nucleus, it has been assumed that electron clouds immediately follow the motion of the nucleus, but in reality it takes some time for the atomic electrons to catch up, resulting in ionization and excitation of the atom. This effect is called the Migdal effect, which was predicted by A. B. Migdal and recently reformulated in the context of Dark Matter searches by Ibe. et al. While the elastic scattering of WIMPs produces nuclear recoils, the Migdal effect predicts secondary electronic recoils that can accompany a nuclear recoil. Unlike nuclear recoils, electronic recoils lose negligible energy as heat, because electrons have small masses compared with xenon nuclei. This results in a lower energy threshold for electronic recoil signals – in XENON1T, down to about 1 keV. Therefore, searching for the electronic recoil signals induced by the Migdal effect enables a significant boost of XENON1T’s sensitivity to low-mass dark matter, based on this lowered threshold. In this search, we adopted an approach that utilizes the ionization signal only (so-called S2-only analysis), as well as both scintillation and ionization signals (S1-S2 analysis), which enables to lower the detection threshold. We interpreted the results in different cases: spin-(in)dependent (SI/SD) WIMP-nucleon interaction and the scenario where the interaction is mediated by a scalar force mediator (light mediator). The results for the spin-(in)dependent WIMP-nucleon interaction are shown in the following figure:
We set the most stringent upper limits on the SI and SD WIMP-nucleon interaction cross-sections for masses below 1.8 GeV and 2 GeV, respectively. Together with the standard nuclear recoil search, XENON1T results have thus reached unprecedented sensitivities to both low-mass (sub-GeV) and high-mass (GeV – TeV) WIMPs. An open access pre-print of the paper can of course be found on the arxiv.