Dark initial state radiation and the kinetic mixing portal

Data from Planck measurements of the cosmic microwave background (CMB) place important constraints on models with light dark matter (DM) and mediators especially when both lie in mass range below $\sim 1 $ GeV. In involving kinetic mixing where photon acts as mediator, these are easily satisfied appropriate DM relic density achievable if is, e.g., a complex scalar, $p$-wave annihilation occurs, or is lighter component split pseudo-Dirac state co-annihilation dominates. cases, although higher order gauge coupling, $g_D$, corresponding processes including initial radiation (ISR) will be dominantly $s$-wave essentially temperature independent cross sections. The rates for ISR associated processes, though not yielding sections large enough to contribute density, can still run into possible conflicts bounds arising CMB. this paper we perform preliminary study present potential future that CMB imposes parameter spaces scenarios due existence ISR. Further analyses effects clearly warranted.