Determination of the Weak Axial Vector Coupling λ = gA/gV from a Measurement of the β-Asymmetry Parameter A in Neutron Beta Decay

We report on a new measurement of the neutron beta-asymmetry parameter A with the instrument PERKEO II. The main enhancements are the high neutron polarization of P = 99.7(1)% from a novel arrangement of supermirror polarizers and reduced background from improvements in beam line and shielding. The leading corrections were thus reduced by a factor of 4, pushing them below the level of statistical error and resulting in a significant reduction of systematic uncertainty compared to our previous experiments. We derive the beta-asymmetry parameter A(0) = -0.11972(45)(stat)((+32)(-44))(sys) = -0.11972((+53)(-65)) and the ratio of the axial vector to the vector coupling constant lambda = g(A)/g(V) = -1.2761(12)(stat)((+9)(-12))(sys) = -1.2761((+14)(-17)). DOI: 10.1103/PhysRevLett.110.172502