Analytic Form of the QCD Instanton Determinant for Small Quark Mass

Abstract

We use a novel method to calculate analytically the QCD instanton prefactor due to a quark field carrying a small mass parameter $m$. In the SU(2) instanton background of size $\rho$, the spinor effective action ${\Gamma }^F$ (in the minimal subtraction scheme), which gives rise to the prefactor $\exp (-{\Gamma }^F)$, is shown to have the small-$m\rho$ behavior \Gamma^F = -\ln (m /\mu) - \ln (\mu\rho)/3 -2 \alpha(1/2) -(m \rho)^2 \{\ln (m \rho /2) +\gamma+1/2\} -2 (m \rho)^4 \{-\ln ^2(m \rho)/4+\ln (m \rho) (1/2-\gamma +\ln 2)/2+C \} +O((m \rho)^6), where $\gamma =\mathrm{0.577216...}$, $\alpha (1/2)=\mathrm{0.145873...}$, and our numerically evaluated value for the constant $C$ is $C=-\mathrm{0.382727...}$. A good agreement between this form and the numerically exact calculation is found if $(m\rho )\lesssim 0.8$.

Comments: 25 pages, 2 figures