Constraints on decaying dark matter from the extragalactic gamma-ray
  background
  If dark matter is unstable and the mass is within GeV-TeV regime, its decays
produce high-energy photons that give contribution to the extragalactic
gamma-ray background (EGRB). We constrain dark matter decay by analyzing the
50-month EGRB data measured with Fermi satellite, for different decay channels
motivated with several supersymmetric scenarios featuring R-parity violation.
We adopt the latest astrophysical models for various source classes such as
active galactic nuclei and star-forming galaxies, and take associated
uncertainties properly into account. The lower limits for the lifetime are very
stringent for a wide range of dark matter mass, excluding the lifetime shorter
than 10^28 s for mass between a few hundred GeV and ~1TeV, e.g., for b\bar{b}
decay channel. Furthermore, most dark matter models that explain the anomalous
positron excess are also excluded. These constraints are robust, being little
dependent on astrophysical uncertainties, unlike other probes such as Galactic
positrons or anti-protons.
