Theory Needs for Gamma-Ray Science in the MeV Gap

Gamma-rays in the MeV gap are a crucial probe of both astrophysical phenomena and fundamental physics. But many of these studies require advances in both astrophysical simulations and fundamental physics models. For example, gamma-rays produced through the decay of radioactive isotopes produced in astrophysical transients provide direct insight into the nature of the explosion mechanisms behind these transients. Gamma-ray line features (typically in the 0.1-10MeV range) provide a direct probe into the yields from these explosions. Doppler measurments of these lines further probe the asymmetries in the explosion. In addition, with detailed astrophysical models, astronomers can use these observations to probe the fundamental physics underpinning these explosions. To fully utilize this probe, we must combine a broad suite of modeling, experimental and theoretical studies from nuclear physics studies at expermental facilities to detailed models of stellar progenitors of supernovae. The MeV gap range provides insight into a broad range of astrophysical phenomena (e.g. supernovae, gamma-ray bursts, blazars) to fundamental physics (e.g. photon splitting, gravitational waves, dense nuclear matter and neutrino physics). Here we review the science behind many of these studies and the theory requirements to truly address these problems.