  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.
