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 fb−1 of data.
Highlights
We present a systematic study of an extension of the Standard Model (SM) with two Higgs doublets and one complex singlet (2HDM+S)
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 fb−1 of data
In order to be compatible with the observed phenomenology, one of the CP-even states must have couplings to pairs of SM particles similar to those of a SM Higgs with a mass of 125 GeV, such that it can be identified with the 125 GeV Higgs boson observed at the LHC
Summary
The mass eigenstates are obtained from the diagonalization of the squared-mass matrix for the Higgs basis states. The Shji (Paji) are obtained from diagonalizing the (symmetric) squared mass matrix for the CP-even (CP-odd) Higgs bosons, M2S (M2P ). In other words, a 125 GeV mass eigenstate approximately aligned with the HSM interaction state. The first option is the so-called alignment without decoupling limit, and is of particular interest for LHC phenomenology This is because the additional CP-even mass eigenstates H and h are not necessarily much heavier than h125 and may be directly accessible at the LHC [18, 19, 23, 52,53,54,55]. And in the following we employ a shorthand notation, sβ ≡ sin β , cβ ≡ cos β , tβ ≡ tan β
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