Improved determination of the 𝛽−‾‾𝜈𝑒 angular correlation coefficient 𝑎 in free neutron decay with the 𝑎⁢SPECT spectrometer

Abstract

We report on a precise measurement of the electron-antineutrino angular correlation (𝑎 coefficient) in free neutron beta decay from the 𝑎⁢SPECT experiment. The 𝑎 coefficient is inferred from the recoil energy spectrum of the protons which are detected in 4⁢𝜋 by the 𝑎⁢SPECT spectrometer using magnetic adiabatic collimation with an electrostatic filter. Data are presented from a 100-day run at the Institut Laue Langevin in 2013. The sources of systematic errors are considered and included in the final result. We obtain 𝑎=−0.10430⁢(84) which is the most precise measurement of the neutron 𝑎 coefficient to date. From this, the ratio of axial vector to vector coupling constants is derived giving |𝜆|=1.2677⁢(28).