Contact Person(s)

Stefania Gori, Jessie Shelton, and Tao Liu More details on this mode may be found in Section 14 of Survey of Exotic Higgs Decays (arXiv:1312.4992).

Theoretical Motivation

h → 4 l+ MET decays, where the leptons are isolated, can arise through the following decay topology : h → X₁ X₂ followed by X₁→ l⁺l⁻ (+ MET), X₂ → l⁺l⁻ + MET, where X_1,2 may or may not be distinct species. Several classes of models can give rise to Higgs decays to 4 isolated leptons +MET :

• (N)MSSM like realizations : h → χ₂ χ₂ followed by χ₂ → χ₁ + l⁺ l⁻, where χ₁, χ₂ are neutralinos with χ₁ being stable (at collider time scales) and the lepton pairs are produced from an off-shell Z boson.
• Fourth-generation neutrino models : Higgs decays are exactly the same as in the (N)MSSM case but the states χ₂, χ₁ are instead the two Majorana-split halves of a Dirac neutrino state [1].
• Hidden sector models : The decay topology is exactly the same as in the previous two cases, but the role of χ₂ and χ₁ is played by meson-like pseudo-scalar states K_v and π_v respectively, the latter being stable. The three body decay in this case is mediated by a kinematically mixed dark photon (see Section SM + Vector). Such decays can also be realized in supersymmetric hidden sector models [2,3].
• Other models with non-trivial flavor structure : Higgs decays into R-parity violating neutralinos can lead to the same final state but the lepton pairs need not be the same flavor. Such a scenario also occurs when the Higgs decays into unstable heavy neutrinos N, which then decay through N→ W^*l→ νl′l [4].

Existing Experimental Constraints

A detailed collider study for the decay channel h→χ₂χ₂→ 4lχ₁χ₁ (where χ₁ is stable and appears as MET) using existing LHC multi-lepton + MET searches can be found in Section 14.2 of arXiv:1312.4992, however the main results are summarized as follows. The bounds for the decay channel under consideration have a strong dependence on the exotic spectrum. To highlight this fact, the authors consider two benchmark scenarios :

• An "optimistic" benchmark scenario with relatively large mass splitting between χ₂ and χ₁, with M₂=55 GeV and M₁=20 GeV.
• A "pessimistic" benchmark scenario with a smaller mass splitting, M₂ = 55 GeV and M₁=35 GeV so that the final state leptons are much softer.

The best limit on the optimistic benchmark, set by recasting the 19.6 fb⁻¹ search performed by CMS in four-lepton final states [5], is Br(h→ 4l+MET) < 1.2×10⁻³, with l = (e,μ,τ) with main contributions from gluon fusion and VBF production modes. This translates into a limit of 5.4×10⁻⁴ if we include only the decay into electron and muons. For the pessimistic benchmark, another CMS multi-lepton search [6] sets the bound : Br(h→ 4l+MET) < 0.011, with l = (e,μ,τ), with the reach almost entirely set by VBF production. As the mass splitting between χ₂ and χ₁ is reduced, VBF and Wh associated production become more important relative to gluon fusion.

Related Decay Modes

Exotic Higgs decays of the kind h → X₁ X₂ can lead to a plethora of experimental signatures depending on the decay of X₂ and/or X₁ : h → 2 ℓ + MET , h → 1 lepton-jet + X, h → 2 lepton-jets + X and  h → 4 ℓ.

Additional Remarks

Since the exotic Higgs decay is invariably a lower-p_T signal than most BSM signals sought in multi-lepton searches, the authors suggest that searches should place only weak requirements on MET, with the transverse m_T (4l, MET) a potentially important variable for distinguishing signal from background.  Searches would also gain sensitivity if optimized for the relatively low-p_T kinematic range characteristic of SM Higgs production.   Alternatively, searches specifically tailored for VBF production, in conjuction with VBF jet cuts, may be promising.

References

[1]W.-Y. Keung and P. Schwaller, Long Lived Fourth Generation and the Higgs, JHEP 1106 (2011) 054, [arXiv:1103.3765].

[2]M. Baumgart, C. Cheung, J. T. Ruderman, L.-T. Wang, and I. Yavin, Non-Abelian Dark Sectors and Their Collider Signatures, JHEP 0904 (2009) 014, [arXiv:0901.0283].

[3]Y. F. Chan, M. Low, D. E. Morrissey, and A. P. Spray, LHC Signatures of a Minimal Supersymmetric Hidden Valley, JHEP 1205 (2012) 155, [arXiv:1112.2705].

[4]L. M. Carpenter and D. Whiteson, Higgs Decays to Unstable Neutrinos: Collider Constraints from Inclusive Like-Sign Dilepton Searches, [arXiv:1107.2123].

[5]Search for rpv susy in the four-lepton final state, Tech. Rep. CMS-PAS-SUS-13-010, CERN, Geneva, 2013.

[6]CMS Collaboration, Search for RPV Supersymmetry with Three or More Leptons and b-Tags, CMS-PAS-SUS-12-027.

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