The mass of the two lightest quarks

Understand

The problems encountered in the determination of m_u and m_d are discussed.

• While their sum is known quite well, the difference m_d-m_u, 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.