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 $76\pm2$ events/(t y keV) between
1 and 30 keV, the data enables sensitive searches for solar axions, an enhanced
neutrino magnetic moment, 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 3.4$\sigma$ significance, and a 3D 90% confidence
surface is reported for axion couplings to electrons, photons, and nucleons.
This surface is inscribed in the cuboid defined by $g_{ae}<3.8 \times
10^{-12}$, $g_{ae}g_{an}^{eff}<4.8\times 10^{-18}$, and
$g_{ae}g_{a\gamma}<7.7\times10^{-22} GeV^{-1}$, 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$\sigma$ with a
trace amount that 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 reduced to 2.0$\sigma$ 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 ($2.3\pm0.2$) keV (68% C.L.) with a 3.0$\sigma$ global
(4.0$\sigma$ local) significance. We also consider the possibility that
$^{37}$Ar may be present in the detector and yield a 2.82 keV peak. Contrary to
tritium, the $^{37}$Ar concentration can be tightly constrained and is found to
be negligible.
