Viability of detection by AMS of sudden features due to dark matter
  annihilation to positrons and electrons
  The Fermi experiment has measured the cosmic ray electron+positron spectrum
and positron fraction [$\Phi_{e^+}/(\Phi_{e^+ + e^-})]$, and PAMELA has
measured the positron fraction with better precision. While the majority of
cosmic ray electrons and positrons are of astrophysical origin, there may also
be a contribution from dark matter annihilation in the galactic halo. The
upcoming results of the AMS experiment will show measurements of these
quantities with far greater precision. One dark matter annihilation scenario is
where two dark matter particles annihilate directly to $e^+$ and $e^-$ final
states. In this article, we calculate the signature "bumps" in these
measurements assuming a given density profile (NFW profile). If the dark matter
annihilates to electrons and positrons with a cross section $\sigma v \sim\,
10^{-26}$ cm$^3$/s or greater, this feature may be discernible by AMS. However,
we demonstrate that such a prominent spectral feature is already ruled out by
the relative smoothness of the positron + electron cosmic ray spectrum as
measured by Fermi. Hence we conclude that such a feature is undetectable unless
the mass is less than $\sim$40 GeV.
