Measurement of the Axial-Vector Coupling Constant $g_A$ in Neutron Beta   Decay
  The matrix element \Vud of the CKM matrix can be determined by two independent measurements in neutron decay: the neutron lifetime $\tau_n$ and the ratio of coupling constants $\lambda=g_A/g_V$, which is most precisely determined by measurements of the beta asymmetry angular correlation coefficient~$A$. We present recent progress on the determination of these coupling constants. 
Bastian Maerkisch, Hartmut Abele
hep-ph