Abstract
Cosmic-ray antiprotons represent an important channel for dark matter indirect-detection studies. Current measurements of the antiproton flux at the top of the atmosphere and theoretical determinations of the secondary antiproton production in the Galaxy are in good agreement, with no manifest deviation which could point to an exotic contribution in this channel. Therefore, antiprotons can be used as a powerful tool for constraining particle dark matter properties. By using the spectrum of PAMELA data from 50 MV to 180 GV in rigidity, we derive bounds on the dark matter annihilation cross section (or decay rate, for decaying dark matter) for the whole spectrum of dark matter annihilation (decay) channels and under different hypotheses of cosmic-rays transport in the Galaxy and in the heliosphere. For typical models of galactic propagation, the constraints are strong, setting a lower bound on the dark matter mass of a ``thermal" relic at about 40–80 GeV for hadronic annihilation channels. These bounds are enhanced to about 150 GeV on the dark matter mass, when large cosmic-rays confinement volumes in the Galaxy are considered, and are reduced to 3–4 GeV for annihilation to light quarks (no bound for heavy-quark production) when the confinement volume is small. Bounds for dark matter lighter than few tens of GeV are due to the low energy part of the PAMELA spectrum, an energy region where solar modulation is relevant: to this aim, we have implemented a detailed solution of the transport equation in the heliosphere, which allowed us not only to extend bounds to light dark matter, but also to determine the uncertainty on the constraints arising from solar modulation modelling. Finally, we estimate the impact of soon-to-come AMS-02 data on the antiproton constraints.
Highlights
Study in detail the way in which a charge dependent solar modulation can affect the antiproton fluxes and the ensuing bounds
Bounds for dark matter lighter than few tens of GeV are due to the low energy part of the PAMELA spectrum, an energy region where solar modulation is relevant: to this aim, we have implemented a detailed solution of the transport equation in the heliosphere, which allowed us to extend bounds to light dark matter, and to determine the uncertainty on the constraints arising from solar modulation modeling
Each set of curves show the current PAMELA bound or the projected AMS02 sensitivity, under three different assumptions on the size of the theoretical uncertainties on the secondary antiproton production: solid, dashed and dot-dashed lines refer to 40%, 20% and 5%, respectively
Summary
Antiprotons can be produced in the Galaxy through two main mechanisms: a primary flux is produced by DM in pair annihilation or decay events, while a secondary flux, which represent the astrophysical background, is produced by the spallation of cosmic rays on the nuclei that populate the interstellar medium (ISM). Σannv is the thermally averaged annihilation cross section, Γdec is the DM decay rate (Γdec = 1/τ with τ the DM lifetime), ρ(r, z) is the DM density profile (in our analysis we will use the profiles listed in Table 1 and we adopt a local DM density of 0.39 GeV cm−3). For the secondary antiproton flux the source term takes into account the hadronic interactions of primary cosmic rays on the ISM: ISM CRs ∞.
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