Non-thermal Dark Matter Production from the Electroweak Phase Transition: Multi-TeV WIMPs and "Baby-Zillas"

TL;DR

The paper explores non-thermal dark matter production at the end of a first-order electroweak phase transition, focusing on energy release from bubble collisions as a source for Higgs-coupled new states. It develops a formalism for producing particles via the Higgs portal, computing production efficiencies for scalars, fermions, and vector bosons, and highlighting backreaction and energy-budget constraints. The authors identify vector boson dark matter with a Higgs coupling as the most viable non-thermal DM, capable of achieving the observed relic density in the multi-TeV range while respecting direct-detection bounds, and they show that very heavy “baby-zilla” DM could arise only under extremely strong transitions and low reheating temperatures. The work delineates two distinct non-thermal pathways (multi-TeV WIMPs and baby-zillas) and discusses the implications for cosmological evolution, wash-out, and experimental prospects. It also examines asymmetric production but finds wash-out challenges that limit its effectiveness.

Abstract

Particle production at the end of a first-order electroweak phase transition may be rather generic in theories beyond the standard model. Dark matter may then be abundantly produced by this mechanism if it has a sizable coupling to the Higgs field. For an electroweak phase transition occuring at a temperature T_EW ~ 50-100 GeV, non-thermally generated dark matter with mass M_X > TeV will survive thermalization after the phase transition, and could then potentially account for the observed dark matter relic density in scenarios where a thermal dark matter component is either too small or absent. Dark matter in these scenarios could then either be multi-TeV WIMPs whose relic abundace is mostly generated at the electroweak phase transition, or "Baby-Zillas" with mass M_GUT >> M_X >> v_EW that never reach thermal equilibrium in the early universe.

Figures (12)

• Figure 1: LEFT: Potential with nearly degenerate minima. RIGHT: Potential with very non-degenerate minima. Each one shows the behaviour of the field immediately after the collision in the region close to the collision point, as described in the text: 1) "Kick" to field values larger than $v(T)$. 2) Large field oscillation, successful (LEFT) or unsuccessful (RIGHT) in driving the field over the potential barrier. 3) Oscillations around the symmetric (LEFT) or broken (RIGHT) minimum.
• Figure 2: Snapshots of the field profile $h(z,t)$ during a bubble collision ($t$ increasing downwards). LEFT: Bubble collision for the potential with nearly degenerate minima (Figure 1 - Left). RIGHT: Bubble collision for the potential with very non-degenerate minima (Figure 1 - Right). In both cases, $\gamma_w = 10^2$, $l_w = 15/T_{\mathrm{EW}}$ and $T_{\mathrm{EW}} = 100$ GeV.
• Figure 3: Particle production efficiency $f(\chi \equiv \omega^2 - p_z^2)$ for $\gamma_w = 10^2$ (LEFT) and $\gamma_w = 10^3$ (RIGHT), $l_w = 15/T_{\mathrm{EW}}$ and $T_{\mathrm{EW}} = 100$ GeV, in the case of a perfectly elastic collision with infinitely thin bubble walls (\ref{['Number7']}) (solid red) and with a finite bubble wall thickness (\ref{['Number7Bis']}) (dashed-black), and in the case of a totally inelastic collision (\ref{['Number7BisBis']}) (solid blue) with $m_h = 125$ GeV. The $\chi$-axis is displayed in units of $(100\,\, \mathrm{GeV})^2$.
• Figure 4: Generated comoving energy density $\Upsilon$ in scalars $S$ (normalized to the observed dark matter comoving energy density) as a function of the scalar mass $M_s$ in the perfectly elastic collision limit (LEFT) and totally inelastic collision limit (RIGHT) for $\gamma_w = 10^8$, $l_w = 15/T_{\mathrm{EW}}$ and $T_{\mathrm{EW}} = 100$ GeV. The solid-black line corresponds to the observed dark matter comoving energy density, and the dashed-black line (not seen in LEFT) corresponds to the maximum possibly generated comoving energy density ($\mathcal{E} = E_w$).
• Figure 5: Generated comoving energy density $\Upsilon$ in fermions $f$ (normalized to the observed dark matter comoving energy density) as a function of the fermion mass $m_f$ in the perfectly elastic collision limit (LEFT) and totally inelastic collision limit (RIGHT) for $\gamma_w = 10^8$, $l_w = 15/T_{\mathrm{EW}}$ and $T_{\mathrm{EW}} = 100$ GeV. Red lines: production in the presence of a direct tree-level Yukawa coupling between fermions and Higgs (\ref{['fermionsProd2']}). Blue lines: production for a tree-level effective coupling (\ref{['fermionsProd5']}), for $\mu_s = M_s=500$ GeV (solid) and $5$ TeV (dashed). Yellow lines: production for a 1-loop effective coupling (\ref{['fermionsProd6']}). The solid-black line corresponds to the observed dark matter comoving energy density, and the dashed-black line corresponds to the maximum possible generated comoving number density ($\mathcal{E} = E_w$).