Observation of Excess Electronic Recoil Events in XENON1T
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 t-y and an unprecedentedly low background rate of $76 \pm 2_{stat}$ events/(t y keV) between 1-30 keV. 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$\sigma$ 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 $g_{ae} < 3.7 \times 10^{-12}$, $g_{ae}g_{an}^{eff} < 4.6 \times 10^{-18}$, and $g_{ae}g_{a\gamma} < 7.6\times10^{-22}~{GeV}^{-1}$, and excludes either $g_{ae}=0$ or $g_{ae}g_{a\gamma}=g_{ae}g_{an}^{eff}=0$. The neutrino magnetic moment signal is similarly favored over background at 3.2$\sigma$ and a confidence interval of $\mu_{\nu} \in (1.4,~2.9)\times10^{-11}\mu_B$ (90% C.L.) is reported. Both results are in strong tension with stellar constraints. The excess can also be explained by $\beta$ decays of tritium at 3.2$\sigma$ significance with a corresponding tritium concentration in xenon of $(6.2 \pm 2.0) \times 10^{-25}$ 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$\sigma$ and 0.9$\sigma$, respectively, if an unconstrained tritium component is included in the fitting. With respect to bosonic dark matter, the excess favors a monoenergetic peak at ($2.3 \pm 0.2$) keV (68% C.L.) with a 3.0$\sigma$ global (4.0$\sigma$ 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$^2$.
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