Fermi-ball dark matter from a first-order phase transition

Abstract

We propose a novel dark matter (DM) scenario based on a first-order phase transition in the early Universe. If dark fermions acquire a huge mass gap between true and false vacua, they can barely penetrate into the new phase. Instead, they get trapped in the old phase and accumulate to form macroscopic objects, dubbed Fermi-balls. We show that Fermi-balls can explain the DM abundance in a wide range of models and parameter space, depending most crucially on the dark-fermion asymmetry and the phase transition energy scale (possible up to the Planck scale). They are stable by the balance between fermion's quantum pressure against free energy release, hence turn out to be macroscopic in mass and size. However, this scenario generally produces no detectable signals (which may explain the null results of DM searches), except for detectable gravitational waves for electroweak scale phase transitions; although the detection of such stochastic gravitational waves does not necessarily imply a Fermi-ball DM scenario.

Dark Matter as dark dwarfs and other macroscopic objects: multiverse relics?

“…In some models with light dark quarks, m Λ, strong dynamics makes it energetically favourable for baryons to stay inside pockets [15]. A similar situation can happen in models with ad-hoc first order phase transitions [32][33][34]. In general, relics remain if friction keeps walls nonrelativistic and if trapped particles are enough heavier outside than inside, so that the relativistic quantum pressure inside gives p quantum = p V with radius R ∼ Q p/4 /Λ, where p = 1 for bosons, p = 4/3 for fermions.…”

“…where p = 4/3, corresponding to relativistic Fermi pressure (bosons give instead p = 1 and a different order unity pre-factor). The term proportional to the wall energy density σ is negligible for large Q. Minimising U gives the radius R ∼ (Q p /∆V ) 1/4 [15,[32][33][34]. Such pockets can be macroscopic objects with super-Planckian mass.…”

Correlated gravitational wave and microlensing signals of macroscopic dark matter

“…If the DM mass in the true vacuum is larger than the critical temperature of the FOPT, then, four-momentum conservation causes the DM to be trapped in the false vacuum. If a DM-antiDM asymmetry exists, then as the false vacuum shrinks, the DM particles are compressed to form macroscopic objects called Fermi balls (FBs), which become the DM relic [1]. Similar ideas have been proposed in refs.…”

“…As the true vacuum expands and the false vacuum shrinks, the χ's aggregate and form a macroscopic FB. For this to occur, there must be a nonzero asymmetry η χ ≡ (n χ −n χ)/s in the number densities in the false vacuum (where s is the entropy density) during the phase transition so that an excess remains after pair annihilation χχ → φφ, and χ must carry a conserved global U(1) Q so that the FB attains stability by accumulating Q-charge [1]. Mechanisms that produce η χ are discussed in the appendix of ref.…”

“…Since the timescale on which the false vacuum bubble shrinks is ∆t = R /v w , V is given by Γ(T )V ∆t ∼ 1. Then, with one FB per critical volume, the number density of FBs n FB | T is determined by [1]:…”