TL;DR
This paper reviews the challenges in precisely determining the masses of the two lightest quarks, u and d, emphasizing recent dispersive analysis methods applied to eta decay to better understand isospin symmetry breaking.
Contribution
It highlights recent advances using dispersive analysis of eta decay to improve estimates of the light quark mass difference, addressing uncertainties in isospin breaking effects.
Findings
Sum of quark masses is well known
Dispersive analysis reduces uncertainties in m_d - m_u
Eta decay provides a clean probe of isospin breaking
Abstract
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.
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