Search for Higgs boson decays to beyond-the-Standard-Model light bosons in four-lepton events with the ATLAS detector at $\sqrt{s}$ = 13 TeV

A search is conducted for a new beyond-the-Standard-Model boson using events where a Higgs boson with mass 125 GeV decays to four leptons (l = e or $\mu$). This decay is presumed to occur via an intermediate state which contains one or two on-shell, promptly decaying bosons: H $\rightarrow$ ZX/XX $\rightarrow$ 4l, where X is a new vector boson Zd or pseudoscalar a with mass between 1 and 60 GeV. The search uses proton-proton collision data collected with the ATLAS detector at the LHC during 2015 and 2016, with an integrated luminosity of 36.1 $fb^{-1}$ at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. No significant excess of events above Standard Model background predictions was observed; therefore, upper limits at 95 \% confidence level are set on model-independent fiducial cross-sections, and on the Higgs boson decay branching ratios to vector and pseudoscalar bosons in two benchmark models.