Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
1996-04-15
Acta Phys.Polon.B27:1439-1446,1996
Physics
High Energy Physics
High Energy Physics - Phenomenology
8 pp, standard LaTeX; Proceedings of the Cracow Epiphany Conference on Proton Structure, Cracow, Jan.\ 5--6, 1996, to appear i
Scientific paper
We discuss the evaluation of the nucleon isoscalar axial coupling, $g_A^{(0)}$, in the instanton vacuum, using the $1/N_c$ expansion. This approach allows a fully consistent treatment of the $U(1)_A$ anomaly. We compute the nucleon matrix element of the topological charge density, $\langle N | F\widetilde{F} | N \rangle$, and show that it reduces to the matrix element of the divergence of the isoscalar axial quark current. Our arguments show that the usual evaluation of $g_A^{(0)}$ in the chiral quark soliton model is consistent with the $U(1)_A$ anomaly in leading order of $1/N_c$. Such calculations give $g_A^{(0)} = 0.36$, which is in agreement with the recent estimate by Ellis and Karliner.
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