This paper presents a search for Higgs bosons decaying to four leptons, either electrons or muons, via one or two light exotic gauge bosons Zd, H→ZZd→4 or H→ZdZd→4. The search was performed using pp collision data corresponding to an integrated luminosity of about 20 fb-1 at the center-of-mass energy of s=8 TeV recorded with the ATLAS detector at the Large Hadron Collider. The observed data are well described by the Standard Model prediction. Upper bounds on the branching ratio of H→ZZd→4 and on the kinetic mixing parameter between the Zd and the Standard Model hypercharge gauge boson are set in the range (1-9)×10-5 and (4-17)×10-2 respectively, at 95% confidence level assuming the Standard Model branching ratio of H→ZZ∗→4, for Zd masses between 15 and 55 GeV. Upper bounds on the effective mass mixing parameter between the Z and the Zd are also set using the branching ratio limits in the H→ZZd→4 search, and are in the range (1.5-8.7)×10-4 for 15<35 GeV. Upper bounds on the branching ratio of H→ZdZd→4 and on the Higgs portal coupling parameter, controlling the strength of the coupling of the Higgs boson to dark vector bosons are set in the range (2-3)×10-5 and (1-10)×10-4 respectively, at 95% confidence level assuming the Standard Model Higgs boson production cross sections, for Zd masses between 15 and 60 GeV.
Aad, G., Abbott, B., Abdallah, J., Abdinov, O., Aben, R., Abolins, M., et al. (2015). Search for new light gauge bosons in Higgs boson decays to four-lepton final states in pp collisions at s =8 TeV with the ATLAS detector at the LHC. PHYSICAL REVIEW D, PARTICLES, FIELDS, GRAVITATION, AND COSMOLOGY, 92(9) [10.1103/PhysRevD.92.092001].