Improved limits on Fierz interference using asymmetry measurements from the Ultracold Neutron Asymmetry (UCNA) experiment

The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the beta-decay asymmetry parameter, A(0), for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their beta-decay electrons are detected with approximate to 4 pi acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to b(n), the Fierz interference term in the neutron beta-decay rate. In this work, we determine b(n) from the energy dependence of A(0) using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 A dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new b(n) measurement using the weighted average of our asymmetry dataset fits, to obtain b(n) = 0.066 f 0.041(stat) +/- 0.024(syst )which corresponds to a limit of -0.012 < b(n) < 0.144 at the 90% confidence level.