We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 tonne-years and an unprecedentedly low background rate of 76 AE 2 stat events=ðtonne × year × keVÞ between 1 and 30 keV, the data enable one of the most sensitive searches for solar axions, an enhanced neutrino magnetic moment using solar neutrinos, and bosonic dark matter. An excess over known backgrounds is observed at low energies and most prominent between 2 and 3 keV. The solar axion model has a 3.4σ significance, and a three-dimensional 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by g ae < 3.8 × 10 −12 , g ae g eff an < 4.8 × 10 −18 , and g ae g aγ < 7.7 × 10 −22 GeV −1 , and excludes either g ae ¼ 0 or g ae g aγ ¼ g ae g eff an ¼ 0. The neutrino magnetic moment signal is similarly favored over background at 3.2σ, and a confidence interval of μ ν ∈ ð1.4; 2.9Þ × 10 −11 μ B (90% C.L.) is reported. Both results are in strong tension with stellar constraints. The excess can also be explained by β decays of tritium at 3.2σ significance with a corresponding tritium concentration in xenon of ð6.2 AE 2.0Þ × 10 −25 mol=mol. Such a trace amount can neither be confirmed nor excluded with current knowledge of its production and reduction mechanisms. The significances of the solar axion and neutrino magnetic moment hypotheses are decreased to 2.0σ and 0.9σ, respectively, if an unconstrained tritium component is included in the fitting. With respect to bosonic dark matter, the excess favors a monoenergetic peak at ð2.3 AE 0.2Þ keV (68% C.L.) with a 3.0σ global (4.0σ local) significance over background. This analysis sets the most restrictive direct constraints to date on pseudoscalar and vector bosonic dark matter for most masses between 1 and 210 keV=c 2. We also consider the possibility that 37 Ar may be present in the detector, yielding a 2.82 keV peak from electron capture. Contrary to tritium, the 37 Ar concentration can be tightly constrained and is found to be negligible.

XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to 12.3 ± 0.6 (keV t y)-1 and (2.2± 0.5)× 10−3 (keV t y)-1, respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of 1.4×10−48 cm2 for a 50 GeV/c2 mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T . In addition, we show that for a 50 GeV/c2 WIMP with cross-sections above 2.6×10−48 cm2 (5.0×10−48 cm2) the median XENONnT discovery significance exceeds 3σ (5σ). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches 2.2×10−43 cm2 (6.0×10−42 cm2).

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