Bremsstrahlung and Gamma Ray Lines in 3 Scenarios of Dark Matter
  Annihilation
  Gamma ray spectral features are of interest for indirect searches of dark
matter (DM). Following Barger et al, we consider 3 simple scenarios of DM that
annihilates into Standard Model (SM) fermion pairs. Scenario 1 is a Majorana DM
candidate coupled to a charged scalar, scenario 2 is a Majorana DM coupled to a
charged gauge boson and scenario 3 is a real scalar DM coupled a charged
vector-like fermion. As shown by Barger et al, these 3 scenarios share
precisely the same internal Bremsstrahlung spectral signature into gamma rays.
Their phenomenology is however distinct. In particular for annihilation into
light SM fermions, in the chiral limit, the 2-body annihilation cross section
is p-wave suppressed for the Majorana candidates while it is d-wave suppressed
for the real scalar. In the present work we study the annihilation into 2
gammas, showing that these three scenarios have distinct, and so potentially
distinguishable, spectral signatures into gamma rays. In the case of the real
scalar candidate we provide a new calculation of the amplitude for annihilation
into 2 gammas.
