Nucleon axial charge in lattice QCD with controlled errors

We report on our calculation of the nucleon axial charge gA in QCD with two flavors of dynamical quarks. A detailed investigation of systematic errors is performed, with a particular focus on contributions from excited states to three-point correlation functions. The use of summed operator insertions allows for a much better control over such contamination. After performing a chiral extrapolation to the physical pion mass, we find gA=1.223±0.063(stat)_-0.060+0.035(syst), in good agreement with the experimental value.