Smooth interpolation between thermal Born and LPM rates

Abstract: In a weakly coupled ultrarelativistic plasma, $1 + n \leftrightarrow 2 + n$ scatterings, with $n \ge 0$, need sometimes to be summed to all orders, in order to determine a leading-order interaction rate. To implement this "LPM resummation", kinematic approximations are invoked. However, in cosmological settings, where the temperature changes by many orders of magnitude and both small and large momenta may play a role, such approximations are not always justified. We suggest a procedure to smoothly interpolate between LPM-resummed $1 + n \leftrightarrow 2 + n$ and Born-level $1 \leftrightarrow 2$ results, rendering the outcome applicable to a broader range of masses and momenta. The procedure is illustrated for right-handed neutrino production from a Standard Model plasma, and dilepton production from a QCD plasma.