# h → 4ℓ (isolated) + MET

## 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 X1l+l (+ MET), X2l+l + MET, where X1,2 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 Kv and πv 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 NW*l→ νll [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.

Since the exotic Higgs decay is invariably a lower-pT 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 mT (4l, MET) a potentially important variable for distinguishing signal from background.  Searches would also gain sensitivity if optimized for the relatively low-pT kinematic range characteristic of SM Higgs production.   Alternatively, searches specifically tailored for VBF production, in conjuction with VBF jet cuts, may be promising.

Here we list new experimental results pertinent to this exotic higgs decay channel.
• ATLAS collaboration, Search for supersymmetry in events with four or more leptons in sqrt s = 8 TeV pp collisions with the ATLAS detector, arxiv:1405.5086
• .
• ATLAS collaboration, Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in sqrt s = 8TeV pp collisions with the ATLAS detector, arxiv:1402.7029
• .
• CMS collaboration, Search for anomalous production of events with three or more leptons in pp collisions at sqrt s = 8 TeV, arxiv:1404.5801
• .
• CMS collaboration, Searches for electroweak production of charginos, neutralinos, and sleptons decaying to leptons and W, Z, and Higgs bosons in pp collisions at 8 TeV, arxiv:1405.7570
• .

## 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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