The Dark Side of the Electroweak Phase Transition

TL;DR

The paper investigates whether a GeV-scale light scalar $s$ coupled to the Higgs can induce a strongly first-order electroweak phase transition, thereby enabling electroweak baryogenesis. By introducing a trilinear Higgs–singlet interaction and analyzing the finite-temperature potential, the authors show that the transition can be strengthened with $v(T_c)/T_c \gtrsim 1$ and can occur at a reduced temperature $T_c$, with viable regions where $m_s \lesssim 12\,\mathrm{GeV}$ satisfy LEP constraints. They connect this scalar sector to a dark sector in which dark matter annihilation proceeds via $\chi\chi \to s a$ with a Sommerfeld enhancement, potentially explaining cosmic-ray leptonic excesses while remaining compatible with direct-detection limits. They compute bubble-nucleation dynamics and the gravitational-wave spectrum, and conclude that the signal is too small to be detected by LISA/BBO in the explored ranges. The work links GeV-scale new physics to EWPT dynamics, dark matter phenomenology, and gravitational-wave signatures, while emphasising experimental tests via light-state searches and decays.

Abstract

Recent data from cosmic ray experiments may be explained by a new GeV scale of physics. In addition the fine-tuning of supersymmetric models may be alleviated by new O(GeV) states into which the Higgs boson could decay. The presence of these new, light states can affect early universe cosmology. We explore the consequences of a light (~ GeV) scalar on the electroweak phase transition. We find that trilinear interactions between the light state and the Higgs can allow a first order electroweak phase transition and a Higgs mass consistent with experimental bounds, which may allow electroweak baryogenesis to explain the cosmological baryon asymmetry. We show, within the context of a specific supersymmetric model, how the physics responsible for the first order phase transition may also be responsible for the recent cosmic ray excesses of PAMELA, FERMI etc. We consider the production of gravity waves from this transition and the possible detectability at LISA and BBO.

Figures (4)

• Figure 1: The critical temperature of the phase transition, for $\overline{m}_H=115~{\rm GeV}$, there is little dependence on $\overline{m}_H$. From the right the lines correspond to $T_c=$$50~{\rm GeV}$ (solid), $40~{\rm GeV}$ (dashes), $30~{\rm GeV}$ (short dashes), $25~{\rm GeV}$ (dot-dashed) respectively. At the boundary on the left the critical temperature grows rapidly. Here the transition becomes a second order phase transition, the minimum near the origin is never the true minimum of the potential.
• Figure 2: Sphaleron energy for $\overline{m}_H=115\,{\rm GeV}$, again there is only weak dependence on Higgs mass. From the top the contours correspond to $E_{sph}/\left(4\pi v/g\right)=1.82,\, 1.84,\, 1.86,\,1.88$ respectively. In the SM, $E_{sph}/\left(4\pi v/g\right)\approx 1.9$.
• Figure 3: For $\overline{m}_H=90\,{\rm GeV}$ (left) and $\overline{m}_H=115\,{\rm GeV}$ (right) we plot the region allowed by LEP constraints and the requirement of a strong enough first order phase transition. The dark green (larger) region assumes the singlet decays to jets or $\tau$'s and so applies the strongest LEP bound whereas the light green (smaller) region assumes the singlet decays some other way and applies only the model independent bound. NPT denotes that for these parameters there is no phase transition, and the red (diagonal) band is the region where the latent heat released during the phase transition is sizeable.
• Figure 4: Plots of $f_{max}$ and $\Omega_{GW}h^2$ for $\overline{m}_H=115\,{\rm GeV}$. In the $f_{max}$ plot, the red (upper) region is in the sensitivity band of LISA and the blue (lower) in that of BBO. In the $\Omega_{GW}h^2$ plot, reading from the top, the contours correspond to $10^{-15}$, $10^{-16}$, $10^{-17}$, $10^{-18}$, and $10^{-20}$ respectively.