Testing Electroweak Baryogenesis with Future Colliders

TL;DR

This work interrogates the testability of Electroweak Baryogenesis (EWBG) by proposing a minimal ‘nightmare’ scenario: a real SM singlet S with a Z2 symmetry that couples through the Higgs portal, with m_S > m_h/2. EWBG can be realized via either a loop-induced one-step first-order phase transition (μ_S^2 > 0) or a tree-induced two-step transition (μ_S^2 < 0), and the authors map these mechanisms onto the (m_S, λ_HS) plane, accounting for perturbativity and vacuum stability constraints. A central result is that a future 100 TeV hadron collider appears necessary—and possibly sufficient—to probe the entire EWBG-favored parameter space of the nightmare scenario, with complementary indirect probes (triple-Higgs coupling and Zh production) at lepton colliders, and direct singlet production via VBF potentially testing the two-step region. Additionally, dark matter constraints (e.g., XENON1T) can exclude much or all of the EWBG-viable region if S is a thermal relic, highlighting the interplay between collider searches and cosmology. Overall, the paper argues that a no-lose test of this EWBG realization is achievable with a future collider program, albeit with sensitivity dependent on the specific signatures and cosmological history assumed for S.

Abstract

Electroweak Baryogenesis (EWBG) is a compelling scenario for explaining the matter-antimatter asymmetry in the universe. Its connection to the electroweak phase transition makes it inherently testable. However, completely excluding this scenario can seem difficult in practice, due to the sheer number of proposed models. We investigate the possibility of postulating a "no-lose" theorem for testing EWBG in future e+e- or hadron colliders. As a first step we focus on a factorized picture of EWBG which separates the sources of a stronger phase transition from those that provide new sources of CP violation. We then construct a "nightmare scenario" that generates a strong first-order phase transition as required by EWBG, but is very difficult to test experimentally. We show that a 100 TeV hadron collider is both necessary and possibly sufficient for testing the parameter space of the nightmare scenario that is consistent with EWBG.

Figures (10)

• Figure 1: The parameter space of the $\mathbb{Z}_2$ symmetric SM+S extension with $m_S > m_h/2$ (our nightmare scenario). Left: The red shaded region indicates when $\mu^2$ is negative. The dotted red contours indicate $\mathrm{Sign}(\mu_S^2)|\mu_S|$. The blue contours show the minimum $S^4$ quartic coupling $\lambda_S$ required for the electroweak symmetry breaking (EWSB) vacuum to be the ground state of the universe, while the green contours show the minimum $\lambda_S$ to avoid negative runaways. Right: Gray regions indicate where theoretical control is lost due to non-perturbative $\lambda_S$. Perturbative analysis of the phase transition breaks down in the blue shaded regions, see Section \ref{['s.ewpt']}. The red and white regions are the possible parameter space of this nightmare scenario.
• Figure 2: Regions in the $(m_S, \lambda_{HS})$ plane with viable EWBG. Red shaded region: for $\mu_S^2 < 0$ it is possible to choose $\lambda_S$ such that EWBG proceeds via a tree-induced strong two-step electroweak phase transition (PT). Orange contours: value of $v_c/T_c$ for $\mu_S^2 > 0$. The orange shaded region indicates $v_c/T_c > 0.6$, where EWBG occurs via a loop-induced strong one-step PT. Above the green dashed line, singlet loop corrections generate a barrier between $h = 0$ and $h = v$ even at $T = 0$, but results in the dark shaded region might not be reliable, see Section \ref{['sss.perturbativereliability']}.
• Figure 3: Comparison of the zero-temperature potential contributions in the SM vs. the SM + singlet with $(m_S, \lambda_{HS}) = (450 \;\mathrm{GeV}, 3.2)$ which has a strong first-order PT with $v_c/T_c > 1$. The one-loop contribution of the singlet reduces the potential difference between the origin and the EWSB vacuum.
• Figure 4: Same as Fig. \ref{['f.vcoverTcplot']}, but with contours of $\Delta \lambda(h=0)$ Eq. (\ref{['e.deltalambda']}) shown in purple. Dark shading above $|\Delta \lambda| = 0.4$ indicates approximately where the results of our analysis in Section \ref{['sss.ewptvialoop']} are not trustworthy due to loss of perturbativity.
• Figure 5: Production cross-sections at hadron colliders for various modes of singlet production with $\lambda_{HS}=2$. These calculations were computed at LO with MadGraph5 Alwall:2011uj