Abstract
We propose an alternate, calculable mechanism of dark matter genesis,"thermal freeze-in," involving a Feebly Interacting Massive Particle (FIMP)interacting so feebly with the thermal bath that it never attains thermalequilibrium. As with the conventional "thermal freeze-out" productionmechanism, the relic abundance reflects a combination of initial thermaldistributions together with particle masses and couplings that can be measuredin the laboratory or astrophysically. The freeze-in yield is IR dominated bylow temperatures near the FIMP mass and is independent of unknown UV physics,such as the reheat temperature after inflation. Moduli and modulinos of stringtheory compactifications that receive mass from weak-scale supersymmetrybreaking provide implementations of the freeze-in mechanism, as do models thatemploy Dirac neutrino masses or GUT-scale-suppressed interactions. Experimentalsignals of freeze-in and FIMPs can be spectacular, including the production ofnew metastable coloured or charged particles at the LHC as well as thealteration of big bang nucleosynthesis.