We report on a new measurement of neutron $\beta$-decay asymmetry. From the result $A_0=-0.1189\mathrm{}\left(7\right)$, we derive the ratio of the axial vector to the vector coupling constant $\lambda {=g}_A{/g}_V=-1.2739\mathrm{}\left(19\right)$. When included in the world average for the neutron lifetime $\tau =885.7\mathrm{}\left(7\right)\mathrm{s}$, this gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix $V_{\mathrm{ud}}$. With this value and the Particle Data Group values for $V_{\mathrm{us}}$ and $V_{\mathrm{ub}}$, we find a deviation from the unitarity condition for the first row of the CKM matrix of $\Delta =0.0083\mathrm{}\left(28\right)$, which is 3.0 times the stated error.

• Received 18 July 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.211801