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 $h \to \phi \phi \to Z'Z'Z'Z'$ with new scalar boson $\phi$ 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 $\phi$ are smaller than ${\cal O}$(10) GeV, because of the highly-boosted effects. Performing the signal and background analyses, we find that the branching ratio of $h \to 4Z'$ 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 $Z'$ ($< 2m_\mu$), 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 $\phi$. 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 $h \to Z'Z'$ in the mass range of $m_{Z'}\lesssim 10$ GeV.