Dark matter freeze-out during matter domination

Abstract

We highlight the general scenario of dark matter freeze-out whilst the energy density of the universe is dominated by a decoupled non-relativistic species. Decoupling during matter domination changes the freeze-out dynamics, since the Hubble rate is parametrically different for matter and radiation domination. Furthermore, for successful Big Bang Nucleosynthesis the state dominating the early universe energy density must decay, this dilutes (or repopulates) the dark matter. As a result, the masses and couplings required to reproduce the observed dark matter relic density can differ significantly from radiation dominated freeze-out.It is accepted that during Big Bang Nucleosynthesis (BBN) the universe was dominated by radiation, and thus a common assumption is that the universe was radiation dominated from the point of inflationary reheating until the period of BBN. Because of this, models of dark matter (DM) freeze-out typically assume that DM decoupling occurs during radiation domination. However, nothing forbids an early period of matter domination provided that radiation domination is restored prior to BBN. Indeed, any long-lived particle species evolving as matter in the early universe will eventually dominate the energy of the universe, and moduli from string theory and extra-dimensional models (Banks-Kaplan-Nelson 1993, Carlos et al 1993) are good candidates for matterlike (non-relativistic decoupled) states that commonly lead to matter domination in the early universe.We consider here a scenario in which the early universe is formed of a thermal bath, comprised of the visible sector states and DM, X, along with a decoupled state φ. Immediately after inflationary reheating all of these states are assumed to be radiation-like, then at some critical temperature T the energy density of φ begins to evolve as matter. If φ is sufficiently long-lived, this leads to an early period of matter domination. Notably, if the DM decouples during matter domination this impacts DM freeze-out, since the expansion rate H is different for matter domination (H ∝ T 3 /2 ) and radiation domination (H ∝ T 2 ).In this work we explore the possibility that DM decoupled whilst the expansion rate scaled as H ∝ T 3 /2 when the universe was dominated by a matter-like field φ. To recover radiation domination at BBN φ must decay and if φ only decays to Standard Model states (as we will assume), this leads to a dilution of the DM freezeout abundance. We highlight that the freeze-out temperature and abundance are distinct from the radiation dominated case, and reproducing the observed DM relic density prefers different DM masses and cross sections.This scenario is similar in spirit to Chung-Kolb-Riotto (1998), Gelmini & Gondolo (2006), Giudice-Kolb-Riotto (2000) which studied DM freeze-out during inflationary reheating, in which case H ∝ T 4 . However, our work dif-fers from these in several ways, most significantly, these other works consider the case in which the radiation bath is initially negligible, whereas here we consider states dec...

Reviving Z and Higgs mediated dark matter models in matter dominated freeze-out

“…The prospect that DM freeze-out may have occurred during a period of matter domination was highlighted in [17], a possibility observed in passing in [34]. For clarity, we note here that some papers (e.g.…”

“…In this section we briefly recap the model independent analysis of matter dominated freezeout which appeared in the short paper [17], using this opportunity to provide additional relevant details. In particular, we expand on [17] by discussing distinctions with the case in which DM freezes-out whilst φ, the matter-like state which dominates the energy density in the early universe, is decaying. This scenario in which DM freeze-out occurs whilst φ decays can no longer be neglected is analogous to the case studied in [1][2][3][4].…”

“…It is common to define the DM yield Y ≡ n χ /s where s is the entropy density s = 2π 2 45 g * S T 3 defined in terms of the bath temperature and the effective entropic degrees of freedom g * S . One can calculate the yield after freeze-out Y (x f ) assuming it occurs during matter domination and for n = 0 (s-wave) one can evaluate this in the general r case [17] Y n=0 (x f ) 45 2π 2…”

“…In [17] matter dominated DM freeze-out was studied in a model independent manner and viable parameter ranges were broadly identified. It was observed that the decay of the matter-like field which leads to the early period of matter domination can dilute the DM, thus allowing for smaller annihilation cross-section or much heavier DM.…”

“…In the interim between the initial letter[17] and thesis of SH[43] and the completion of this work, two articles appeared[20,21] which study aspects of matter dominated freeze-out via the scalar Higgs portal.…”