Light Dark Matter Search with 7.8 Tonne-Year of Ionization-Only Data in XENONnT

We report on a blinded search for dark matter (DM) using ionization-only (S2-only) signals in XENONnT with a total exposure of 7.83     Tonne-year over 579 days in three science runs. Dedicated background suppression techniques and the first complete S2-only background model in XENONnT provide sensitivity to nuclear recoils of [ 0.5 , 5.0 ]     keV nr and electronic recoils of [ 0.04 , 0.7 ]     keV ee . No significant excess over the expected background is observed, and we set 90% confidence level upper limits on spin-independent DM-nucleon and spin-dependent DM-neutron scattering for DM masses between 3 and 8     GeV / c 2 , as well as on DM-electron scattering, axionlike particles, and dark photons, improving on previous constraints. For spin-independent DM-nucleon scattering, we exclude cross sections above 6.0 × 10 − 45     cm 2 at a DM mass of 5     GeV / c 2 , pushing the XENONnT sensitivity closer to the region where coherent elastic neutrino-nucleus scattering becomes an irreducible background.