Dark Matter production from relativistic bubble walls

TL;DR

The paper introduces Bubble Expansion (BE) as a non-thermal DM production mechanism active during strong first-order phase transitions with ultra-relativistic bubble walls. It derives the production probability for wall-induced decays, estimates the resulting DM density, and analyzes how late-time annihilation, supercooling, and reheating modify the relic abundance, including the possibility of super-heavy DM. The EWPT and Higgs-portal DM realizations are explored, showing viable regions where BE dominates or coexists with freeze-out, and predicting a characteristic stochastic gravitational-wave background as a detector-rich signature. This framework broadens DM model-building beyond thermal freeze-out and links DM genesis to observable gravitational waves, potentially enabling tests with future GW observatories and direct/indirect detection experiments.

Abstract

In this paper we present a novel mechanism for producing the observed Dark Matter(DM) relic abundance during the First Order Phase Transition (FOPT) in the early universe. We show that the bubble expansion with ultra-relativistic velocities can lead to the abundance of DM particles with masses much larger than the scale of the transition. We study this non-thermal production mechanism in the context of a generic phase transition and the electroweak phase transition. The application of the mechanism to the Higgs portal DM as well as the signal in the Stochastic Gravitational Background are discussed.

Figures (8)

• Figure 1: The unprocessed final relic abundance coming from FO and BE process with $T_{\text{nuc}} = T_{\text{reh}}$ and $v = 200$ GeV. The blue shading gives the value of $\Omega_{\text{BE}}^{\text{today}}$. The red lines $\Omega^{\text{today}}_{\text{BE},\phi} = \Omega^{\text{today}}_{\text{FO},\phi}$, $\Omega^{\text{today}}_{\text{BE},\phi} = \Omega^{\text{today}}_{\text{obs}}$ and $\Omega^{\text{today}}_{\text{BE},\phi} = \Omega^{\text{today}}_{\text{obs}}$ define 4 regions. In I, BE abundance is dominant and FO is not enough to account for the observation. In II, FO is too large, but BE is still dominant. In III, both BE and FO are too large, but FO is dominant. Finally, in IV, FO is dominant, and BE is not enough to account for $\Omega^{\text{today}}_{\text{obs}}$.
• Figure 2: The evolution of the energy density of the Dark Higgs portal DM, with $v=T_{\rm reh}=100 {\rm ~GeV} ,$$M_\phi=1 {\rm ~TeV} (5 {\rm ~TeV} ,5 {\rm ~TeV} )$, and $\lambda\simeq 0.63( 4.3, 0.1)$ with large initial number density in the left top (right top, bottom) panel, which corresponds to late time FO (late time annihilation, satisfied stability condition)
• Figure 3: The parameter region of the Dark Higgs portal DM with non-thermal over-production at $v=T_{\rm reh}=50 {\rm ~GeV} ,100 {\rm ~GeV} , 200 {\rm ~GeV} , 400 {\rm ~GeV}$ from left to right [Black line]. $g_4=1/4.$ We neglect the mass of the dark Higgs boson. The orange dashed line indicates the FO prediction.
• Figure 4: The parameter region of the SM Higgs portal dark matter with non-thermal over production for $T_{\rm reh}=50 {\rm ~GeV} ,100 {\rm ~GeV} , 200 {\rm ~GeV}$ from left to right [Black line]. $v=174\,$GeV, $m_{\rm mes}=125\,$GeV, and $g_4=1.$ The orange dashed line indicates the FO prediction. The purple region above the purple solid line may be excluded by XENON1T experiment Aprile:2019dbj. The green dashed and blue dotted lines represent the future reaches of the XENONnT Aprile:2020vtw and DARWIN Aalbers:2016jon, respectively. The lines are adopted from Arcadi:2019lka. The Cerenkov Telescope Array (CTA) reach (by assuming the NFW distribution of DM) is adopted from Beniwal:2015sdl.
• Figure 5: Values of $M_\phi$ and $\lambda$ providing the observed DM relic abundance today in the Dark Higgs portal model, for values of supercooling $\frac{T_{\text{reh}}}{T_{\text{nuc}}} = (10, 10^{1.5},10^2)$, $v = 2000$ GeV, $g_4 = 1$. Each plot corresponds to a different value of the reheating temperature $T_{\text{reh}} = 2000, 500, 50$ GeV. The Red lines correspond to contributions from FO and BE providing the observed DM abundance and that do not undergo annihilation after the transition. The black line is the result of DM annihilation, as in Section \ref{['sec:co_ann']}. Roughly when $M_\phi < 20 T_{\text{reh}}$, the DM comes back to equilibrium after the transition and the final parameters compatible parameters are given by the orange dotted line. Let us also emphasize that we assumed runaway regime bubble, with the maximal DM mass given by Eq.(\ref{['eq:MAXmass_2']})