Strongly-interacting massive particle and dark photon in the era of
  intensity frontier
  A strongly interacting massive particle (SIMP) is an interesting candidate
for dark matter (DM) because its self-interaction cross section can be
naturally strong enough to address the astrophysical problem of small-scale
structure formation. A simple model was proposed by assuming a monopole
condensation, where composite SIMP comes from a "strongly interacting"
U(1)$_{\rm d}$ gauge theory. In the original model, the DM relic abundance is
determined by the $3\to2$ annihilation process via the Wess-Zumino-Witten term.
In this letter, we discuss that the DM relic abundance is naturally determined
also by a semi-annihilation process via a kinetic mixing between the
hypercharge gauge boson and the dark U(1)$_{\rm d}$ gauge boson (dark photon).
The dark photon can be discovered by LDMX-style missing momentum experiments in
the near future.
