FLAVOR-SINGLET g_A FROM LATTICE QCD

We report on a lattice QCD calculation of the flavor-singlet axial coupling constant $g_A$ of the proton from the axial-vector current. The simulation is carried out at $\beta = 6$ on a quenched $16^3 \times 24$ lattice. An extrapolation to the chiral limit shows that the connected insertion (valence and cloud parts) is $0.62 \pm 0.09$, which is close to $g_A^8$. The disconnected insertion (vacuum polarization from the sea quark) for the $u$ or $d$ quark is $- 0.12\pm 0.01 $ and the $s$ quark also contributes $-0.12\pm 0.01$. The total $g_A^1$ is thus $0.25 \pm 0.12$ which is in agreement with experiments. In addition, we find $g_A^3 = 1.20 \pm 0.11$, $g_A^8 = 0.61 \pm 0.13$ and $F_A/D_A = 0.60 \pm 0.02$, also in agreement with experiments.