  The PADME experiment at the DA$\Phi$NE Beam-Test Facility (BTF) of the INFN
Laboratory of Frascati is designed to search for invisible decays of dark
sector particles produced in electron-positron annihilation events with a
positron beam and a thin fixed target, by measuring the missing mass of
single-photon final states. The presence of backgrounds originating from beam
halo particles can significantly reduce the sensitivity of the experiment. To
thoroughly understand the origin of the beam background contribution, a
detailed Geant4-based Monte Carlo simulation has been developed, containing a
full description of the detector together with the beam line and its optical
elements. This simulation allows the full interactions of each particle to be
described, both during beam line transport and during detection, a possibility
which represents an innovative way to obtain reliable background predictions
