Electroweak First-Order Phase Transition Triggered by Non-Gaussian Fluctuations of a $\mathbb{Z}_2$-Symmetric Spectator Scalar

Author(s)

Lu, Bo-Qiang

Abstract

We propose a novel mechanism to trigger a first-order cosmological electroweak phase transition using non-Gaussian primordial fluctuations of a $\mathbb{Z}_2$-symmetric spectator scalar field. We show that the large fluctuations of the spectator field can modify the Higgs thermal mass and enhance the thermal barrier, thereby enabling a strong first-order phase transition. Non-Gaussianities in the primordial fluctuation spectrum significantly increase the probability of large-amplitude fluctuations, allowing a substantial fraction of the Universe to undergo the transition. The spectator field also naturally serves as a cold dark matter candidate through its coherent oscillations, reproducing the observed relic abundance. The resulting stochastic gravitational wave background peaks in the $10^{-3}$-$10^{-1}$ Hz band, making it detectable by future space-based interferometers.

Figures

The effective potential as a function of the Higgs field value. We take $s=2\times 10^{8}$~GeV, $\kappa=10^{-12}$, $\Lambda=10^8$~GeV, and $Z_{\infty}=0.1$.
Caption The effective potential as a function of the Higgs field value. We take $s=2\times 10^{8}$~GeV, $\kappa=10^{-12}$, $\Lambda=10^8$~GeV, and $Z_{\infty}=0.1$.
The blue points represent the saddle point $\sqrt{\kappa}s_*$ as a function of the coupling $\kappa$. The orange and green lines represent the upper and lower bounds from conditions $m_{\rm loc}^2>0$ and $\Delta>0$, respectively. We take ($\Lambda=10^6$~Gev, $Z_{\infty}=0.1$) and ($\Lambda=10^8$~Gev, $Z_{\infty}=0.1$) for left and right panels, respectively. The temperature is fixed at T = 150 GeV.
Caption The blue points represent the saddle point $\sqrt{\kappa}s_*$ as a function of the coupling $\kappa$. The orange and green lines represent the upper and lower bounds from conditions $m_{\rm loc}^2>0$ and $\Delta>0$, respectively. We take ($\Lambda=10^6$~Gev, $Z_{\infty}=0.1$) and ($\Lambda=10^8$~Gev, $Z_{\infty}=0.1$) for left and right panels, respectively. The temperature is fixed at T = 150 GeV.
The fraction of the Universe that undergoes a first-order phase transition, $f_{\rm FOPT}$, in $H_{\rm inf}-\kappa$ space. We fix the non-Gaussian parameter $f_{\rm NL}=5$ and the asymptotic value $Z_{\infty}=0.1$. The new physics scale $\Lambda$ is taken as $10^6$~GeV and $10^8$~GeV for the left and right panels, respectively.
Caption The fraction of the Universe that undergoes a first-order phase transition, $f_{\rm FOPT}$, in $H_{\rm inf}-\kappa$ space. We fix the non-Gaussian parameter $f_{\rm NL}=5$ and the asymptotic value $Z_{\infty}=0.1$. The new physics scale $\Lambda$ is taken as $10^6$~GeV and $10^8$~GeV for the left and right panels, respectively.
Scatter points distribution in $H_{\rm inf}-\kappa$ plane. The colorbar denotes the values of the nucleation temperature $T_n$. We take the new physics scale $\Lambda=10^{6}$~GeV and $10^8$~GeV for the left and right panels, respectively. The scalar mass $m_s$ is determined by requiring the correct DM relic abundance $\Omega_s h^2 = 0.12$.
Caption Scatter points distribution in $H_{\rm inf}-\kappa$ plane. The colorbar denotes the values of the nucleation temperature $T_n$. We take the new physics scale $\Lambda=10^{6}$~GeV and $10^8$~GeV for the left and right panels, respectively. The scalar mass $m_s$ is determined by requiring the correct DM relic abundance $\Omega_s h^2 = 0.12$.
Scatter plot in the $H_{\rm inf}$-$\kappa$ plane. The colorbar indicates the nucleation temperature $T_n$. The left and right panels correspond to new physics scales $\Lambda = 10^6$~GeV and $10^8$~GeV, respectively. The scalar mass $m_s$ is determined by requiring the correct DM relic abundance $\Omega_s h^2 = 0.12$.
Caption Scatter plot in the $H_{\rm inf}$-$\kappa$ plane. The colorbar indicates the nucleation temperature $T_n$. The left and right panels correspond to new physics scales $\Lambda = 10^6$~GeV and $10^8$~GeV, respectively. The scalar mass $m_s$ is determined by requiring the correct DM relic abundance $\Omega_s h^2 = 0.12$.
SGWB spectra from first-order phase transitions are shown as functions of frequency $f$. The detection ranges of future space-based detectors, namely LISA (blue), TianQin (orange), Taiji (cyan), DECIGO (green), BBO (red), and aLIGO (purple), are also indicated.
Caption SGWB spectra from first-order phase transitions are shown as functions of frequency $f$. The detection ranges of future space-based detectors, namely LISA (blue), TianQin (orange), Taiji (cyan), DECIGO (green), BBO (red), and aLIGO (purple), are also indicated.
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