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A new electron scattering experiment (E12-21-003) to verify and understandthe nature of hidden sector particles, with particular emphasis on theso-called X17 particle, has been approved at Jefferson Lab. The search forthese particles is motivated by new hidden sector models introduced to accountfor a variety of experimental and observational puzzles: excess in $e^+e^-$pairs observed in multiple nuclear transitions, the 4.2$\sigma$ disagreementbetween experiments and the standard model prediction for the muon anomalousmagnetic moment, and the small-scale structure puzzle in cosmologicalsimulations. The aforementioned X17 particle has been hypothesized to accountfor the excess in $e^+e^-$ pairs observed from the $^8$Be M1, $^4$He M0, and,most recently, $^{12}$C E1 nuclear transitions to their ground states observedby the ATOMKI group. This experiment will use a high resolution electromagneticcalorimeter to search for or set new limits on the production rate of the X17and other hidden sector particles in the $3 - 60$ MeV mass range via their$e^+e^-$ decay (or $\gamma\gamma$ decay with limited tracking). In thesemodels, the $1 - 100$ MeV mass range is particularly well-motivated and thelower part of this range still remains unexplored. Our proposed directdetection experiment will use a magnetic-spectrometer-free setup (the PRadapparatus) to detect all three final state particles in the visible decay of ahidden sector particle for an effective control of the background and willcover the proposed mass range in a single setting. The use of thewell-demonstrated PRad setup allows for an essentially ready-to-run anduniquely cost-effective search for hidden sector particles in the $3 - 60$ MeVmass 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 betweenhidden and visible sectors.

A new direct detection electron scattering experiment to search for the X17 particle