Limit on the Fierz Interference Term $b$ from a Measurement of the Beta Asymmetry in Neutron Decay

In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Nonzero scalar or tensor interactions would imply an additional contribution to the differential decay rate of the neutron, the Fierz interference term. We derive a limit on this hypothetical term from a measurement using spin-polarized neutrons. This method is statistically less sensitive than the determination from the spectral shape but features much cleaner systematics. We obtain a limit of $b=0.017(21)$ at 68.27% C.L., improving the previous best limit from neutron decay by a factor of four.

Figure 1

Example of measured calibration spectra and the simultaneous fit performed to extract the free parameters of the detector response model. The dashed vertical lines indicate the fit regions. The spectra are practically free of $\gamma$ background from the sources. The discrepancies at low energy outside the fit range mostly stem from uncertainties in the exact shape of the trigger function. We only show every other data point to improve the presentation.

Figure 2

Fit to the data at the chopper frequency of 83 Hz measured by the downstream detector. The fit includes theoretical corrections as well as the full model of the detector response. Corrections for detector nonuniformity and undetected backscattering are not included. The data is rebinned by a factor of 2 for presentation. The fit range is indicated by the dashed vertical lines.

Figure 3

68.27% confidence region for the beta asymmetry parameter $A$ and Fierz interference parameter $b$ and the contributions by statistics and systematics.