Flavour - singlet g A and the QCD sum rule incorporating instanton effects Tetsuo NISHIKAWA

Flavour-singlet gA and the QCD sum rule incorporating instanton effects Tetsuo NISHIKAWA Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, 1-1, Ooho, Tsukuba, Ibaraki, 305-0801, Japan (Dated: February 2, 2008) Abstract We derive a QCD sum rule for the flavour-singlet axial coupling constant g (0) A from a two point correlation function of flavour-singlet axial vector currents in a one-nucleon state. In evaluating the correlation function by an operator product expansion we take into account the terms up to dimension 6. This correlation function receives an additional two-loop diagram which comes from an (anti-)instanton. If we do not include it, g (0) A is estimated to be 0.8. However, the additional diagram due to instantons contributes negatively and reduces g (0) A towards the experimental value.