Estimates of Light Quark Masses from Lattice QCD and QCD Sum rules

Abstract

This talk reviews the progress made in the determination of the light quark masses using lattice QCD and QCD sum rules. Based on preliminary calculations with three flavors of dynamical quarks, the lattice estimate is $m_s = 75(15)$ MeV, a tantalizingly low value. On the other hand the leading estimates from scale and pseudo-scalar sum rules are $99(16)$ and $100(12)$ MeV respectively. The $\tau$-decay sum rule estimates depend very sensitively on the value of $|V_{us}|$. The central values from different analyses lie in the range 115-120 MeV if unitarity of CKM matrix is imposed, and in the range 100-105 MeV if the Particle Data Group values for $|V_{us}|$ are used. I also give my reasons for why the lattice result is not yet in conflict with rigorous lower bounds from sum rule analyses.