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In our quest to understand the nature of dark matter and discover itsnon-gravitational interactions with ordinary matter, we propose an experimentusing a \pbo ~calorimeter to search for or set new limits on the productionrate 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) thehypothetical X17 particle, claimed in multiple recent experiments. The searchfor these particles is motivated by new hidden sector models and dark mattercandidates introduced to account for a variety of experimental andobservational puzzles: the small-scale structure puzzle in cosmologicalsimulations, anomalies such as the 4.2$\sigma$ disagreement between experimentsand the standard model prediction for the muon anomalous magnetic moment, andthe excess of $e^+e^-$ pairs from the $^8$Be M1 and $^4$He nuclear transitionsto 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 thisrange still remains unexplored. Our proposed direct detection experiment willuse a magnetic-spectrometer-free setup (the PRad apparatus) to detect all threefinal state particles in the visible decay of a hidden sector particle allowingfor an effective control of the background and will cover the proposed massrange in a single setting. The use of the well-demonstrated PRad setup allowsfor an essentially ready-to-run and uniquely cost-effective search for hiddensector particles in the $3 - 60$ MeV mass range with a sensitivity of8.9$\times$10$^{-8}$ - 5.8$\times$10$^{-9}$ to $\epsilon^2$, the square of thekinetic mixing interaction constant between hidden and visible sectors. Thisupdated proposal includes our response to the PAC49 comments.

A Direct Detection Search for Hidden Sector New Particles in the 3-60 MeV Mass Range