Dark Matter interpretations of the excesses after FERMI

Abstract

The cosmic-ray excess observed by PAMELA in the positron fraction and by FERMI and HESS in e − + e + can be interpreted in terms of DM annihilations or decays into leptonic final states. Final states into τ 's or 4µ give the best fit to the excess. However, in the annihilation scenario, they are incompatible with photon and neutrino constraints, unless DM has a quasi-constant density profile. Final states involving e's are less constrained but poorly fit the excess, unless hidden sector radiation makes their energy spectrum smoother, allowing a fit to all the data with a combination of leptonic modes. In general, DM lighter than about a TeV cannot fit the excesses, so PAMELA should find a greater positron fraction at higher energies.The DM interpretation can be tested by FERMI γ observations above 10 GeV: if the e ± excess is everywhere in the DM halo, inverse Compton scattering on ambient light produces a well-predicted γ excess that FERMI should soon detect.

Diffuse gamma ray constraints on annihilating or decaying Dark Matter after Fermi

“…[57,12,13]; for related studies with current data see [58,67]). These signals have a weaker dependence on the extrapolations of the profile in the inner region of the Galaxy than most previously cited constraints.…”

“…It is worth stressing that, even before such new Fermi data, the annihilating DM interpretation of the leptonic CR spectra was challenged by the constraints from gamma rays and radio waves from the galactic center [46,47,48], from the integrated cosmological flux of ICS photons [49,50,51], from neutrinos from the galactic center [52,13] and from Big Bang Nucleosynthesis [53]. Among the most robust constraints are those from CMB observations and the reionization history of the Universe [54,55,14,51,56], which rule out (if barely) the best fit regions of PAMELA+Fermi+HESS.…”

“…1 We compute the IC gamma ray spectrum as discussed in detail in [12], adopting in particular the approximation of neglecting the diffusion processes of the e ± injected by Dark Matter. While this can have an impact on small internal galactic regions (where the DM density has the highest gradient), this is instead a very good approximation for large observational regions away from the GC (amounting to a correction of less than a factor of 2 on the amplitude of the flux already for intermediate latitudes [12,13]). The total spectrum dN/d is strictly independent of direction only for the prompt component, while for the IC it carries a dependence on the characteristics of the background light on which the IC scattering occurs.…”

“…As mentioned in the Introduction, the anomalous PAMELA, Fermi and HESS data have been interpreted in terms of DM annihilations [8,13] or decay [32]. We recall here briefly the main features of the experimental data and of their DM interpretations, without entering in the details of any specific particle physics model.…”

Decaying into the hidden sector

“…A DM explanation of the electronic CR excess requires a DM mass greater than a TeV [6,7], and predominantly leptonic production [8]. Consequently, the vanilla MSSM WIMP scenario is disfavored, and many new models have been proposed, bifurcating into annihilating models [9,10] and decaying models [11,12,13,14,15].…”