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 (GWs) for electroweak scale phase transitions; although the detection of such stochastic GWs does not necessarily imply a Fermi-ball DM scenario.
Highlights
The particle origin of dark matter (DM) is a longstanding mystery
We propose a novel dark matter (DM) scenario based on a first-order phase transition in the early Universe
Cosmological observations show that DM contributes ∼27% of the total energy of the Universe [1], but none of the Standard Model (SM) particles can serve as DM candidates [2]
Summary
The particle origin of dark matter (DM) is a longstanding mystery. Cosmological observations show that DM contributes ∼27% of the total energy of the Universe [1], but none of the Standard Model (SM) particles can serve as DM candidates [2]. We propose a new mechanism in which during an FOPT dark fermions are trapped inside the false vacuum to subsequently form compact macroscopic DM candidates, which we call “Fermi-balls.”. This scenario requires the following three conditions to be satisfied:. The Fermi-ball has several novelties compared to other similar mechanisms It is made of fermions, while a Q-ball [49]—which localizes conserved charges in small objects—is made of scalars. VII. ħ 1⁄4 c 1⁄4 1 is adopted throughout this paper
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