Compton Polarimetry for Neutral Weak Form Factor Measurements in 208Pb and 48Ca

Atomic nuclei contain a number of protons and neutrons. Most heavy nuclei contain more neutrons than protons in order to remain stable. The nuclear proton distribution can be measured via elastic scattering of charged particles, but the neutron distribution remains poorly constrained. Nuclear structure theory also predicts a "neutron skin" ΔRnp where the mean radii of neutrons in the nucleus exceeds that of protons. Two experiments were run in 2019 and 2020 to measure the neutron skin in two different nuclei. The first experiment, PREX-II, was designed to measure the neutron skin in 208Pb, while the second experiment, CREX, was designed to measure the neutron skin in 48Ca. Both experiments employed the parity-violating electron scattering (PVES) technique in which longitudinally spin-polarized electrons are scattered off an unpolarized target. The parity-violating asymmetry Apv is then measured which is the normalized difference of scattering cross section from electrons in alternating positive and negative helicity states. PREX-II performed this experiment with 950 MeV electrons scattered at a 5° angle with Q^2=0.00616 +/- 0.00004 (GeV/c)^2, while CREX used 2182 MeV electrons at the same angle with Q^2=0.0297 +/- 0.0002 (GeV/c)^2.

One of the potentially largest sources of systematic uncertainty for both experiments arises from the measurement precision of the beam polarization Pe. To accurately measure Pe a Compton scattering polarimeter was used. The electron beam for the main experiment was passed through a resonant laser optical cavity. The Compton backscattered photons then entered the photon detector where their asymmetry was measured yielding Pe. The Compton polarimetry data from PREX-II lacked the statistical precision necessary to make a final polarimetry measurement. However, for CREX the Compton polarimetry measurement was Pe=(87.115 +/- 0.453)% with negligible contribution from statistical uncertainty.

The measurement of Pe was then incorporated into both experiments' calculation of Apv. Additionally, both experiments used a dynamic beam correction system to correct for any sources of false asymmetry. For PREX-II the asymmetry was Apv=550 +/- 16 (stat) +/- (syst) ppb, which corresponds to ΔRnp(208)=0.278 +/- 0.078 (exp.) +/- 0.012 (theo.) fm. The CREX asymmetry was Apv=2658.6 +/- 106.1 (stat) +/- 39.3 (syst) ppb, although the analysis for ΔRnp(48) is not yet complete. The results of these two experiments will have significant implications for the theory of nuclear structure, and even for the nuclear equation of state which governs neutron stars.