Scalar Dark Matter Production from Preheating and Structure Formation Constraints

Abstract

We investigate the out-of-equilibrium production of scalar dark matter (DM) from the inflaton condensate during inflation and reheating. We assume that this scalar couples only to the inflaton via a direct quartic coupling and is minimally coupled to gravity. We consider all possible production regimes: purely gravitational, weak direct coupling (perturbative), and strong direct coupling (nonperturbative). For each regime, we use different approaches to determine the dark matter phase space distribution and the corresponding relic abundance. For the purely gravitational regime, scalar dark matter quanta are copiously excited during inflation resulting in an infrared (IR) dominated distribution function and a relic abundance which overcloses the universe for a reheating temperature T reh > 34 GeV. A non-vanishing direct coupling induces an effective DM mass and suppresses the large IR modes in favor of ultraviolet (UV) modes and a minimal scalar abundance is generated when the interference between the direct and gravitational couplings is maximal. For large direct couplings, backreaction on the inflaton condensate is accounted for by using the Hartree approximation and lattice simulation techniques. Since scalar DM candidates can behave as non-cold dark matter, we estimate the impact of such species on the matter power spectrum and derive the corresponding constraints from the Lyman-α measurements. We find that they correspond to a lower bound on the DM mass of 3 × 10 −4 eV for purely gravitational production, and 20 eV for direct coupling production. We discuss the implications of these results.

Higgs boson induced reheating and ultraviolet frozen-in dark matter

“…Therefore we find it more appropriate to express the whole r.h.s. of (47) in terms of the inflaton density ρ φ adopting (24). Solutions of the Boltzmann equations in both cases approximately coincide during reheating when the "width" is smaller than the Hubble rate.…”

“…Since the DM interactions with the SM and inflaton are assumed to be Planck mass suppressed, therefore vector DM is not in thermal equilibrium with the SM bath. We study DM production through its gravitational interactions with the inflaton background field [17][18][19][20][21][22][23][24] as well as through the annihilation of SM particles, also known as the gravitational freeze-in mechanism [25][26][27][28][29][30]. Moreover, DM production through direct inflaton decay is also investigated.…”

“…Using the continuity equation ( 25) together with the definition (24) and (29) of the scale factor a reads…”

“…In this subsection, we calculate the energy gain E of the SM radiation due to the Higgs pair production at the expense of the inflaton field. The process that is considered is a quantum production from a vacuum state, |0 , to the two-Higgs-boson final state, |hh , in the presence of the classical inflaton field, see also [11,20,24,40,41]. Due to the energy conservation, the energy density gained by the Higgs pair production from the vacuum must be equal to the energy loss of the inflaton field.…”

“…Note that the definition of the envelope function(24) implies that d ρ φ /dt = ρφ assuming periodicity and neglecting cosmological time evolution while averaging over a period.…”