First Search for Light Dark Matter in the Neutrino Fog with XENONnT

We search for dark matter (DM) with a mass [3,12] GeV/c² using an exposure of 3.51 tonne year with the XENONnT experiment. We consider spin-independent DM-nucleon interactions mediated by a heavy or light mediator, spin-dependent DM-neutron interactions, momentum-dependent DM scattering, and mirror DM. Using a lowered energy threshold compared to the previous weakly interacting massive particle search, a blind analysis of [0.5, 5.0] keV nuclear recoil events reveals no significant signal excess over the background. XENONnT excludes spin-independent DM-nucleon cross sections >2.5×10⁻⁴⁵ cm² at 90% confidence level for 6 GeV/c² DM. In the considered mass range, the DM sensitivity approaches the "neutrino fog," the limitation where neutrinos produce a signal that is indistinguishable from that of light DM-xenon nucleus scattering.