Improved neutron lifetime measurement with UCN$\tau$

Abstract

We report an improved measurement of the free neutron lifetime ${\tau }_n$ using the UCN$\tau$ apparatus at the Los Alamos Neutron Science Center. We counted a total of approximately $38\times 10^6$ surviving ultracold neutrons (UCN) after storing in UCN$\tau$'s magneto-gravitational trap over two data acquisition campaigns in 2017 and 2018. We extract ${\tau }_n$ from three blinded, independent analyses by both pairing long and short storage-time runs to find a set of replicate ${\tau }_n$ measurements and by performing a global likelihood fit to all data while self-consistently incorporating the $\beta$-decay lifetime. Both techniques achieve consistent results and find a value ${\tau }_n=877.75\pm 0.28_{\text{ stat}}+0.22/-0.16_{\text{ syst}}$~s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.

Comments: 7 pages, 2 figures, 3 tables