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 $7.83\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{n}\mathrm{e}\times \mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}$ 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] $\mathrm{k}\mathrm{e}{\mathrm{V}}_{\mathrm{n}\mathrm{r}}$ and electronic recoils of [0.04, 0.7] $\mathrm{k}\mathrm{e}{\mathrm{V}}_{\mathrm{e}\mathrm{e}}$. 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 $\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$, as well as on DM–electron scattering, axion-like particles, and dark photons, improving on previous constraints. For spin-independent DM–nucleon scattering, we exclude cross sections above $6.0\times {10}^{-45}$cm${}^2$ at a DM mass of 5 $\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$, pushing the XENONnT sensitivity closer to the region where coherent elastic neutrino-nucleus scattering ($\text{CE}\nu \text{NS}$) becomes an irreducible background.