Antihelium from dark matter

Cosmic-ray antinuclei provide a promising discovery channel for the indirect detection of particle dark matter. Hadron showers produced by the pair annihilation or decay of Galactic dark matter generate antinucleons which can in turn form light antinuclei. Previous studies have only focused on the spectrum and flux of low energy antideuterons which, although very rarely, are occasionally also produced by cosmic-ray spallation. Heavier elements ($A\ge 3$) have instead entirely negligible astrophysical background and a primary yield from dark matter which could be detectable by future experiments. Using a Monte Carlo event generator and an event-by-event phase space analysis, we compute, for the first time, the production spectrum of ${}^3\overline{\mathrm{He}}$ and ${}^3\overline{\mathrm{H}}$ for dark matter annihilating or decaying to $b\overline{b}$ and $W^{+}{W}^{-}$ final states. We then employ a semianalytic model of interstellar and heliospheric propagation to calculate the ${}^3\overline{\mathrm{He}}$ flux as well as to provide tools to relate the antihelium spectrum corresponding to an arbitrary antideuteron spectrum. Finally, we discuss prospects for current and future experiments, including GAPS and AMS-02.

Figure 1

Ratios of the production of (${}^3\overline{\mathrm{He}}+{}^3\overline{\mathrm{H}}$) to $\overline{\mathrm{D}}$ for Majorana dark matter annihilating to $b\overline{b}$ (left column) and $W^{+}{W}^{-}$ (right column) final states integrated over the energy bands for the proposed GAPS (LDB) instrument. For each species, these bands were shifted for solar modulation according to a 500 MV Fisk potential. The solid blue vertical lines show nominal values for $p_0^{A=2}$ with uncertainties (vertical shaded) while the horizontal lines show the $A=3$ coalescence momentum extrapolated using the nuclear binding energy (blue dashed) and heavy-ion data (black dot dashed). White regions with no contours contained no Monte Carlo events.

Figure 2

Propagation ratios $R(T)=P_{\text{num}}^{\overline{\mathrm{He}}}/P_{\text{num}}^{\overline{\mathrm{D}}}$ for [$P_{\text{num}}$ in Eq. (6)] which show the enhancement or suppression of the antihelium flux with respect to antideuterons. MIN/MED/MAX interstellar propagation models are shown in blue/red/green for two values of the propagation cross section: Annihilation only (solid lines) and total inelastic (dashed lines). Also shown is the ratio of solar propagation functions in the force field approximation (black solid).

Figure 3

Proton-antinuclei inelastic scattering cross sections as parametrized in Ref. [28]. The nonannihilating inelastic cross section for antideuterons is taken from Ref. [7]. Shown are the total inelastic (black), nonannihilating inelastic (red), and annihilation cross sections for antihelium (solid) and antideuterons (dashed).

Figure 4

Flux of ${}^3\overline{\mathrm{He}}$ at the top of the atmosphere produced by dark matter annihilating to $W^{+}{W}^{-}$ (top) and $b\overline{b}$ (bottom) final states assuming an NFW dark matter density profile. Flux is multiplied by 100 for $W^{+}{W}^{-}$ with $m_{\chi }=1$, 2 TeV. The shaded vertical bands represent the energy bands and proposed sensitivities for various GAPS and AMS-02 observations. Shaded uncertainty bands represent the MIN/MAX interstellar propagation models, although these are reduced to within a factor 4 of the central value after applying constraints from PAMELA measurements of the $\overline{p}$ spectrum. Nuclear physics uncertainties are not shown.