Measuring the Fierz Interference Term in Beta-Decay using Ultracold Neutrons

It is theorized that contributions to the Fierz interference term from scalar interactions beyond the Standard Model could be detectable in the spectrum of neutron beta-decay, manifest as a nonzero value for the so-called $b$ parameter. Some supersymmetric models could have $b$ as large as $10^{-3}$, which is within reach for measurement, but below the current limits set by superallowed $0+ \to 0+$ nuclear $\beta$-decays. We present a new experiment that uses the Ultracold Neutron (UCN) source at LANSCE in which UCN, which can be trapped inside material bottles, are guided to a $4\pi$ scintillator beta calorimeter, shielded from background generated by the pulsed spallation target.