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_h²/v_S²) ×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_iχ_j. The relevant vertex couplings for a singlino-like χ₁ and a bino-like χ₂ are

(4)

For m_χ2 <~100 and m_χ1  ∼ O(1−10 GeV) the off-diagonal decay h → χ₁ χ₂ can be kinematically accessible with an O(0.1) branching fraction. The purely invisible decay h → χ₁ χ₁ is suppressed by a factor of  ∼ λ/(g₁ tanβ) relative to the off-diagonal decay, ignoring phase space factors. Meanwhile, Higgs decay to a pair of bino-like χ₂ also scales as a single factor of the bino-Higgsino mixing angle, C_hχ2χ2 ∼ O (g₁ /λ) C_hχ1χ2 and if h→χ₂χ₂ 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 χ₂→χ₁ a or χ₂→χ₁s [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 χ₂ is the three-body decay χ₂→(a,s)^*χ₁ → (xx) χ₁, while the radiative mode χ₂ →χ₁ γ may become significant, with Br(h → χ₁ χ₁γ) as high as O(0.1). On-shell χ₂ → χ₁Z 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.     Summary: The PQ-limit of the NMSSM yields semi-invisible exotic Higgs decays into pairs of light neutralinos, most typically h→ χ₂χ₁ or h→ χ₂χ₂, with χ₂ → χ₁ a, χ₁ 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

[2]J. Huang, T. Liu, L.-T. Wang, and F. Yu, Supersymmetric Exotic Decays of the 125 GeV Higgs Boson, Phys. Rev. Lett. 112 (2014) 221803, [arXiv:1309.6633]. [3]P. Ciafaloni and A. Pomarol, Dynamical determination of the supersymmetric Higgs mass, Phys.Lett. B404 (1997) 83-88, [hep-ph/9702410]. [4]D. Miller, S. Moretti, and R. Nevzorov, Higgs bosons in the NMSSM with exact and slightly broken PQ-symmetry, [hep-ph/0501139]. [5]B. A. Dobrescu and K. T. Matchev, Light Axion Within the Next-to-Minimal Supersymmetric Standard Model, JHEP 0009 (2000) 031, [hep-ph/0008192]. [6]R. Dermisek and J. F. Gunion, The NMSSM Close to the R-symmetry Limit and Naturalness in h → aa Decays for m_a < 2m_b, Phys.Rev. D75 (2007) 075019, [hep-ph/0611142]. [7]www.th.u psud.fr/NMHDECAY/nmssmtools.html. [8]U. Ellwanger and C. Hugonie, NMSPEC: A Fortran code for the sparticle and Higgs masses in the NMSSM with GUT scale boundary conditions, Comput.Phys.Commun. 177 (2007) 399-407, [hep-ph/0612134]. [9]M. Muhlleitner, A. Djouadi, and Y. Mambrini, SDECAY: A Fortran code for the decays of the supersymmetric particles in the MSSM, Comput.Phys.Commun. 168 (2005) 46-70, [hep-ph/0311167]. [10]D. Das, U. Ellwanger, and A. M. Teixeira, NMSDECAY: A Fortran Code for Supersymmetric Particle Decays in the Next-to-Minimal Supersymmetric Standard Model, Comput.Phys.Commun. 183 (2012) 774-779, [arXiv:1106.5633].

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