On the Antimatter Signatures of the Cosmological Dark Matter Subhalos

While the PAMELA collaboration has recently confirmed the cosmic ray positron excess, it is interesting to review the effects of dark matter (DM) subhalos on the predicted antimatter signals. We recall that, according to general subhalo properties as inferred from theoretical cosmology, and for DM with constant annihilation cross section, the enhancement cannot be & 20 for the antimatter yield. This bound is obviously different from that found for γ-rays. We also recall some predictions for supersymmetric benchmark models observable at the LHC and derived in the cosmological N-body framework, showing in the meantime the existing discrepancy between profiles derived from N-body experiments and the current observations of the Milky Way. The positron (e) cosmic ray (CR) fraction has recently drawn considerable attention due to the confirmation of a e excess reported by the PAMELA collaboration up to 270 GeV (Adriani et al., 2008). While the amplitude of this excess is unclear according to the most recent predictions of the secondary e flux at the Earth (Delahaye et al., 2008, where we have emphasized the role of the e’s), dark matter (DM) annihilation has been again proposed as a source of such es, as it was already the case with the less clear HEAT excess ten years ago (Barwick et al., 1997; Baltz & Edsjö, 1998). However, this primary contribution usually needs to be amplified to fit the data, and most of the authors have invoked some ad hoc boost factor coming from the presence of DM substructures in the Milky Way (MW), which should indeed increase the average annihilation rate. Such subhalos are expected in the frame of hierarchical structure formation, and well resolved down to ∼ 10M⊙ in cosmological N-body simulations as performed in the Λ-CDM scheme. Here we shortly review the effects of DM clumps on the antimatter yield, focusing on es and antiprotons (ps). We will not discuss the PAMELA e fraction, since it is worth waiting for the release of the e spectra themselves. The results and figures that we present here have been derived in (Lavalle et al., 2007, 2008b,a), to which we refer the 1 Dipartimento di Fisica Teorica, Università di Torino & INFN, Via Giuria 1, 10125 Torino – Italia c © EDP Sciences 2008 DOI: (will be inserted later) 2 Title : will be set by the publisher reader for more details and references. Relevant scales for es and ps. As the DM distribution is usually found to exhibit a steep spatial dependence in Nbody simulations, the knowledge of the typical CR propagation scale is useful: it characterizes the spatial extension over which the observed signal at the Earth is integrated, and depends on both the propagation model and the CR species. In the GeV-TeV energy range, CRs diffuse on the Galactic magnetic turbulences, which ensure their confinement in the MW in a region which can be extended by a few kpcs above and below the Galactic disk. They interact through various processes with the interstellar medium (ISM) and/or the interstellar radiation field (ISRF). When propagating, ps mostly experience spallation in the disk, where the ISM is located, and convection outward the disk. These processes are efficient at low energies . 5 GeV, while energy losses are almost irrelevant; reacceleration can also be neglected. es obey to the same spatial diffusion as ps, but lose very quickly their energy, mostly through scattering on the ISRF. Their propagation is thus mainly set by the synchrotron and inverse Compton energy losses, of typical timescale τ ∼ 300 Myr, which occur in the whole diffusion zone. Other processes can be neglected without loss of accuracy above a few GeV. This allows to infer the characteristic propagation lengths for es and ps: λp̄ = K(E) Vc and λe+ = 4K0τ 1−δ {