Excess electronic recoil events in XENON1T

Abstract

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.

Undulating dark matter

“…Recently XENON1T has reported an excess in just such a region [8]. While this excess most likely has it's origins in more mundane effects, it is interesting to speculate if it could be the harbinger of a DM discovery.…”

“…Even if the background rate were only poorly understood, we find that an undulating contribution to the signal can be constrained several orders of magnitude better than a constant contribution, for a reasonably wide window of frequencies. 1 This strategy could boost discovery potential in high background search regions, such as the single-electron event bins of the upcoming SENSEI [2][3][4] and SuperCDMS experiments [5,6], and the electronic recoil mode of the XENON1T experiment [7] (in which an excess of events has recently been observed [8]).…”

“…[22], where α is the fine-structure constant and µ χe is the reduced mass between the electron and χ. 8 The data from the 1e − bin is significantly less constraining than the multi-electron bins because the background rate is high, with the ≥ 3e − bin being essentially background-free. On the other hand, due to kinematic thresholds, the multi-electron bins lose sensitivity for dark matter masses lighter than 1 MeV or so.…”

“…Other reasonable models give no bounds on the DM EDM[37,38] 7. At the time of writing, these constraints are currently world-leading for DM masses mχ ∈ [0.6, 5] MeV, as well as in the low mass window mχ ∈ [1.2, 12.8] eV 8. For these calculations we assume the local dark matter density is ρ DM = 0.3GeV/cm 3 , the dark matter escape velocity is 600 km/s, the mean local velocity of dark matter is 230 km/s and the average Earth velocity is 240 km/s.…”

Exothermic dark matter for XENON1T excess

“…Quite recently, a tantalizing hint has been announced for the potential dark matter signals from the electron recoil events in the recoil energy, E R = 1 − 10 keV, from XENON1T experiment [1]. The origin of the Xenon excess has attracted interest from several groups [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”