We report on the search for x-ray radiation as predicted from dynamical quantum collapse with low-energy electronic recoil data in the energy range of 1-140 keV from the first science run of the XENONnT dark matter detector. Spontaneous radiation is an unavoidable effect of dynamical collapse models, which were introduced as a possible solution to the long-standing measurement problem in quantum mechanics. The analysis utilizes a model that for the first time accounts for cancellation effects in the emitted spectrum, which arise in the x-ray range due to the opposing electron-proton charges in xenon atoms. New world-leading limits on the free parameters of the Markovian continuous spontaneous localization and Diósi-Penrose models are set, improving previous best constraints by two orders of magnitude and a factor of five, respectively. For the strength and correlation length of the continuous spontaneous localization model, values in the originally proposed parameter ranges are experimentally excluded for the first time.

We report the measurement of the 214Bi𝛽-decay spectrum to the ground state of 214Po using the XENONnT detector. This decay is classified as first-forbidden nonunique, for which theoretical predictions require detailed nuclear structure modeling. A dedicated identification algorithm isolates a high-purity sample of ground-state 𝛽 decays, explicitly excluding events with associated 𝛾-ray emission. By comparing the measured spectrum, which covers energies up to 3.27MeV, with several nuclear models, we find that the prediction based on the conserved vector current hypothesis provides the best description of the data. Using this dataset, we additionally derive charge and light yield curves for electronic recoils, extending detector response modeling up to the megaelectronvolt scale.

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.

The XLZD collaboration is developing a two-phase xenon time projection chamber with an active mass of 60–80 t capable of probing the remaining weakly interacting massive particle-nucleon interaction parameter space down to the so-called neutrino fog. In this work we show that, based on the performance of currently operating detectors using the same technology and a realistic reduction of radioactivity in detector materials, such an experiment will also be able to competitively search for neutrinoless double beta decay in ¹³⁶Xe using a natural-abundance xenon target. XLZD can reach a 3σ discovery potential half-life of 5.7 × 10²⁷ years (and a 90% CL exclusion of 1.3 × 10²⁸ years) with 10 years of data taking, corresponding to a Majorana mass range of 7.3–31.3 meV (4.8–20.5 meV). XLZD will thus exclude the inverted neutrino mass ordering parameter space and will start to probe the normal ordering region for most of the nuclear matrix elements commonly considered by the community.

The XENONnT experiment has achieved an exceptionally low ²²²Rn activity concentration within its inner 5.9 tonne liquid xenon detector of (0.90±0.02 stat±0.07 syst)  μ⁢Bq kg⁻¹, equivalent to about 430 ²²²Rn atoms per tonne of xenon. This was achieved by active online radon removal via cryogenic distillation after stringent material selection. The achieved ²²²Rn activity concentration is 5 times lower than that in other currently operational multitonne liquid xenon detectors engaged in dark matter searches. This breakthrough enables the pursuit of various rare event searches that lie beyond the confines of the standard model of particle physics, with world-leading sensitivity. The ultralow ²²²Rn levels have diminished the radon-induced background rate in the detector to a point where it is for the first time comparable to the solar neutrino-induced background, which is poised to become the primary irreducible background in liquid xenon-based detectors.

We report on a blinded search for dark matter with single- and few-electron signals in the first science run of XENONnT relying on a novel detector response framework that is physics model dependent. We derive 90% confidence upper limits for dark matter-electron interactions. Heavy and light mediator cases are considered for the standard halo model and dark matter up-scattered in the Sun. We set stringent new limits on dark matter-electron scattering via a heavy mediator with a mass within 10–20  MeV/𝑐2 and electron absorption of axionlike particles and dark photons for 𝑚𝜒 below 0.03  keV/𝑐2.

Radiogenic neutrons emitted by detector materials are one of the most challenging backgrounds for the direct search of dark matter in the form of weakly interacting massive particles (WIMPs). To mitigate this background, the XENONnT experiment is equipped with a novel gadolinium-doped water Cherenkov detector, which encloses the xenon dual-phase time projection chamber (TPC). The neutron veto (NV) can tag neutrons via their capture on gadolinium or hydrogen, which release γ-rays that are subsequently detected as Cherenkov light. In this work, we present the first results of the XENONnT NV when operated with demineralized water only, before the insertion of gadolinium. Its efficiency for detecting neutrons is (82 ± 1)%, the highest neutron detection efficiency achieved in a water Cherenkov detector. This enables a high efficiency of (53 ± 3)% for the tagging of WIMP-like neutron signals, inside a tagging time window of 250 μs between TPC and NV, leading to a livetime loss of 1.6% during the first science run of XENONnT.

This report describes the experimental strategy and technologies for XLZD, the next-generation xenon observatory sensitive to dark matter and neutrino physics. In the baseline design, the detector will have an active liquid xenon target of 60 tonnes, which could be increased to 80 tonnes if the market conditions for xenon are favorable. It is based on the mature liquid xenon time projection chamber technology used in current-generation experiments, LZ and XENONnT. The report discusses the baseline design and opportunities for further optimization of the individual detector components. The experiment envisaged here has the capability to explore parameter space for Weakly Interacting Massive Particle (WIMP) dark matter down to the neutrino fog, with a 3σ evidence potential for WIMP-nucleon cross sections as low as 3×10−49cm2 (at 40 GeV/c2 WIMP mass). The observatory will also have leading sensitivity to a wide range of alternative dark matter models. It is projected to have a 3σ observation potential of neutrinoless double beta decay of 136Xe at a half-life of up to 5.7×1027 years. Additionally, it is sensitive to astrophysical neutrinos from the sun and galactic supernovae.

We report on a search for weakly interacting massive particle (WIMP) dark matter (DM) via elastic DM-xenon-nucleus interactions in the XENONnT experiment. We combine datasets from the first and second science campaigns resulting in a total exposure of 3.1 tonne-years. In a blind analysis of nuclear recoil events with energies above 3.8  keVNR, we find no significant excess above background. We set new upper limits on the spin-independent WIMP-nucleon scattering cross section for WIMP masses above 10  GeV/𝑐2 with a minimum of 1.7×10−47  cm2 at 90% confidence level for a WIMP mass of 30  GeV/𝑐2. We achieve a best median sensitivity of 1.4×10−47  cm2 for a 41  GeV/𝑐2 WIMP. Compared to the result from the first XENONnT science dataset, we improve our sensitivity by a factor of up to 1.8.

The XENONnT experiment, located at the INFN Laboratori Nazionali del Gran Sasso, Italy, features a 5.9 tonne liquid xenon time projection chamber surrounded by an instrumented neutron veto, all of which is housed within a muon veto water tank. Because of extensive shielding and advanced purification to mitigate natural radioactivity, an exceptionally low background level of (15.8±1.3)  events/(tonne·year·keV) in the (1,30) keV region is reached in the inner part of the time projection chamber. XENONnT is, thus, sensitive to a wide range of rare phenomena related to dark matter and neutrino interactions, both within and beyond the Standard Model of particle physics, with a focus on the direct detection of dark matter in the form of weakly interacting massive particles. From May 2021 to December 2021, XENONnT accumulated data in rare-event search mode with a total exposure of one tonne·year. This paper provides a detailed description of the signal reconstruction methods, event selection procedure, and detector response calibration, as well as an overview of the detector performance in this time frame. This work establishes the foundational framework for the “blind analysis” methodology we are using when reporting XENONnT physics results.