  This document complements and completes what was submitted last year to PAC45
as an update to the proposal PR12-16-001 "Dark matter search in a Beam-Dump
eXperiment (BDX)" at Jefferson Lab submitted to JLab-PAC44 in 2016. Following
the suggestions contained in the PAC45 report, in coordination with the lab, we
ran a test to assess the beam-related backgrounds and validate the simulation
framework used to design the BDX experiment. Using a common Monte Carlo
framework for the test and the proposed experiment, we optimized the selection
cuts to maximize the reach considering simultaneously the signal, cosmic-ray
background (assessed in Catania test with BDX-Proto) and beam-related
backgrounds (irreducible NC and CC neutrino interactions as determined by
simulation). Our results confirmed what was presented in the original proposal:
with 285 days of a parasitic run at 65 $\mu$A (corresponding to $10^{22}$ EOT)
the BDX experiment will lower the exclusion limits in the case of no signal by
one to two orders of magnitude in the parameter space of dark-matter coupling
versus mass.
