Measurement of the Electron-Antineutrino Angular Correlation in Neutron $\beta$ Decay

We report the first result for the electron-antineutrino angular correlation ($a$ coefficient) in free neutron $\beta$ decay from the aCORN experiment. aCORN uses a novel method in which the $a$ coefficient is proportional to an asymmetry in proton time of flight for events where the $\beta$ electron and recoil proton are detected in delayed coincidence. Data are presented from a 15 month run at the NIST Center for Neutron Research. We obtained $a=-0.1090\pm 0.0030(\mathrm{stat})\pm 0.0028(\mathrm{sys})$, the most precise measurement of the neutron $a$ coefficient reported to date.

Figure 1

The aCORN “wishbone” histogram plot of proton time of flight vs $\beta$ energy for delayed coincidence neutron decay events. Blue pixels are positive and red are negative (due to the background subtraction). Inset: Monte Carlo simulation.

Figure 2

A rendering of the aCORN apparatus indicating the major components and arrangement.

Figure 3

Top: The wishbone energy spectrum, a histogram of total wishbone events vs $\beta$ energy, fit to the theoretical $\beta$ spectrum modified by the aCORN momentum acceptances. Bottom: Fit residuals (data—fit). Error bars are statistical uncertainty.

Figure 4

Open circles: The measured, uncorrected, wishbone asymmetry $X(E)$ for each magnetic field direction. Solid points: The same data including the corrections ${\delta }_1(E)$, ${\delta }_2(E)$, and the energy-dependent systematic corrections. Error bars are statistical uncertainty. Solid curves: The product $a{f}_a(E)$, where $a$ is the best fit value of the $a$ coefficient in each case.