In June of this year, physicists working on the XENON1T dark-matter detector announced the measurement of a curious signal in their experiment — which comprises 2 tonne of ultrapure xenon. The signal had a statistical significance of 3.5σ or less, which is well below the 5σ level that is usually required for a discovery in particle physics.
In a preprint published at the time, the team suggested that the signal – an excess in low-energy electron recoil events – could have three explanations. The most mundane is that it was caused by contamination of the ultrapure xenon by radioactive tritium.
A more intriguing explanation, they said, is that the signal is the detection of hypothetical particles called axions that could be emitted the Sun. The third possibility is that the excess is caused by neutrinos interacting with the xenon in an unexpected way – which would also be very interesting.
Now, the original preprint has been published in Physical Review D and over in Physical Review Letters, five theoretical papers put forth a wide range of tantalizing explanations for the excess.
Working in Japan, Fuminobu Takahashi, Masaki Yamada, and Wen Yin say that the signal could be related to a hypothetical axionlike particle (ALP) with a mass of a few keV/c2 that interacts with electron. As well as explaining the XENON1T signal, such an ALP could be a constituent of dark matter and its existence could explain an anomaly in the observed cooling of white dwarf and red giant stars.
Meanwhile in Germany, Andreas Bally, Sudip Jana, and Andreas Trautner reckon the mystery signal could be the work of a hypothetical gauge boson that mediates a new interaction between solar neutrinos and electrons.
A paper written by Nicole Bell et al. makes the case for a “relatively low-mass luminous dark matter candidate,” as the source of the excess. They suggest that this dark-matter particle could enter the detector in a “light state” and be scattered into a “heavy state” that would decay by emitting a photon. This photon would then interact with an electron in the detector to create the observed signal.
Another dark-matter proposal comes from Bartosz Fornal and colleagues who suggest that otherwise sluggish cold-dark-matter particles could get a boost of energy from the galactic centre and collide with XENON1T electrons.
The fifth idea comes from Joseph Bramante and Ningqiang Song in Canada, who argue that the signal could come from the scattering of a type of dark matter that is a thermal relic from the early universe.
They can’t all be right, and it will be very interesting to see if a similar signal shows up in future dark-matter experiments.
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±2stat events/(tonne×year×keV) 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 gae<3.8×10−12, gaegeffan<4.8×10−18, and gaegaγ<7.7×10−22 GeV−1, and excludes either gae= or gaegaγ=gaegeffan=. The neutrino magnetic moment signal is similarly favored over background at 3.2σ, and a confidence interval of μν∈(1.4,2.9)×10−11 μB (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 (6.2±2.0)×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 (2.3±0.2) 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/c2. We also consider the possibility that 37Ar may be present in the detector, yielding a 2.82 keV peak from electron capture. Contrary to tritium, the 37Ar concentration can be tightly constrained and is found to be negligible.