Abstract
The hypothetical $SU(3)$ flavor-singlet dibaryon state $S$ with strangeness $\ensuremath{-}2$ has been discussed as a dark-matter candidate capable of explaining the curious 5-to-1 ratio of the mass density of dark matter to that of baryons. We study the early-universe production of dibaryons and find that irrespective of the hadron abundances produced by the QCD quark/hadron transition, rapid particle reactions thermalized the $S$ abundance, and it tracked equilibrium until it ``froze out'' at a tiny value. For the plausible range of dibaryon masses (1860--1890 MeV) and generous assumptions about its interaction cross sections, $S$'s account for at most ${10}^{\ensuremath{-}11}$ of the baryon number and, thus, cannot be the dark matter. Although it is not the dark matter, if the $S$ exists, it might be an interesting relic.
Highlights
The most striking thing we know about dark matter is that it is nonbaryonic,1 and a new kind of matter. This links the fields of cosmology and particle physics and, today, the nature of dark matter is one of the most pressing problems in each field
There is a 50σ discrepancy between the baryon density, ΩBh2 1⁄4 0.0222Æ 0.0002, and the total matter density, ΩMh2 1⁄4 0.142Æ 0.0013, inferred from BBN and the CMB; see e.g., Ref. [1]. This fact leads to a puzzle: the unchanging ratio between the dark-matter density and the density of baryons is about 5-to-1, order unity, rather than being very small or very large [2]
One dark-matter candidate that did address this issue was quark nuggets [3], where the dark matter was supposed to exist in the form of macroscopic, stable quark states with very large baryon number, which formed in a first-order QCD phase transition
Summary
The most striking thing we know about dark matter is that it is nonbaryonic, and a new kind of matter. (We will use the name S to refer to the compact dibaryon that plausibly evades the stringent constraints on the larger, heavier dibaryon and is proposed as the dark-matter candidate.) We believe that what we know with certainty about the putative S, summarized below, is sufficient to determine its cosmological production: (1) Its mass must be greater than mp þ mn − me − 2BE ≃ 1860 MeV (BE ∼ 8 MeV is the binding energy of a nucleon in a nucleus) to guarantee nuclear stability [6].2 (2) If its mass is greater than 2mp þ2me ≃1878 MeV, it can decay to two nucleons; for mS < 2055 MeV 1⁄4 mp þ me þ mΛ, this is a doubly weak process with a decay width Γ > G4Fδm (δm 1⁄4 mS − 1878 MeV); for mS < 1890 MeV the lifetime of the S is greater than the age of the Universe. Farrar [12] has argued for hadronic cross-sections of the order of 10−30 cm or so These facts point to the mass range 1860 to 1890 MeV, where the dibaryon is stable or long-lived and can plausibly escape the very stringent constraints on a much heavier dibaryon. Our calculations show that the freeze-out abundance of the S’s, which determines the present-day relic abundance, is at most 10−11 that of nucleons, largely independent of the dibaryon mass and the strength of its interactions
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