Non-perturbative determination of the axial current normalization   constant in O(a) improved lattice QCD
  A finite-size technique is employed to compute the normalization constant $Z_A$ of the isovector axial current in lattice QCD. The calculation is carried out in the quenched approximation for values of the bare gauge coupling $g_0$ ranging from 0 to 1. In the lattice action and the lattice expression for the axial current we include the counterterms required for O(a) improvement, with non-perturbatively determined coefficients. With little additional work the normalization constant $Z_V$ of the improved isospin current is also obtained. 
Martin L\"uscher, Stefan Sint, Rainer Sommer and Hartmut Wittig