Stimulated emission of dark matter via thermal scattering: novel limits for freeze-in and eV cold dark matter

A bstract 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 [1]. In this paper, we show that in a 2 → 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. We also study the dark matter mass effect for the stimulated emission.