Improved determination of the β-(ν)over-bare angular correlation coefficient a in free neutron decay with the aSPECT spectrometer

We report on a precise measurement of the electron-antineutrino angular correlation (a coefficient) in free neutron beta decay from the aSPECT experiment. The a coefficient is inferred from the recoil energy spectrum of the protons which are detected in 4 pi by the aSPECT 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 = -0.104 30(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 vertical bar lambda vertical bar = 1.2677(28).