Scalar Dark Matter Production from Preheating and Structure Formation Constraints

TL;DR

This paper explores how scalar dark matter can be produced during inflation and reheating through gravitational and direct couplings, analyzing its relic abundance and effects on structure formation to set mass constraints.

Contribution

It provides a comprehensive analysis of scalar dark matter production regimes, including gravitational and non-perturbative effects, and derives new constraints from structure formation data.

Findings

Purely gravitational production overcloses the universe for reheating temperatures above 34 GeV.

Direct coupling suppresses IR modes and alters relic abundance.

Lower bound on scalar DM mass is approximately 3×10^{-4} eV for gravitational and 20 eV for direct coupling production.

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 (non-perturbative). 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 $T_\text{reh}>34 ~\text{GeV}$. 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-$\alpha$ measurements. We find that they correspond to a lower bound on the DM mass of $\gtrsim 3\times 10^{-4} \, \rm{eV}$ for purely gravitational production, and $\gtrsim 20 \, \rm {eV}$ for direct coupling production. We discuss the implications of these results.