Multi-lepton jets from quadruple Z′ via the Higgs decay at LHC

We investigate multi-lepton jet events from the decay of the 125 GeV Higgs boson (h) into quadruple new gauge bosons (Z′) at the LHC. Such an exotic decay is realized via the process of h → ϕϕ → Z′Z′Z′Z′ with new scalar boson ϕ in models with an additional U(1) gauge symmetry. Charged leptons coming from the Z′ decay tend to be observed as lepton-jets rather than isolated leptons when the masses of Z′ and ϕ are smaller than O(10) GeV, because of the highly-boosted effects. Performing the signal and background analyses, we find that the branching ratio of h → 4Z′ is maximally constrained to be smaller than of order 10⁻⁶ (10⁻⁷) by using the muonic-lepton jets assuming the integrated luminosity of 140 fb⁻¹ (3000 fb⁻¹) at LHC. For lighter Z′ (< 2m_μ), we can use the electronic-lepton jets instead of the muon-jets, by which the upper limit on the branching ratio is obtained to be of order 10⁻⁶–10⁻⁵. These bounds can be converted into the constraint on model parameters such as a mixing angle between h and ϕ. It is shown that stronger bounds on the mixing angle are obtained in the dark photon case as compared with the previous constraints given by flavor experiments and the Higgs decay h → Z′Z′ in the mass range of mZ′ ≲ 10 GeV.