A strong electroweak phase transition in the 2HDM after LHC8

TL;DR

The paper investigates whether two-Higgs-doublet models can sustain a strongly first-order electroweak phase transition consistent with LHC data. Using a comprehensive 1-loop finite-temperature potential with daisy resummation, the authors perform a large parameter scan over masses and mixing angles to identify strong PT points, applying electroweak precision and flavour constraints. They find that a SM-like light Higgs (α ≈ β) with tanβ ≈ 1, a light H^0 around 200 GeV, and a heavy A^0 (m_{A^0} ≳ 400 GeV) favors a strong EWPT, while h^0 → γγ can be enhanced in Type II/Y scenarios. These results support 2HDMs as viable frameworks for electroweak baryogenesis and outline specific mass-coupling patterns testable at the LHC.

Abstract

The nature of the electroweak phase transition in two-Higgs-doublet models is revisited in light of the recent LHC results. A scan over an extensive region of their parameter space is performed, showing that a strongly first-order phase transition favours a light neutral scalar with SM-like properties, together with a heavy pseudo-scalar (m_A^0 > 400 GeV) and a mass hierarchy in the scalar sector, m_H^+ < m_H^0 < m_A^0. We also investigate the h^0 -> gamma gamma decay channel and find that an enhancement in the branching ratio is allowed, and in some cases even preferred, when a strongly first-order phase transition is required.

Figures (8)

• Figure 1: Scatter plot in $\tan\beta\times m_{H^\pm}$ showing the exclusion regions from $\bar{B}\to X_s\gamma$ (red/dashed) and $B^0-\bar{B}^0$ mixing (black/full) for Types I/X. Blue/dark-grey points are physical, while green/light-grey ones have a strong phase transition.
• Figure 2: Scatter plot in $\tan\beta\times m_{H^\pm}$ showing the exclusion regions from $\bar{B}\to X_s\gamma$ (red/dashed) and $B^0-\bar{B}^0$ mixing (black/full) for Types II/Y. Blue/dark-grey points are physical, while green/light-grey ones have a strong phase transition.
• Figure 3: Counting rates for physical (blue/dark) and strong phase transition (green/light) points, and their ratio, as a function of $\mu$ (top) and $\tan\beta$ (bottom) for Types I/X (left) and II/Y (right).
• Figure 4: Counting rates for physical (blue/dark) and strong phase transition (green/light) points, and their ratio, as a function of $\beta-\alpha$ for Types I/X (left) and Types II/Y (right).
• Figure 5: Counting rates and ratio for points subject only to type-independent constraints from $B^0-\bar{B}^0$ mixing. (a) $m_{H^\pm}$ hardly influences the phase transition. (b) Preference for $m_{H^0}\approx 200$ GeV; (c--e) Strong phase transitions prefer a scalar mass hierarchy $m_{A^0}>m_{H^0}\gtrsim m_{H^\pm}$. (f) Large pseudo-scalar masses, $m_{A^0}\gtrsim 400$ GeV, are also favoured.