  In our quest to understand the nature of dark matter and discover its
non-gravitational interactions with ordinary matter, we propose an experiment
using a \pbo ~calorimeter to search for or set new limits on the production
rate of i) hidden sector particles in the $3 - 60$ MeV mass range via their
$e^+e^-$ decay (or $\gamma\gamma$ decay with limited tracking), and ii) the
hypothetical X17 particle, claimed in multiple recent experiments. The search
for these particles is motivated by new hidden sector models and dark matter
candidates introduced to account for a variety of experimental and
observational puzzles: the small-scale structure puzzle in cosmological
simulations, anomalies such as the 4.2$\sigma$ disagreement between experiments
and the standard model prediction for the muon anomalous magnetic moment, and
the excess of $e^+e^-$ pairs from the $^8$Be M1 and $^4$He nuclear transitions
to their ground states observed by the ATOMKI group. In these models, the $1 -
100$ MeV mass range is particularly well-motivated and the lower part of this
range still remains unexplored. Our proposed direct detection experiment will
use a magnetic-spectrometer-free setup (the PRad apparatus) to detect all three
final state particles in the visible decay of a hidden sector particle allowing
for an effective control of the background and will cover the proposed mass
range in a single setting. The use of the well-demonstrated PRad setup allows
for an essentially ready-to-run and uniquely cost-effective search for hidden
sector particles in the $3 - 60$ MeV mass range with a sensitivity of
8.9$\times$10$^{-8}$ - 5.8$\times$10$^{-9}$ to $\epsilon^2$, the square of the
kinetic mixing interaction constant between hidden and visible sectors. This
updated proposal includes our response to the PAC49 comments.
