MicrOMEGAs5.0 : Freeze-in

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We present a major upgrade of the micrOMEGAs dark matter code to compute the abundance of feebly interacting dark matter candidates through the freeze-in mechanism in generic extensions of the Standard Model of particle physics. We develop the necessary formalism in order to solve the freeze-in Boltzmann equations while making as few simplifying assumptions as possible concerning the phase-space distributions of the particles involved in the dark matter production process. We further show that this formalism allows us to treat different freeze-in scenarios and discuss the way it is implemented in the code. We find that, depending on the New Physics scenario under consideration, the effect of a proper treatment of statistics on the predicted dark matter abundance can range from a few percent up to a factor of two, or more. We moreover illustrate the underlying physics, as well as the various novel functionalities of micrOMEGAs, by presenting several example results obtained for different dark matter models. Program summaryProgram title: micrOMEGAs5.Program Files doi:http://dx.doi.org/10.17632/4ck6jf5vxf.2Licensing provisions: GNU General Public License 3 (GPL)Programming language: C and FortranJournal reference of previous version: Comput. Phys. Commun. 222 (2018) 327.Does the new version supersede the previous version?: YesReasons for the new version: Previous versions of micrOMEGAs worked within the assumption that dark matter is in thermal equilibrium with the standard model particles in the early Universe. For several classes of dark matter models this condition is not fulfilled. This new version allows to treat such cases, in particular the one where dark matter is composed of feebly interacting massive particles (FIMPs) that obtain their relic density via the freeze-in mechanism.Summary of revisions: This version includes new routines to compute the abundance of feebly interacting dark matter candidates through the freeze-in mechanism in generic extensions of the Standard Model of particle physics. A proper treatment of the phase-space distribution functions for bosons and fermions is included. The user must specify which particles are to be considered as FIMPs as well as the reheating temperature, that is the temperature at which dark matter formation starts. The relic density of the (next-to) lightest dark sector particle can also be computed via the freeze-out mechanism. This version includes three new sample models in which dark matter production proceeds through freeze-in.Nature of problem: Dark matter candidates that satisfy cosmological constraints cover a wide range of masses and interaction strength. One reason for dark matter particles to have escaped all direct, indirect and collider searches so far could be that they are feebly interacting. We provide the first public code to perform a precise computation of the relic density of FIMPs in generic extensions of the standard model in order to check agreement with the value of the relic density extracted from cosmological observations.Solution method: We solve the freeze-in Boltzmann equations while making as few simplifying assumptions as possible concerning the phase-space distribution of the particles involved in the dark matter production process. We include the case where dark matter is produced through the two-body decay of a particle whether or not it is in thermal equilibrium with the thermal bath as well as the one where dark matter is produced through 2→2 annihilations of a pair of bath particles. Two numerical methods are provided — one where the collision term in the Boltzmann equation is integrated directly and a more efficient one where some of the integrals are performed analytically.