Determination of the Axial-Vector Weak Coupling Constant with Ultracold Neutrons

A precise measurement of the neutron decay $\beta$ asymmetry $A_0$ has been carried out using polarized ultracold neutrons from the pulsed spallation ultracold neutron source at the Los Alamos Neutron Science Center. Combining data obtained in 2008 and 2009, we report $A_0=-0.11966\pm 0.000{89}_{-0.00140}^{+0.00123}$, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon $g_A/g_V=-1.275{90}_{-0.00445}^{+0.00409}$.

Figure 1

Panel (a): schematic of the experiment and definition of the asymmetry. Panel (b): background subtracted electron $E_{\mathrm{recon}}$ spectrum (solid circles with the uncertainty of $E_{\mathrm{recon}}$ reflected by the horizontal error bars), combining both sides of the detectors and two spin states, overlaid with the Monte Carlo spectrum (histogram). The open circles represent the measured ambient background spectrum. Panel (c): $A_0$ vs $E_{\mathrm{recon}}$, combining all four geometries. The horizontal line represents the extracted $A_0$ within [275, 625] keV. Drawn on the same scale below the graph are four sets of bands representing the sum of energy-dependent backscattering and angle effect corrections (light color), the positive sign indicating a larger $|A_0|$, and their uncertainties (dark) for the four geometries. The positive (negative) correction at low (high) energy is a consequence of the backscattering (angle effect) dominating in this energy region. The number of $\beta$ events for each geometry is also indicated in the figure.

Figure 2

Ideogram comparing the previous measurements that are included in the 2009 Particle Data Group best value for $A_0$ along with the new result reported here. The dashed line is the ideogram with the previous four data points, and the solid line shows the result of including the present data.