Abstract
Searches are conducted for new spin-0 or spin-1 bosons using events where a
Higgs boson with mass $125$ GeV decays into four leptons ($\ell =$ $e$,$\mu$).
This decay is presumed to occur via an intermediate state which contains two
on-shell, promptly decaying bosons: $H \rightarrow XX/ZX \rightarrow 4\ell$,
where the new boson $X$ has a 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 139 fb$^{-1}$ at a centre-of-mass energy $\sqrt{s}=13$ TeV. The
data are found to be consistent with Standard Model expectations. Limits are
set on fiducial cross sections and on the branching ratio of the Higgs boson to
decay into $XX/ZX$, improving those from previous publications by a factor
between two and four. Limits are also set on mixing parameters relevant in
extensions of the Standard Model containing a dark sector where $X$ is
interpreted to be a dark boson.