Upper and lower bounds of the lightest CP-even Higgs boson in the two-Higgs-doublet model

TL;DR

This paper derives bounds on the lightest CP-even Higgs boson mass in the two-Higgs-doublet model, considering theoretical and experimental constraints up to high energy scales, revealing a significantly reduced lower bound compared to the Standard Model.

Contribution

It provides the first comprehensive evaluation of Higgs mass bounds in 2HDM with all free parameters and experimental constraints, extending previous analyses.

Findings

Upper bound on Higgs mass similar to SM

Lower bound significantly reduced in 2HDM

Small Higgs masses excluded in Model II by b→sγ constraints

Abstract

By imposing validity of the perturbation and stability of vacuum up to an energy scale $\Lambda$ ($\leq 10^{19}$ GeV), we evaluate mass bounds of the lightest CP-even Higgs-boson mass ($m_h$) in the two-Higgs-doublet model (2HDM) with a softly-broken discrete symmetry. The most general bounds are obtained as a function of $\Lambda$ vering all the free parameters under the experimental constraint from the $rho$ parameter and the b \to s \gamma$ results. We find that, while the upper bound is almost the same as in the SM, the lower bound is significantly reduced. In the decoupling regime where the model behaves like the SM at low energy, the lower bound is given, for example, by about 100 GeV for $\Lambda = 10^{19}$ GeV and $m_t = 175$ GeV, which is smaller by about 40 GeV than the corresponding lower bound in the SM. In general case, the $m_h$ is no longer bounded from below by these conditions. If we consider the experimental $b \to s \gamma$ constraint, small $m_h$ are excluded in Model II of the 2HDM.