Two Higgs Doublets and a Complex Singlet: Disentangling the Decay Topologies and Associated Phenomenology

TL;DR

This paper systematically studies a 2HDM+S extension of the Standard Model, analyzing its parameter space, phenomenology, and collider signatures, especially Higgs cascade decays, to evaluate LHC discovery prospects up to 1 TeV masses.

Contribution

It introduces a re-parameterization of the 2HDM+S model for better physical insight and explores the collider phenomenology, including Higgs cascade decays, with projections for LHC searches.

Findings

Alignment without decoupling is necessary for SM-like Higgs properties.

Higgs cascade decays are common and enhance discovery potential.

LHC with 3000 fb^{-1} can probe Higgs masses up to 1 TeV.

Abstract

We present a systematic study of an extension of the Standard Model (SM) with two Higgs doublets and one complex singlet (2HDM+S). In order to gain analytical understanding of the parameter space, we re-parameterize the 27 parameters in the Lagrangian by quantities more closely related to physical observables: physical masses, mixing angles, trilinear and quadratic couplings, and vacuum expectation values. Embedding the 125\,GeV SM-like Higgs boson observed at the LHC places stringent constraints on the parameter space. In particular, the mixing of the SM-like interaction state with the remaining states is severely constrained, requiring approximate alignment without decoupling in the region of parameter space where the additional Higgs bosons are light enough to be accessible at the LHC. In contrast to 2HDM models, large branching ratios of the heavy Higgs bosons into two lighter Higgs bosons or a light Higgs and a $Z$ boson, so-called Higgs cascade decays, are ubiquitous in the 2HDM+S. Using currently available limits, future projections, and our own collider simulations, we show that combining different final states arising from Higgs cascades would allow to probe most of the interesting region of parameter space with Higgs boson masses up to 1\,TeV at the LHC with $L=3000\,{\rm fb}^{-1}$ of data.