  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 25 Milky Way dwarf spheroidal satellite galaxies based on 4
years of Fermi Large Area Telescope (LAT) data. None of the dwarf galaxies are
significantly detected in gamma rays, and we present gamma-ray flux upper
limits between 500 MeV and 500 GeV. We determine the dark matter content of 18
dwarf spheroidal galaxies from stellar kinematic data and combine LAT
observations of 15 dwarf galaxies to constrain the dark matter annihilation
cross section. We set some of the tightest constraints to date on the the
annihilation of dark matter particles with masses between 2 GeV and 10 TeV into
prototypical Standard Model channels. We find these results to be robust
against systematic uncertainties in the LAT instrument performance, diffuse
gamma-ray background modeling, and assumed dark matter density profile.
