Constraints on light scalars in the Aligned-two-Higgs-doublet model

TL;DR

This paper investigates the parameter space of the flavor-aligned two-Higgs-doublet model with light scalars, using Bayesian global fits to incorporate theoretical and experimental constraints from various sources.

Contribution

It provides the first comprehensive Bayesian analysis of light-scalar scenarios in the A2HDM, considering all relevant experimental and theoretical constraints.

Findings

Certain light-scalar configurations are strongly constrained or excluded.

The analysis delineates viable regions of parameter space for light scalars.

Results inform future experimental searches for light scalars in extended Higgs sectors.

Abstract

In this work, we explore the viability of light-scalar scenarios within the $\mathcal{CP}$-conserving, flavour-aligned two-Higgs-doublet model (A2HDM), where at least one additional neutral or charged scalar is assumed to be lighter than the 125 GeV Higgs boson. The A2HDM naturally avoids flavour-changing neutral currents via Yukawa alignment, providing a more flexible framework than conventional $Z_2$-symmetric models. Employing the $\texttt{HEPfit}$ package within a Bayesian statistical approach, we perform global fits across all distinct light-scalar configurations, incorporating theoretical requirements and experimental constraints from electroweak precision observables, flavour data, Higgs measurements, and direct searches at the LEP and the LHC.