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. Non-zero 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.