  Detection of gamma rays from dark matter annihilation in the galactic center
is one of the feasible techniques to search for dark matter. We evaluate the
gamma ray flux in the case that the dark matter has an electroweak SU(2)_L
charge. Such dark matter is realized in the minimal supersymmetric standard
model (MSSM) when the lightest SUSY particle is the Higgsino- or Wino-like
neutralino. When the dark matter is heavy compared to the weak gauge bosons,
the leading-order calculation of the annihilation cross sections in
perturbation breaks down due to a threshold singularity. We take into account
non-perturbative effects by using the non-relativistic effective theory for the
two-body states of the dark matter and its SU(2)_L partner(s), and evaluate
precise cross sections relevant to the gamma ray fluxes. We find that the
annihilation cross sections may be enhanced by several orders of magnitude due
to resonances when the dark matter mass is larger than 1 TeV. Furthermore, the
annihilation cross sections in the MSSM may be changed by factors even when the
mass is about 500 GeV. We also discuss sensitivities to gamma ray signals from
the galactic center in the GLAST satellite detector and the large Air Cerenkov
Telescope arrays.
