Freeze-In Production of FIMP Dark Matter

TL;DR

This paper introduces a new dark matter production mechanism called thermal freeze-in, where FIMPs are produced in the early universe without reaching thermal equilibrium, offering testable predictions and independence from unknown high-energy physics.

Contribution

It presents the calculable thermal freeze-in mechanism for FIMP dark matter, distinct from freeze-out, with implications for experimental detection and cosmological observations.

Findings

Freeze-in yield is IR dominated near the FIMP mass.

Relic abundance depends on measurable particle properties.

Mechanism is independent of reheat temperature after inflation.

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 thermal equilibrium. As with the conventional "thermal freeze-out" production mechanism, the relic abundance reflects a combination of initial thermal distributions together with particle masses and couplings that can be measured in the laboratory or astrophysically. The freeze-in yield is IR dominated by low temperatures near the FIMP mass and is independent of unknown UV physics, such as the reheat temperature after inflation. Moduli and modulinos of string theory compactifications that receive mass from weak-scale supersymmetry breaking provide implementations of the freeze-in mechanism, as do models that employ Dirac neutrino masses or GUT-scale-suppressed interactions. Experimental signals of freeze-in and FIMPs can be spectacular, including the production of new metastable coloured or charged particles at the LHC as well as the alteration of big bang nucleosynthesis.