Two Higgs doublets and a complex singlet: disentangling the decay topologies and associated phenomenology

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.

Enlarging the scope of resonant di-Higgs searches: hunting for Higgs-to-Higgs cascades in 4b final states at the LHC and future colliders

“…At the same time, it is well-motivated as a simplified version of the NMSSM and as a portal to a dark matter sector [68][69][70]. As emphasized in [23], in this class of extended Higgs sectors the Higgs-to-Higgs cascade decays we discuss in this work are ubiquitous.…”

“…In this case, the decays of H 2 into SM particles are simply controlled by the mixing between the singlet S and the heavy 2HDM state H (see section 5.2), which we parametrise by sin θ. Note that, contrary to the pseudoscalar case, the ρ parameter does not necessarily imply constraints 22 See [23,73] for the alignment limit conditions in the 2HDM + S. on this angle. These can be avoided in the alignment limit by having m A m ± H and the cancellation is no longer spoiled by a − A mixing.…”

“…We now consider the addition of a real scalar/pseudoscalar singlet field S to the above 2HDM (for a detailed discussion of the 2HDM + complex singlet, see [23]), assuming for simplicity CP conservation in the scalar potential. This scenario captures the features of richer, more complicated scalar sectors than those analyzed in sections 5.1 and 5.2.…”

“…These can be avoided in the alignment limit by having m A m ± H and the cancellation is no longer spoiled by a − A mixing. Again, we consider the case where the 2HDM states A and H ± are decoupled 23 . The leading decays of the state H 1 are H 1 → H 2 H 2 , H 1 → h SM h SM (this decay is possible in the alignment limit if µ S , µ S 1 and/or µ S 2 in (5.4) are non-vanishing) and decays into SM fermions.…”

“…In such a case, Higgs-to-Higgs decays with both the parent particle and its decay products as BSM states are possible, and may constitute the most promising avenue for their discovery. This has been emphasized in the literature for certain processes within the two-Higgs-doublet model (2HDM) [18][19][20][21][22], the 2HDM plus a scalar singlet [23], the next-to-minimal supersymmetric Standard Model (NMSSM) [24][25][26][27][28] and the SM extended by several singlet scalars [29].…”