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.

Keywords: baryon asymmetry; cosmic ray experiments; cosmic ray theory; dark matter experiments.

Figure 1.

The BESS-Polar II experiment consists of inner drift chambers and a jet-type drift tracking chamber, surrounded by a solenoid magnet, aerogel Cherenkov counter, and time-of-flight system. Reprinted with permission from [53].

Figure 2.

AMS-02 is composed of a solenoid magnet and a series of detectors: the transition radiation detector, silicon tracker, anticoincidence counters, ring-imaging Cherenkov detector, electromagnetic calorimeter, and time-of-flight system. Reprinted with permission from [55].

Figure 3.

The GAPS detection scheme relies on a tracker, which is composed of ten layers of Si(Li) strip detectors (only two layers shown), surrounded by a plastic scintillator time-of-flight system.

Figure 4.

The regions of DM parameter space favored (within 2σ) by the AMS-02 antiproton spectrum (green closed) and the Galactic Center gamma-ray excess (red closed) for the case of annihilations into $b\overline{b}$, in the plane of velocity averaged annihilation cross section 〈σv〉 and DM mass m_DM [29].

Figure 5.

The regions of DM parameter space favored by recent analyses of AMS-02 antiproton data for the case of annihilations into $b\overline{b}$, in the plane of velocity averaged annihilation cross section 〈σv〉 and DM mass m_DM. The contours of Cuoco, et al. 2019 [28] (red) and Cholis, et al. 2019 [29] (green) refer to the 2σ best-fit region in the frequentist interpretation, while the contour of Cui, et al. 2018 [70] (blue) corresponds to a 95% C.L. in Bayesian interpretation. The exclusion limits from four recent analysis are also shown [27, 29, 70, 72]. We note that the analysis of Reinert, et al. 2017 [27] does not find a significant DM signal, but the derived limit weakens at m_DM ≈ 80 GeV such that all indicated DM signal regions lie partly below this limit. For a thermal WIMP one would expect a annihilation cross section at the order of 〈σv〉 ≈ 3 × 10⁻²⁶ cm³/s (black dashed line). The signal regions and limits have been rescaled to a local DM density of ρ₀ = 0.3 GeV/cm³. Note that the different analyses use slightly different values for the halo half-height which affects the value of 〈σv〉.

Figure 6.

Antihelium-3 flux predictions from different DM model and astrophysical background calculations [38, 41, 48, 51, 52, 92, 93]. The error bands illustrate uncertainties in the coalescence momentum, but also include propagation uncertainties.

Figure 7.

Antiproton flux data from AMS-02 [8], BESS-Polar I/II [4, 107], and PAMELA [6], as well as projections for the GAPS [91] antiproton flux measurements after 40 days, in comparison with the GALPROP plain diffusion prediction [108]. Also shown are the predicted antideuteron flux corresponding to DM parameters indicated by AMS-02 antiproton signal, interpreted as annihilation into purely $b\overline{b}$ [38, 100]), as well as the predicted secondary and tertiary astrophysical antideuteron flux. The anticipated sensitivity of GAPS [57] for a 3 σ discovery and the BESS 97–00 95% C.L. exclusion limits are indicated [54]. Solar modulation is treated in the force-field approximation with a potential of 500 MV. All antideuteron fluxes are derived in the analytic coalescence model with a coalescence momentum of 160 GeV [101] for the lower edge of the band and with a higher coalescence momentum of 248 GeV [102] for the upper edge of the band.

Figure 8.

The predicted antideuteron flux corresponding to the observed AMS-02 antiproton excess, interpreted as DM annihilation into purely $b\overline{b}$, two gluons, two Z-bosons, two Higgs-bosons, or $t\overline{t}$, divided by the expected GAPS sensitivity [57]. A clear antideuteron signal is also expected in GAPS for all annihilation channels. For the Z-boson decay, one of the two bosons might be produced off-shell, which is denoted with a star superscript. The shaded regions correspond to the 2σ on the best-fit DM mass and annihilation cross section from antiproton data [100]. Figure from [38].

Figure 9.

Antiproton production cross section in proton-proton interactions [24].

Figure 10.

$p_0^{\prime }$ for antideuterons as function of kinetic energy of the incoming proton for two different hadronic generators. The solid lines show empiric fits to the best-fit $p_0^{\prime }$ values for the corresponding generator. The dashed red lines indicate the one-sigma uncertainty range for EPOS-LHC [121].

Figure 11.

Top panel: antiproton flux for solar minimum and maximum from a 3D numerical model for cosmic-ray propagation through the Heliosphere. The solar minimum and solar maximum spectra has been evaluated for positive and negative polarity of the HMF (A > 0 and A < 0) to show the effect of the charge sign solar modulation. Middle panel: ratio between the LIS and the modulated spectra for the period of maximum and minimum solar activity (A < 0). Bottom panel: ratio between the modulated spectra for opposite polarity of the HMF for the period of minimum and maximum solar activity.

Figure 12.

The GRAMS detector consists of a segmented liquid-argon time projection chamber surrounded by plastic scintillators.

Figure 13.

The ADHD instrument is made of a 400 bar helium-gas calorimeter (HeCal) surrounded by a time-of-flight system with three scintillator layers. Antideuterons are detected as a single in-going prompt track (yellow points) followed by several out-going delayed pions (light-red points).

Figure 14.

The AMS-100 instrument consists of a central calorimeter surrounded by a silicon-strip and scintillating-fiber tracker, all within a high-temperature superconducting solenoid magnet. Figure from [204].