Jessie Shelton, David McKeen, and Tao Liu More details on this mode may be found in Sec. 13 of the Survey of Exotic Higgs Decays, arXiv:1312.4992.

Theoretical Motivation

Here, we consider the decay h→ 2γ+MET. This signature can be realized in several ways.

• Non-Resonant:First, consider the non-resonant signature where the photons come from opposite sides of the initial two-body decay, h→ X X, followed by X→ γY on each side of the event with Y a detector-stable, neutral particle.
  This can occur in gauge-mediated SUSY-breaking models where the lightest neutralino is mainly bino, and decays via χ₁ ⁰→ γ~G. Minimal models of gauge mediation make it difficult to obtain a bino with m_~B < m_h/2 while keeping winos and gluinos sufficiently heavy to avoid constraints. However, more general models of gauge mediation [1] can allow this spectrum to be realized [2].
  More generally, this signature may be realized by having two new (Majorana) fermions χ₁ and χ₂, with a dipole coupling

  (1)

  and a dimension five Higgs portal coupling c₂₂ | H | ²( χ₂χ₂ + χ₂^† χ₂^†). In this case, both m_χ1 and m_χ2 are parameters of the model. It is natural to extend this simple model to include in addition off-diagonal couplings c₁₂ | H | ²( χ₂ χ₁ + χ₂^† χ₁^†) and couplings of the Higgs directly to the lighter of the two new fermions, c₁₁| H | ²( χ₁ χ₁ + χ₁^† χ₁^†). This generic model would then also yield h→ 1 γ+ MET and h→ MET signatures with relative branching fractions uniquely determined by the c_ij. Previous study of this topology in the MSSM has been performed in [2] and, for the heavier MSSM Higgses, in [3].
• Resonant: Second is the case where the photons reconstruct an intermediate resonance, h→ X X, with X→ γγ on one side and X→ invisible on the other.
  This can be simply realized in a bottom-up fashion by introducing a renormalizable Higgs portal interaction leading to a coupling of a to h, λ|H|²a², and also coupling a to photons and to a neutral, detector-stable particle χ via, e.g.,

  (2)

  M and M′ are the scales of the two dimension-five operators, and we have assumed that a is a real pseudoscalar and that χ is a Dirac fermion for definiteness. For some regions of parameter space, a→γγ and a→―χχ can have comparable branching fractions, making h→ 2γ+MET an important final state.
  Unlike the non-resonant case, the resonant signature has the useful additional handle that the two photons should reconstruct m_a, improving the search prospects.
  This simplified model can be trivially generalized to the case that the Higgs decays to two distinct states, a₁ and a₂, with a₁→γγ and a₂→inv. This can proceed through a dimension-four Higgs portal interaction, λ₁₂ | H | ²a₁a₂, if a₁ couples to photons while a₂ decays invisibly. This decay mode can dominate over h→inv. or h→4γ if λ₁₂ >> λ_11,22 where λ_11,22 are the coupling constants of the other allowed Higgs portal interactions, λ₁₁ | H | ²a₁²+λ₂₂ | H | ²a₂². While, in this resonant case, we limit our study to the situation m_a1 ≅ m_a2 ≡ m_a, the two intermediate particles having different masses is a well-motivated possibility.
• Cascade: The last decay topology we consider involves the initial decay h→ XY, followed by X→ YZ, Z→γγ with Y again appearing as missing energy in the detector.
  A concrete model for this involves neutral (Majorana) fermions χ₁ and χ₂ coupled to the Higgs through c₁₂|H|²(χ₂χ₁+χ₂^†χ₁^†), with χ₁ stable on detector scales, and a scalar s that can decay to two photons through the dimension-five operator sF_μνF^μν.¹ The decay can proceed through h→χ₁χ₂, and χ₂→ sχ₁, s→γγ if there is also a Yukawa interaction between s and χ_1,2,given by y₁₂s(χ₂χ₁+χ₂^†χ₁^†).

These different cases may arise in different theoretical models, and require related but distinct strategies to observe at colliders, as we discuss below.

Existing Experimental Searches and Limits

We focus on direct limits on the simplified models described above, ignoring model-dependent indirect limits on searches for e.g., gauginos produced via Drell-Yan. GMSB searches at the LHC have good prospects for discovering or excluding exotic Higgs decays into 2γ+MET, in both the resonant and non-resonant scenarios. The ATLAS search for 2γ+MET using 7 TeV data [4] has some sensitivity, setting limits of <~15% on the exotic Higgs branching fraction over much of the parameter space. The more recent CMS study using 4.04 fb⁻¹ of 8 TeV data [5] sets the current best limits. This search selects events with at least two photons and at least one central jet, and bins events in 5 exclusive MET bins beginning from a minimum of 50 GeV. We show the reach of this search in the resonant and non-resonant cases in Figs. 1 and 2 (left), as a function of m_χ1 in the non-resonant topology and m_a in the resonant topology. In Fig. 2 (right) we show the reach in the case of the cascade topology as a function of m_s, setting m_χ1=0 and m_χ2=60 GeV. We find that the limit obtained in this case is not very sensitive to the value of m_χ2=60 GeV chosen. In all three topologies the Br(h→2γ+MET) can be constrained at the level of a few percent over much of the parameter space.     Since searches using only 4 fb⁻¹ of 8 TeV data and optimized for other signatures are already able to place limits as stringent as O( 5%) on the Higgs branching fraction into this mode, 2 photons + MET is a good candidate for searches in the near future. The reach could be easily extended by requiring the transverse mass of the photons and MET to be bounded from above, as consistent with resonant origin from the 125 GeV Higgs. In the resonant case, looking for a peak in the γγ spectrum could offer another useful handle.

References

[1] P. Meade, N. Seiberg, and D. Shih, General Gauge Mediation,  Prog.Theor.Phys.Suppl. 177 (2009) 143-158, [arXiv:0801.3278].

[2] J. D. Mason, D. E. Morrissey, and D. Poland, Higgs Boson Decays to Neutralinos in Low-Scale Gauge Mediation, Phys.Rev. D80 (2009) 115015, [arXiv:0909.3523].

[3] J. L. Diaz-Cruz, D. K. Ghosh, and S. Moretti, The Diphoton signature of Higgs bosons in GMSB models at the CERN LHC, Phys.Rev. D68 (2003) 014019, [hep-ph/0303251].

[4] ATLAS Collaboration, G. Aad et. al., Search for diphoton events with large missing transverse momentum in 7 TeV proton-proton collision data with the ATLAS detector, Phys.Lett. B718 (2012) 411-430, [arXiv:1209.0753].

[5] CMS Collaboration, Search for supersymmetry in events with photons and missing energy.

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Footnotes:

¹The s F_μνF^μν operator could arise through mixing between s and h, see for example Sec. , although that would lead to a very suppressed h→ 2γ+MET branching ratio compared to final states like bb+MET. For 2γ+MET to be dominant, the s F_μνF^μν operator would have to be generated by a direct coupling of s to electrically-charged matter, e.g. (heavy) vector-like leptons. For a similar model, see the γγjj page.

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