Searching for Fierz Interference in Beta-Decay of Ultracold Neutrons

It is theorized that contributions from scalar and tensor interactions from physics beyond the Standard Model could be detectable in the spectrum of neutron beta-decay, manifest as a nonzero value for the so-called neutron Fierz interference parameter, denoted as $b_n$. Some models have $b_n$ as large as $10^{-3}$, which is within reach for measurement, but below the current limits set for the Fermi component $b_F$ measured by superallowed $0+ \to 0+$ nuclear $\beta$-decays. We present progress on the UCNb experiment that uses the Ultracold Neutron (UCN) source at LANSCE to trap UCN in a $4\pi$ beta calorimeter. We also report limits on $b_n$ extracted from data collected from the 2010 run of the UCNA experiment.