Determination of the Weak Axial Vector Coupling $\lambda \mathbf{=}{g}_A/g_V$ from a Measurement of the $\beta$-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 $A_0=-0.11972(45{)}_{\mathrm{stat}}{(}_{-44}^{+32}{)}_{\mathrm{sys}}=-0.11972{(}_{-65}^{+53})$ and the ratio of the axial vector to the vector coupling constant $\lambda =g_A/g_V=-1.2761(12{)}_{\mathrm{stat}}{(}_{-12}^{+9}{)}_{\mathrm{sys}}=-1.2761{(}_{-17}^{+14})$.

Figure 1

Neutron polarization in the center of the Perkeo II beam. The ${}^3\mathrm{He}$ cells had lengths of 25 or 10 cm. In the legend, the effective pressures for a 10 cm cell are given.

Figure 2

Difference of the spectra with the second beam line shutter closed and with the first beam line shutter closed, a measure for the background produced by the collimation system. The main part of the background is low energetic.

Figure 3

Neutron ${\beta }^{-}$ spectra after adding or subtracting opposite neutron spin orientations $N^{\uparrow }$ and $N^{\downarrow }$ for detector 1 and detector 2, respectively. The solid curves show fits in the indicated ranges, and the dotted curves show their extrapolations. The spectra ($N^{\uparrow }-N^{\downarrow }$) are intrinsically free of (spin-independent) background.

Figure 4

Fit to the experimental asymmetry $A_{\exp }$ for detector 1 (${\chi }^2/\mathrm{dof}=64/83$) and detector 2 (${\chi }^2/\mathrm{dof}=82/81$), with solid and dotted curves as in Fig. 3.