Green Bank Telescope Constraints on Dark Matter Annihilation in Segue I
  We use a non-detection in $\nu = 1.4\,$GHz Green Bank Telescope observations
of the ultra-faint dwarf spheroidal galaxy Segue I, which could be immersed in
a non-negligible halo magnetic field of the Milky Way, to place bounds on
particle dark matter properties. We model the galaxy using an Einasto dark
matter profile, and compute the expected synchrotron flux from dark matter
annihilation as a function of the magnetic field strength $B$, diffusion
coefficient $D_0$, and particle mass $m_\chi$ for different annihilation
channels. The data strongly disfavor annihilations to $e^+e^-$ for $m_\chi
\lesssim 50\,$GeV, but are not sensitive to the $b \bar b$ channel. Adopting a
fiducial $B \sim 2\,\mu$G inferred from Segue I's proximity to the Milky Way,
our models of annihilation to $\tau^+\tau^-$ with $m_\chi = 30\,$GeV require an
intermediate value of $D_0$ for consistency with the data. The most compelling
limits are obtained for WIMP annihilation to $\mu^+\mu^-$: we exclude $m_\chi
\lesssim 30\,$GeV$\,\rightarrow\mu^+\mu^-$ at 95% confidence, unless $D_0$
exceeds the Milky Way value or $B$ is significantly smaller than we have
assumed.
