The field content of the NMSSM is very similar to the MSSM; it differs merely by the addition of a singlet superfield S, which is introduced to address the μ-problem of the MSSM (for an exhaustive review of the NMSSM see e.g. ). The superpotential and soft supersymmetry-breaking terms of the Higgs sector are given by
The phenomenology of this model can be easily connected to the simplified models that we have reviewed previously. If we disregard the Higgsinos and singlino (which if heavy are largely irrelevant for Higgs phenomenology) the Higgs sector of the NMSSM is essentially that of a Type II `2HDM + Scalar' model, where we can immediately identify Hd, Hu as H1,H2.The singlet scalar S = [1/(√2)](SR + i SI) can obtain a vacuum expectation value 〈S 〉 = vs, generating an effective μ parameter μeff = λvs. The presence of additional light singlet scalars, pseudoscalars, and fermions allows for exotic Higgs decays within the NMSSM. On this page we discuss decays to light CP-even scalars s or pseudoscalars a of the form
(3)Decays to fermions are covered here. There are three ways of realizing the above decays within the NMSSM. In each case, the exotic Higgs decay phenomenology is a subset of the Type II 2HDM+S discussed in §, with some additional restrictions (like −π/2 < α < 0). The first is an accidental cancellation resulting in a light singlet-like s or a. Recent examples of such models have been found in a parameter scan . By choosing λ, κ ∼ 0.5, |Aλ| <~150 GeV and Aκ ∼ 0 the lightest pseudoscalar can satisfy ma < mh/2 for a SM-like Higgs h, with Br(h → a a) or Br(h → Z a) ∼ O(0.1). On the other hand, λ, κ ∼ 0.5, Aλ ∼ 0−200 GeV and Aκ ∼ − 500 GeV can result in a singlet-like light Higgs satisfying ms < mh/2 with Br(h → s s) ∼ O(0.1). There are also two symmetry limits resulting in light pseudoscalars, namely the R-limit and the PQ-limit of the NMSSM. The R-symmetry limit is realized for Aλ , Aκ → 0 [3,4,5], defined by the scalar field transformations
(4)This global symmetry is spontaneously broken by the Higgs vacuum expectation values vu, vd, vs, which results in a massless Nambu-Goldstone boson (the R-axion) appearing in the spectrum:
In most of the parameter space vs = [(μeff)/(λ)] >> vsin2 β, making AR mostly singlet-like. To avoid cosmological constraints on a massless axion and to help stabilize the vacuum, the R-symmetry is usually taken to be approximate. This leads to a light, mostly singlet-like pseudo-goldstone boson, and depending on the exact parameters chosen opens up the possibility of h→ aa for a = AR. Through its A component, a then decays to SM fermions, dominantly b―b and τ+ τ− above the respective thresholds. For κ, Aκ → 0 [6,7,8,9,10,11,12,13,14,15], there is an approximate PQ-symmetry:
(6)The PQ-symmetry limit is also shared by some other singlet-extensions of the MSSM, including the nearly-MSSM (nMSSM)  and the general NMSSM (e.g., see ). Analogously to the R-limit there is a PQ-axion,
(7)Exotic Higgs decays to this pseudoscalar, and even the singlet-like scalar, are in principle possible. However, for mh = 125 GeV, exotic Higgs decays to (pseudo-)scalars are generically not dominant in the PQ-limit. Instead, decays to binos and singlinos can dominate. This is discussed here.
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