Isospin Breaking in the Nucleon Isovector Axial Charge from QCD Sum Rules

Abstract

The isospin breaking in the nucleon isovector axial charge, $g_A^3$, are calculated within the external field QCD sum-rule approach. The isospin violations arising from the difference in up and down current quark masses and in up and down quark condensates are included; electromagnetic effects are not considered. We find $\delta g_A^3/g_A^3\approx (0.5-1.0)\times 10^{-2}$, where $\delta g_A^3=(g_A^3{)}_p+(g_A^3{)}_n$ and $g_A^3=[(g_A^3{)}_p-(g_A^3{)}_n]/2$. Using the Goldberger-Treiman relation, we also obtain an estimate of the isospin breaking in the pion-nucleon coupling constant, $(g_{pp{\pi }_0}-g_{nn{\pi }_0})/g_{NN\pi }\approx (2-7)\times 10^{-3}$.

Comments: 11 pages in RevTeX including one PS figure, uses epsf.sty and rotate.sty (included), uuencoded