Gamma-rays from Dark Showers with Twin Higgs Models
  We consider a twin WIMP scenario whose twin sector contains a full dark copy
of the SM hadrons, where the lightest twin particles are twin pions. By analogy
to the standard WIMP paradigm, the dark matter (DM) freezes out through twin
electroweak interactions, and annihilates into a dark shower of light twin
hadrons. These are either stable or decay predominantly to standard model (SM)
photons. We show that this 'hadrosymmetric' scenario can be consistent with all
applicable astrophysical, cosmological and collider constraints. In order to
decay the twin hadrons before the big-bang nucleosynthesis epoch, an additional
portal between the SM and twin sector is required. In most cases we find this
additional mediator is within reach of either the LHC or future intensity
frontier experiments. Furthermore, we conduct simulations of the dark shower
and consequent photon spectra. We find that fits of these spectra to the
claimed galactic center gamma-ray excess seen by Fermi-LAT non-trivially
coincide with regions of parameter space that both successfully generate the
observed DM abundance and exhibit minimal fine-tuning.
