Cosmic-ray antinuclei as messengers of new physics: status and outlook for the new decade

Abstract

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.

Formation of light (anti)nuclei

“…These particles are of particular importance in cosmic ray studies due to their low expected background at small kinetic energies, for a recent review see Ref. [1]. This makes them a promising detection channel for exotic physics, such as dark matter annihilations and decays.…”

“…Currently, the measurement of astrophysical antinuclei is performed by the AMS-02 experiment on-board the International Space Station, while the balloon-borne GAPS experiment is expected to be launched in December 2022 [2,3]. Unfortunately, the AMS-02 collaboration has not yet published any results regarding antinuclei measurements, even though the expected antideuteron flux from secondary production and optimistic dark matter models is close to its estimated sensitivity [1]. In heavy ion collisions, (anti)nuclei measurements are of particular interest due to their small binding energies.…”

Understanding the background in dark matter searches by studying antinucleosynthesis in the laboratory with ALICE

“…After the propagation through space towards the earth, these particles could be observed as an excess of the light antinuclei flux above the expectations from interactions of primary cosmic rays (CR) and the interstellar medium (ISM) at low kinetic energies by satellite-born or balloon experiments. As indicated in Figure 1, which shows the predicted flux of 3 He for different dark matter models and decays as well as different background calculations, the exact energy and strength of the excess related to dark matter depends on its characteristics [1]. Thus, it is important to be able to precisely predict the expected background from reactions of primary cosmic rays and the interstellar medium, which both consist mostly of protons and 4 He [2,3].…”

“…Secondary antinuclei are expected to be produced via the decay or annihilation of dark matter particles in space [1]. After the propagation through space towards the earth, these particles could be observed as an excess of the light antinuclei flux above the expectations from interactions of primary cosmic rays (CR) and the interstellar medium (ISM) at low kinetic energies by satellite-born or balloon experiments.…”

“…Thus annihilation would be limited to their surface, allowing survival The error bands illustrate uncertainties in the coalescence momentum (see text for details), but also include propagation uncertainties. This figure is taken from [1].…”