Improved determination of the π½ββΎβΎππ angular correlation coefficient π in free neutron decay with the πβ’SPECT spectrometer
We report on a precise measurement of the electron-antineutrino angular correlation (π coefficient) in free neutron beta decay from the πβ’SPECT experiment. The π coefficient is inferred from the recoil energy spectrum of the protons which are detected in 4β’π by the πβ’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 π=β0.10430β’(84) which is the most precise measurement of the neutron π coefficient to date. From this, the ratio of axial vector to vector coupling constants is derived giving |π|=1.2677β’(28).