A new electron scattering experiment (E12-21-003) to verify and understand
the nature of hidden sector particles, with particular emphasis on the
so-called X17 particle, has been approved at Jefferson Lab. The search for
these particles is motivated by new hidden sector models introduced to account
for a variety of experimental and observational puzzles: excess in e+eβ
pairs observed in multiple nuclear transitions, the 4.2Ο disagreement
between experiments and the standard model prediction for the muon anomalous
magnetic moment, and the small-scale structure puzzle in cosmological
simulations. The aforementioned X17 particle has been hypothesized to account
for the excess in e+eβ pairs observed from the 8Be M1, 4He M0, and,
most recently, 12C E1 nuclear transitions to their ground states observed
by the ATOMKI group. This experiment will use a high resolution electromagnetic
calorimeter to search for or set new limits on the production rate of the X17
and other hidden sector particles in the 3β60 MeV mass range via their
e+eβ decay (or Ξ³Ξ³ decay with limited tracking). In these
models, the 1β100 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 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 3β60 MeV
mass range with a sensitivity of 8.9Γ10β8 - 5.8Γ10β9 to
Ο΅2, the square of the kinetic mixing interaction constant between
hidden and visible sectors.Comment: 6 pages, 7 figures. arXiv admin note: substantial text overlap with
arXiv:2108.1327