For more information about the NMSSM, click here. While both the R- and the PQ-limit lead to a light pseudoscalar as discussed elsewhere, the PQ-limit with m

_{h}= 125 GeV typically leads to different exotic Higgs decay phenomenology, in which decays to fermions can be as or more important than decays to scalars [1,2]. When v

_{s}>> v

_{u}, v

_{d}, the dominant tree-level contributions to the masses of the singlet-like scalars and singlino-like fermion ~S are [3,4,1]

(1)

The pseudoscalar a is light in both the R- and PQ-limits, but in the PQ-limit s and ~S must be light as well. This cannot be realized in the R-limit, since vacuum stability for small κ requires A_{λ}∼ μtanβ, strongly breaking R-symmetry. This abundance of possible light singlet-like states opens up many different exotic Higgs decays, giving phenomenology that is qualitatively unlike the decays in the R-limit. In the R-limit, the coupling of the SM-like Higgs to the R-axion eigenstate is g

_{h a a}∼

*O*(m

_{h}

^{2}/v

_{S}

^{2}) ×v [5,6], and v

_{s}as large as 10 m

_{h}can still yield a sizeable branching fraction Br(h→aa) ∼ 0.1. The corresponding couplings in the PQ-limit instead scale as [1,2]

(2)

where(3)

is required by vacuum stability (avoiding a runaway in the S-direction). For a given μ_{eff}, small λ corresponds to small singlet-doublet mixing and mostly SM-like Higgs phenomenology. Correspondingly, parameter scans using NMSSMTools [7,8,9,10] indicate that λ <~0.3 dominates the surviving parameter space in the PQ-limit (κ << λ) (see App. ). It is thus common in the PQ-limit to obtain g

_{haa}, g

_{hss}<< v, suppressing exotic Higgs decays to (pseudo-)scalars. However, the PQ-limit allows the SM-like Higgs boson to decay into a pair of light neutralinos h → χ

_{i}χ

_{j}. The relevant vertex couplings for a singlino-like χ

_{1}and a bino-like χ

_{2}are

(4)

For m_{χ2}<~100 and m

_{χ1}∼

*O*(1−10 GeV) the off-diagonal decay h → χ

_{1}χ

_{2}can be kinematically accessible with an

*O*(0.1) branching fraction. The purely invisible decay h → χ

_{1}χ

_{1}is suppressed by a factor of ∼ λ/(g

_{1}tanβ) relative to the off-diagonal decay, ignoring phase space factors. Meanwhile, Higgs decay to a pair of bino-like χ

_{2}also scales as a single factor of the bino-Higgsino mixing angle, C

_{hχ2χ2}∼

*O*(g

_{1}/λ) C

_{hχ1χ2}and if h→χ

_{2}χ

_{2}is kinematically available, this branching fraction can be important. For m

_{χ2}−m

_{χ1}> min {m

_{s}, m

_{a}}, the heavier neutralino can decay via χ

_{2}→χ

_{1}a or χ

_{2}→χ

_{1}s [1,2]. This leads to a plethora of possible h→ (xx) + MET or h → (xx)(yy) + MET decays, where x, y are SM partons (most likely b, τ, or light jets, see §) that reconstruct the singlet boson mass a or s. If m

_{χ2}− m

_{χ1}< min {m

_{s}, m

_{a}}, the principal decay mode of χ

_{2}is the three-body decay χ

_{2}→(a,s)

^{*}χ

_{1}→ (xx) χ

_{1}, while the radiative mode χ

_{2}→χ

_{1}γ may become significant, with Br(h → χ

_{1}χ

_{1}γ) as high as

*O*(0.1). On-shell χ

_{2}→ χ

_{1}Z does not occur until m

_{χ2}− m

_{χ1}> m

_{Z}. Given that we require m

_{χ2}− m

_{χ1}< m

_{h}−2 m

_{χ1}, these points are sparse. Fig. 1 shows the corresponding exotic decay topologies.

Figure 1: Two significant fermionic decay topologies of the SM-like Higgs boson in the

PQ symmetry limit.

**Left (a):**depending on whether min{m_{s}, m_{a}} exceeds m_{χ2}− m_{χ1}, a(s) may or may not be on shell.**Right (b):**to be non-negligible, the radiative χ_{2}decay requires min{m_{s}, m_{a}} > m_{χ2}− m_{χ1}.)**Summary:**The PQ-limit of the NMSSM yields semi-invisible exotic Higgs decays into pairs of light neutralinos, most typically h→ χ

_{2}χ

_{1}or h→ χ

_{2}χ

_{2}, with χ

_{2}→ χ

_{1}a, χ

_{1}s, and a,s→ (f―f, gg, γγ). This yields final states of the form (b―b)+MET, (ττ)+MET, (b―b) (b―b)+MET, (ττ)(ττ))+MET, and the rarer but cleaner γ+MET, (2,4)μ+MET, (μμ)(b―b)+MET. Depending on the spectrum, the visible particles may be collimated or isolated. Current experimental constraints and future prospects for a subset of these decays are discussed on the γ+MET, bb+MET, 2γ+MET, ττ+MET, and collimated/resonant leptons pages.

## References

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