Search

The precise measurement of cosmic-ray antinuclei serves as an important means for identifying the nature of dark matter and other new astrophysical phenomena, and could be used with other cosmic-ray species to understand cosmic-ray production and propagation in the Galaxy. For instance, low-energy antideuterons would provide a "smoking gun" signature of dark matter annihilation or decay, essentially free of astrophysical background. Studies in recent years have emphasized that models for cosmic-ray antideuterons must be considered together with the abundant cosmic antiprotons and any potential observation of antihelium. Therefore, a second dedicated Antideuteron Workshop was organized at UCLA in March 2019, bringing together a community of theorists and experimentalists to review the status of current observations of cosmic-ray antinuclei, the theoretical work towards understanding these signatures, and the potential of upcoming measurements to illuminate ongoing controversies. This review aims to synthesize this recent work and present implications for the upcoming decade of antinuclei observations and searches. This includes discussion of a possible dark matter signature in the AMS-02 antiproton spectrum, the most recent limits from BESS Polar-II on the cosmic antideuteron flux, and reports of candidate antihelium events by AMS-02; recent collider and cosmic-ray measurements relevant for antinuclei production models; the state of cosmic-ray transport models in light of AMS-02 and Voyager data; and the prospects for upcoming experiments, such as GAPS. This provides a roadmap for progress on cosmic antinuclei signatures of dark matter in the coming years.

The search for neutrinoless double beta decay probes the fundamentalproperties of neutrinos, including whether or not the neutrino and antineutrinoare distinct. Double beta detectors are large and expensive, so backgroundreduction is essential for extracting the highest sensitivity. Theidentification, or 'tagging', of the $^{136}$Ba daughter atom from double betadecay of $^{136}$Xe provides a technique for eliminating backgrounds in thenEXO neutrinoless double beta decay experiment. The tagging scheme studied inthis work utilizes a cryogenic probe to trap the barium atom in solid xenon,where the barium atom is tagged via fluorescence imaging in the solid xenonmatrix. Here we demonstrate imaging and counting of individual atoms of bariumin solid xenon by scanning a focused laser across a solid xenon matrixdeposited on a sapphire window. When the laser sits on an individual atom, thefluorescence persists for $\sim$30~s before dropping abruptly to the backgroundlevel, a clear confirmation of one-atom imaging. No barium fluorescencepersists following evaporation of a barium deposit to a limit of $\leq$0.16\%.This is the first time that single atoms have been imaged in solid nobleelement. It establishes the basic principle of a barium tagging technique fornEXO.

The coherent elastic scattering of neutrinos off nuclei has eluded detection for four decades, even though its predicted cross section is by far the largest of all low-energy neutrino couplings. This mode of interaction offers new opportunities to study neutrino properties and leads to a miniaturization of detector size, with potential technological applications. We observed this process at a 6.7σ confidence level, using a low-background, 14.6-kilogram CsI[Na] scintillator exposed to the neutrino emissions from the Spallation Neutron Source at Oak Ridge National Laboratory. Characteristic signatures in energy and time, predicted by the standard model for this process, were observed in high signal-to-background conditions. Improved constraints on nonstandard neutrino interactions with quarks are derived from this initial data set.