Observation of Excess Electronic Recoil Events in XENON1T

June 17, 2020


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 ± 2 stat events/(tonne × year × keV) between 1–30 keV, the data enables competitive searches for solar axions, an enhanced neutrino magnetic moment using solar neutrinos, and bosonic dark matter. An excess over known backgrounds is observed below 7 keV, rising towards lower energies and prominent between 2–3 keV. The solar axion model has a 3.5 σ 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 gae < 3.7 × 10−12 , gaeg eff an < 4.6 × 10−18 , and gaegaγ < 7.6 × 10−22 GeV−1 , and excludes either gae = 0 or gaegaγ = gaeg eff an = 0. The neutrino magnetic moment signal is similarly favored over background at 3.2 σ and a confidence interval of µν ∈ (1.4, 2.9)×10−11 µB (90% C.L.) is reported. Both results are in tension with stellar constraints. The excess can also be explained by β decays of tritium, which was initially not considered, at 3.2 σ significance with a corresponding tritium concentration in xenon of (6.2 ± 2.0) × 10−25 mol/mol. Such a trace amount can be neither confirmed nor excluded with current knowledge of production and reduction mechanisms. The significances of the solar axion and neutrino magnetic moment hypotheses are decreased to 2.1 σ and 0.9 σ, respectively, if an unconstrained tritium component is included in the fitting. This analysis also sets the most restrictive direct constraints to date on pseudoscalar and vector bosonic dark matter for most masses between 1 and 210 keV/c2.

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