Contact Person(s)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 MotivationHere, 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 < mh/2 while keeping winos and gluinos sufficiently heavy to avoid constraints. However, more general models of gauge mediation  can allow this spectrum to be realized .
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 mχ1 and mχ2 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 cij. Previous study of this topology in the MSSM has been performed in  and, for the heavier MSSM Higgses, in .
- 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 ma, 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 >> λ11,22 where λ11,22 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 ma1 ≅ ma2 ≡ ma, 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 sFμνFμν.1 The decay can proceed through h→χ1χ2, and χ2→ sχ1, s→γγ if there is also a Yukawa interaction between s and χ1,2,given by y12s(χ2χ1+χ2†χ1†).
Existing Experimental Searches and LimitsWe 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  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  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 mχ1 in the non-resonant topology and ma in the resonant topology. In Fig. 2 (right) we show the reach in the case of the cascade topology as a function of ms, setting mχ1=0 and mχ2=60 GeV. We find that the limit obtained in this case is not very sensitive to the value of mχ2=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.
Figure 1: Approximate 95% C.L. upper limit on (σ/σSM) ×Br(h→ χ2χ2→ 2γ+MET) from the results of Ref. , for multiple values of mχ2 as indicated by the text labeling the different curves. Solid lines correspond to 100% photon efficiency, and dashed lines to a (flat) 80% photon efficiency.
Figure 2: Approximate 95% C.L. upper limit on (σ/σSM)× Br(h→ aa→ 2γ+MET) from the 2γ+MET search in Ref. . The solid line corresponds to 100% photon efficiency, and the dashed line to a (flat) 80% photon efficiency. Left: Resonant case, where h→ aa, one a decays to γγ and the other decays invisibly. Right: Cascade case, where h→χ1χ2, χ2→ sχ1, s→γγ. Here mχ1=0 and mχ2=60 GeV (the limit is insensitive to the particular value of mχ2 as long as it is kinematically allowed).
 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]. CMS Collaboration, Search for supersymmetry in events with photons and missing energy.
Footnotes:1The s FμνFμν 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 FμνFμν 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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