The mass of the two lightest quarks
Abstract
The problems encountered in the determination of mu and md are discussed. While their sum is known quite well, the difference md-mu, which measures the breaking of isospin symmetry in the QCD Lagrangian, is still subject to significant uncertainties. I focus on recent work based on the dispersive analysis of the decay eta -> 3 pi, which offers a good handle on isospin breaking, because in that transition, the contributions from the e.m. interaction are suppressed.
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