Recoil-order and radiative corrections to the aCORN experiment

Abstract

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 ‾𝑎=−0.10779±0.00125⁢(stat)±0.00134⁢(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) ].