The galactic antiproton spectrum at high energies: background expectation vs. exotic contributions

TL;DR

The paper analyzes the galactic antiproton spectrum at high energies, separating the standard secondary background produced by cosmic-ray spallation from potential primary contributions from dark matter annihilation. It uses a semi-analytic two-zone diffusion model constrained by low-energy data to quantify background uncertainties, and explores TeV-scale DM benchmarks (LSP Higgsino, LSP Wino, LKP B^(1)) with various halo profiles and boost scenarios. The results show that, while the background is largely fixed up to roughly 100 GeV, DM annihilation could dominate at higher energies under plausible boost factors—especially in IMBH scenarios—providing a path to indirect DM detection but with challenges in model discrimination due to backgrounds. Overall, the work guides interpretation of upcoming PAMELA/AMS-02 data and highlights complementary opportunities with gamma-ray probes and cross-correlations across cosmic-ray species for DM searches.

Abstract

A new generation of upcoming space-based experiments will soon start to probe the spectrum of cosmic ray antiparticles with an unprecedented accuracy and, in particular, will open up a window to energies much higher than those accessible so far. It is thus timely to carefully investigate the expected antiparticle fluxes at high energies. Here, we perform such an analysis for the case of antiprotons. We consider both standard sources as the collision of other cosmic rays with interstellar matter, as well as exotic contributions from dark matter annihilations in the galactic halo. Up to energies well above 100 GeV, we find that the background flux in antiprotons is almost uniquely determined by the existing low-energy data on various cosmic ray species; for even higher energies, however, the uncertainties in the parameters of the underlying propagation model eventually become significant. We also show that if the dark matter is composed of particles with masses at the TeV scale, which is naturally expected in extra-dimensional models as well as in certain parameter regions of supersymmetric models, the annihilation flux can become comparable to - or even dominate - the antiproton background at the high energies considered here.

Figures (7)

• Figure 1: The various contributions to secondary antiprotons from the spallation of the interstellar medium by cosmic rays. Here, we took the 'medium' configuration of propagation parameters from Table \ref{['tab_prop']} and $T_{\bar{p}}^\mathrm{TOA}$ denotes the antiproton kinetic energy as measured at the top of the atmosphere. For reference, we also show the existing low-energy data on the antiproton flux at the top of the atmosphere bess1bess2capriceams98.
• Figure 2: Theoretical uncertainties in the secondary flux in antiprotons, taking into account the whole range of propagation parameters that is allowed by the existing B/C data, again featured together with the existing low-energy data. In the right panel of this figure, we have plotted $T_{\bar{p}}^3\Phi_{\bar{p}}$ in order to better illustrate the expected near $T_{\bar{p}}^{-3}$ scaling of the flux at high energies.
• Figure 3: Boost factor for an average Milky Way halo population of IMBHs as a function of the antiproton kinetic energy. The two panels on the left do not include the contribution from tertiaries, energy losses and diffusive reacceleration. The long dashed lines have been obtained by integrating directly the antiproton Green function $G_{\hbox{$\bar{\rm p}$}}$ over the diffusive halo. In the right panels, tertiaries, energy losses and diffusive reacceleration have been taken into account. An isothermal sphere -- top panels -- as well as an NFW profile -- bottom panels -- have been considered for the DM smooth distribution.
• Figure 4: The primary flux in antiprotons, as compared to the background in secondaries, for the set of benchmark models specified in Table \ref{['tab_models']}. From bottom to top, the different curves correspond to the isothermal sphere, NFW, Moore 04 and Moore 99 profiles, respectively; for the diffusion parameters we adopt the 'medium' configuration of Table \ref{['tab_prop']}.
• Figure 5: Same as Fig. \ref{['fig_prof']}, now for an NFW halo profile; the diffusion parameters are varied from the 'minimal' to the 'maximal' configurations of Table \ref{['tab_prop']}.