Dark matter filtering-out effect during a first-order phase transition

Abstract

If the mass of dark matter is generated from a cosmological phase transition involving the nucleation of bubbles, the corresponding bubble walls can filter out dark matter particles during the phase transition. Only particles with sufficient momentum to overcome their mass inside the bubbles can pass through the walls. As a result, the dark matter number density after the phase transition has a suppression factor expð−M χ =2γTÞ, where M χ is the dark matter mass, andγ and T are the Lorentz factor and temperature of the incoming fluid in the bubble wall rest frame, respectively. Under certain assumptions, we show that the filtering-out process can naturally provide a large suppression consistent with the observed dark matter density for a wide range of dark matter masses up to the Planck scale. Since the first-order phase transition is the decisive ingredient in our mechanism, a new connection is made between heavy dark matter scenarios and gravitational wave observations.

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

“…When the balance between them P ¼ ΔU Ã is achieved, the bubble reaches its terminal velocity v b . It can be solved numerically for a given model [21,[69][70][71][72], but in this paper it will be treated as a free parameter of Oð0.1Þ. If the bubble radius is much larger than wall thickness, the vicinity of the wall can be treated as a onedimensional problem: the wall is parallel to the Oxy plane and moving along the z-axis with the velocity v b .…”

“…Only χ with −p z > M Ã χ can pass across the wall, due to the energy conservation. The particle current per unit area and unit time is then [21]…”

“…Given a scalar potential Uðϕ; TÞ, it is convenient to rewrite Fermi-ball profiles in Eqs. (19) and (21) in terms of the action at T Ã . Since ΓðT Ã Þ=T 4 Ã ≈ e −S 3 ðT Ã Þ=T Ã , the mass, radius and charge of a Fermi-ball are rewritten as…”

“…QB [23], different from Eq. (21). This means that a Fermiball is typically heavier and larger than a Q-ball for a given amount of localized Q-charge.…”

“…Recently, there has been a growing number of studies on the DM generated in association with a first-order cosmic phase transition (FOPT). During an FOPT, the discontinuity of the scalar vacuum expectation value (VEV) could be crucial in DM physics, by altering the decay of DM [7][8][9], by generating asymmetric DM [10][11][12][13][14][15][16][17][18], by producing DM nonthermally [19], by filtering DM to the true vacuum [20][21][22], by condensing particles into the false vacuum to form (scalar) Q-ball DMs [23,24] or quark (or quark-like fermion) nuggets [25][26][27][28][29][30][31][32], and by producing the primordial black holes [33,34]. 1 In this paper, 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."…”