Precision measurement of the neutron $\beta$-decay asymmetry

A new measurement of the neutron $\beta$-decay asymmetry $A_0$ has been carried out by the UCNA Collaboration using polarized ultracold neutrons (UCNs) from the solid deuterium UCN source at the Los Alamos Neutron Science Center. Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to $A_0=-0.11954{\left(55\right)}_{\mathrm{stat}}{\left(98\right)}_{\mathrm{syst}}$, corresponding to the ratio of axial-vector to vector coupling $\lambda \equiv g_A/g_V=-1.2756\left(30\right)$.

Figure 1

Top: background subtracted electron energy spectrum, combining both detector sides and spin states, overlaid with the Monte Carlo prediction. The measured background spectrum is also shown. Middle: $A_0$ vs $E_{\mathrm{recon}}$, shown with statistical error bars, and fit to a constant from 220 to 670 keV. Bottom: corrections and their uncertainties (band) excluding polarization and theory contributions; positive sign indicating a larger $|A_0|$.

Figure 2

Ideogram of values for $A_0$ from this work (filled square) and recent measurements (open circles) [7, 9, 28, 29, 30, 31, 32], arranged by year of publication. To account for correlated systematic errors in sequential measurements, the ideogram (solid curve) was constructed using the combined result from [31] and [32] of $-0.11951\left(50\right)$ reported in [32], and the combined result of [7, 9] and this work of $-0.11956\left(110\right)$, as discussed in the text. The gray band indicates the PDG 2012 average value of $A_0=-0.1176\left(11\right)$ [33], which includes the results of [7, 9, 28, 29, 30, 31], but does not include [32] or the work reported here.

Figure 3

Light quark weak coupling $V_{\mathrm{ud}}$ vs $\lambda$. $V_{\mathrm{ud}}=0.97425\left(22\right)$ from $0^{+}\to 0^{+}$ decays and the neutron lifetime ${\tau }_n=880.1\left(1.1\right)$ s are from PDG 2012 [33]. Values of $\lambda$ are the UCNA result from this paper, and the perkeo ii combined result $\lambda =-1.2755\left(13\right)$ from [32].