New Limits on Dark Matter Annihilation from Alpha Magnetic Spectrometer Cosmic Ray Positron Data

The Alpha Magnetic Spectrometer experiment onboard the International Space Station has recently provided cosmic ray electron and positron data with unprecedented precision in the range from 0.5 to 350 GeV. The observed rise in the positron fraction at energies above 10 GeV remains unexplained, with proposed solutions ranging from local pulsars to TeV-scale dark matter. Here, we make use of this high quality data to place stringent limits on dark matter with masses below $\sim 300\mathrm{GeV}$, annihilating or decaying to leptonic final states, essentially independent of the origin of this rise. We significantly improve on existing constraints, in some cases by up to 2 orders of magnitude.

Figure 1

The $e^{\pm }$ spectrum from annihilating DM, after propagation, for different annihilation final states, assuming $⟨\sigma v⟩=3\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. Solid lines refer to reference diffusion zone ($L=4\mathrm{kpc}$) and energy loss assumptions ($U_{\mathrm{rad}}+U_B=1.7\mathrm{eV}{\mathrm{cm}}^{-3}$). Dashed (dotted) lines show the effect of a different scale height $L=8(2)\mathrm{kpc}$. The dotted-dashed line shows the impact of increasing the local radiation plus magnetic field density to $U_{\mathrm{rad}}+U_B=2.6\mathrm{eV}{\mathrm{cm}}^{-3}$.

Figure 2

The AMS positron fraction measurement [2] and $\mathrm{\text{background}}+\mathrm{\text{signal}}$ fit for DM annihilating directly to $e^{+}{e}^{-}$, for $m_{\chi }=10\mathrm{GeV}$ and 100 GeV. The normalization of the DM signal in each case was chosen such that it is barely excluded at the 95% C.L. For better visibility, the contribution from DM (lower lines) has been rescaled as indicated.

Figure 3

Upper limits (95% C.L.) on the DM annihilation cross section, as derived from the AMS positron fraction, for various final states (this work), WMAP7 (for ${\ell }^{+}{\ell }^{-}$) [44], and Fermi LAT dwarf spheroidals (for ${\mu }^{+}{\mu }^{-}$ and ${\tau }^{+}{\tau }^{-}$) [43]. The dotted portions of the curves are potentially affected by solar modulation. We also indicate $⟨\sigma v{⟩}_{\mathrm{therm}}\equiv 3\times {10}^{-26}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$. The AMS limits are shown for reasonable reference values of the local DM density and energy loss rate (see text), and can vary by a factor of a few, as indicated by the hatched band (for clarity, this band is only shown around the $e^{+}{e}^{-}$ constraint).