Recoil-order and radiative corrections to the aCORN experiment

The aCORN experiment measures the electron-antineutrino correlation a coefficient in free neutron decay. We update the previous aCORN results [G. Darius et al., Phys. Rev. Lett. 119, 042502 (2017) and M. T. Hassan et al., Phys. Rev. C 103, 045502 (2021)] to include radiative and recoil corrections to first order, and discuss a key issue in the comparison of results from different a-coefficient experimental methods when these effects are considered. The corrected combined result is ā=-0.10779±0.001250.16em0ex(stat)±0.001340.16em0ex(sys), averaged over the full Fermi neutron beta spectrum. The corresponding corrected result for the ratio of weak coupling constants λ=GA/GV is λ=-1.2712±0.0061, and the corresponding value of a₀, useful for comparing to proton recoil measurements, is a₀=-0.1053±0.0018. This improves agreement with previous a-coefficient experiments, in particular the 2020 aSPECT result [M. Beck et al., Phys. Rev. C 101, 055506 (2020)].