Improved Neutron Lifetime Measurement with UCNτ

Abstract

We report an improved measurement of the free neutron lifetime ${\tau }_n$ using the $\mathrm{UCN}\tau$ apparatus at the Los Alamos Neutron Science Center. We count a total of approximately $38\times {10}^6$ surviving ultracold neutrons (UCNs) after storing in $\mathrm{UCN}\tau$’s magnetogravitational 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 ${\tau }_n=877.75\pm 0.28_{\mathrm{stat}}+0.22/-0.16_{\mathrm{syst}}\mathrm{s}$. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.