Axial coupling constant in a magnetic background

The nucleon axial coupling constant $g_A$ is calculated in the presence of an external uniform magnetic field using finite energy sum rules. The correlation function of proton, neutron and axial-vector currents is calculated both the hadronic and the QCD sector. Once the axial contribution in the form factor is isolated, a double sum rule is considered, i.e., the usual QCD contour integration for the external moments of both the proton and neutron current. The effects of the external magnetic field come mainly through the in-medium current-nucleon coupling constants $\lambda_N(B)$ and the hadronic thresholds $s_0(B)$ provided by the nucleon-nucleon correlators. As a result, the axial coupling constant decreases in the presence of the magnetic field.