Isospin Symmetry Breaking in the Quark Condensates, $<\overline {d}d> \neq <\overline {u}u>$, as Estimated from QCD Sum Rules

Isospin symmetry breaking in the quark condensates, $ \not= $, is a fundamental parameter in both the QCD sum rule studies and the chiral perturbation theory. In this article, we apply the QCD sum rule method to treat the hyperon mass difference ${1\over 2}(m_{\Sigma^-}+m_{\Sigma^0})-m_{\Sigma^+}$ in order to obtain a reasonable estimate on such isospin symmetry breaking parameter $\gamma$ defined as $(< to the particular mass difference are expected to cancel almost completely. Using the light-quark mass difference $m_d-m_u=4\pm 1$ MeV and the experimental data ${1øver 2}(m_{\Sigma^-}+m_{\Sigma^0})-m_{\Sigma^+