Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam

We present a precision measurement of the axial-vector coupling constant $g_A$ in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain $\lambda =g_A/g_V=-1.27641(45{)}_{\mathrm{stat}}(33{)}_{\mathrm{sys}}$, which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of $A_0=-0.11985(17{)}_{\mathrm{stat}}(12{)}_{\mathrm{sys}}$. We discuss implications on the Cabibbo-Kobayashi-Maskawa matrix element $V_{ud}$ and derive a limit on left-handed tensor interaction.

Figure 1

Schematic of the spectrometer setup Perkeo iii installed at PF1B. The magnetic field is indicated in red. The cold neutron beam from the reactor source enters the beam line on the left-hand side. Picture from [35].

Figure 2

Number of decay events in the electron energy window 300 to 700 keV as a function of time of flight of the neutron pulse through the spectrometer for a chopper frequency of 83 Hz. Background is subtracted. The shape of the curves for the detectors is caused by the magnetic mirror effect. The vertical lines indicate the time windows used for signal and background extraction. (Inset) Enlargement of the background time window.

Figure 3

Measured asymmetry $A_{\exp }$ for both detectors as a function of time of flight of the neutron pulse. When averaging over both detectors, the dominant effect of the magnetic mirror effect cancels and only a small correction remains, see Table 1. The vertical dashed lines indicate the time window used for the asymmetry analysis, and the curves indicate the calculated mirror effect.

Figure 4

The experimental asymmetry $|A_{\exp ,i}(E_e)|$ and fits with a single free parameter $\lambda$ for both detectors and both chopper frequencies. The solid part of the line indicates the fit interval that was chosen to balance statistics and effects due to detector nonlinearity and unrecognized backscattering.