Stimulated Emission of Dark Matter via Thermal Scattering: Novel Limits for Freeze-In and eV Cold Dark Matter

Recently, one of the present authors noticed a stimulated emission process of bosonic dark matter via the two-body decay of a mother particle in a thermal plasma similar to the operation principle of a laser in 2301.08735. In this paper, we show that in a $2 \to 2$ process, including a bosonic final particle (e.g., an axion or dark photon), the stimulated emission occurs as well due to a small angle scattering of the thermal mother particles and thus the phenomenon is more universal. Two important conclusions follow: (1) Care must be taken when studying the freeze-in production of a bosonic dark matter, as the abundance and momentum distribution of dark matter can differ significantly due to this effect. (2) eV-mass-range bosonic dark matter is special and theoretically well-motivated because models for freeze-in or other thermal production of dark matter include the parameter region of cold eV dark matter. We also study the dark matter mass effect for the stimulated emission.