  The dwarf spheroidal satellite galaxies of the Milky Way are some of the most
dark-matter-dominated objects known. Due to their proximity, high dark matter
content, and lack of astrophysical backgrounds, dwarf spheroidal galaxies are
widely considered to be among the most promising targets for the indirect
detection of dark matter via gamma rays. Here we report on gamma-ray
observations of Milky Way dwarf spheroidal satellite galaxies based on 6 years
of Fermi Large Area Telescope data processed with the new Pass 8 reconstruction
and event-level analysis. None of the dwarf galaxies are significantly detected
in gamma rays, and we present upper limits on the dark matter annihilation
cross section from a combined analysis of the 15 most promising dwarf galaxies.
The constraints derived are among the strongest to date using gamma rays and
lie below the canonical thermal relic cross section for WIMPs of mass $\lesssim
100~GeV$ annihilating via the $b \bar b$ and $\tau^{+}\tau^{-}$ channels.
