The XENONnT experiment, operated at INFN Laboratori Nazionali del Gran Sasso, is designed to search for Dark Matter (DM), primarily in the form of Weakly Interacting Massive Particles (WIMPs), with masses in the GeV to TeV range. Its sensitivity to the lower end of this range diminishes, in standard analyses, largely because the latter are typically optimized for higher-mass candidates, depositing more energy in the detector.

In this work, we take full advantage of the large exposures accumulated with XENONnT to perform a blind search for light dark matter models interacting via nuclear recoils. To achieve this, we have developed an innovative analysis that enables us to perform our dark matter search on smaller energy depositions, which are typically excluded by the standard approach. This has allowed us to explore several light dark matter models in previously untested regions. In the absence of a statistically significant excess over the background, we have set world-leading constraints on spin-independent DM-nucleon interactions mediated by a heavy or light mediator, spin-dependent DM-neutron interactions, momentum-dependent DM scattering, and mirror DM. In doing so, we have, for the first time in the field, en- countered a “new” irreducible background of astrophysical origin: ⁸B neutrinos from the Sun, undergoing coherent elastic neutrino-nucleus scattering (CEνNS) with xenon atoms, producing a nuclear recoil spectrum that is essentially indistinguishable from that expected from certain light dark matter model.

Entering this new regime, often referred to as the “neutrino fog”, implies that, with the results from this search, XENONnT has nearly reached its maximum sensitivity to light dark matter models interacting via nuclear recoil. Consequently, further advancement in analysis techniques or realistic larger exposures is unlikely to lead to major sensitivity improvements. Despite this, XENONnT continues to produce cutting-edge science, as searches for DM masses above 10 GeV – where CEνNS from solar neutrinos is negligible – remain a couple of orders of magnitude away from reaching the corresponding atmospheric neutrino fog.