Enhancement of Majorana Dark Matter Annihilation Through Higgs
  Bremsstrahlung
  For Majorana dark matter, gauge boson bremsstrahlung plays an important role
in enhancing an otherwise helicity-suppressed s-wave annihilation
cross-section. This is well known for processes involving a radiated photon or
gluon together with a Standard Model fermion-antifermion pair, and the case of
massive electroweak gauge bosons has also recently been studied. Here we show
that internal Higgs bremsstrahlung also lifts helicity suppression and could be
the dominant contribution to the annihilation rate in the late Universe for
dark matter masses below ~1 TeV. Using a toy model of leptophilic dark matter,
we calculate the annihilation cross-section into a lepton-antilepton pair with
a Higgs boson and investigate the energy spectra of the final stable particles
at the annihilation point.
