Full calculation of clumpiness boost factors for antimatter cosmic rays in the light of ΛCDM N-body simulation results. Abandoning hope in clumpiness enhancement?

International audienceContext: Anti-proton and positron Galactic cosmic ray spectra are among the key targets for indirect detection of dark matter. The boost factors, corresponding to an enhancement of the signal, and linked to the clumpiness properties of the dark matter distribution, have been taken as high as thousands in the past. The dramatic impact of these boost factors for indirect detection of antiparticles, for instance with the PAMELA satellite or the coming AMS-02 experiment, asks for their detailed calculation. Aims: We take into account the state-of-the-art results of high resolution N-body dark matter simulations to calculate the most likely energy dependent boost factors, which are linked to the cosmic ray propagation properties, for anti-protons and positrons. The results from extreme, but still possible, configurations of the clumpy dark matter component are also discussed. Methods: Starting from the mass and space distributions of sub-halos, the anti-proton and positron propagators are used to calculate the mean value and the variance of the boost factor for the primary fluxes. We take advantage of the statistical method introduced in Lavalle et al. (2007) and cross-check the results with Monte Carlo computations. Results: By spanning some extreme configurations of sub-halo and propagation properties, we find that the average contribution of the clumps is negligible compared to that of the smooth dark matter component. Dark matter clumps do not lead to enhancement of the signals, unless they are taken with some extreme (unexpected) properties. This result is independent of the nature of the self-annihilating dark matter candidate considered, and provides precise estimates of the theoretical and the statistical uncertainties of the antimatter flux from sub-halos. Conclusions: Spectral distortions can still be expected in antimatter flux measurements, but scenarios invoking large and even mild clumpiness boost factors are strongly disfavoured by our analysis. Some very extreme configurations could still lead to large enhancements, e.g. (i) very small clumps with masses ⪉10-6 M&sun; following a M-α mass distribution with α⪆ 2, highly concentrated with internal r-β profiles with β⪆ 1.5, and spatially distributed according to the smooth component; or (ii) a big sub-halo of mass ⪆ 07 M&sun; within a distance of ⪉ 1 kpc from the Earth. However, they are very unlikely from either theoretical or statistical arguments. Appendices A and B are only available in electronic form at http://www.aanda.or