Search for Higgs boson Decays to Beyond-the-Standard-Model Light Bosons in Four-Lepton Final States with the ATLAS Detector at the LHC

Hidden sector or dark sector states appear in many extensions to the Standard Model (SM), to provide particle mediators for dark matter in the universe. A new probe of this hypothetical hidden or dark sector may have become available at the energy frontier opened up by the LHC with the Higgs boson and its distinct couplings to SM particles. A search is conducted for a beyond-the-Standard-Model vector boson using events where a Higgs boson with mass 125 GeV decays to four leptons. This decay is presumed to occur via an intermediate state which contains one or two decaying light exotic bosons, $H \to Z(Z_d)Z_d \to 4l(l = e, \mu)$, where $Z_d$ is a new vector boson with mass between 1 and 60 GeV. The search uses pp collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 36 fb$^{-1}$ at the center- of-mass energy of 13 TeV. No significant excess of events above SM background predictions is observed; therefore, upper bounds on the branching ratios BR$(H \to Z_dZ_d \to 4l)$ are set as a function of the mass of the dark vector boson at 95% confidence level.