2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/178782A standing mystery in the Standard Model is the unnatural smallness of the strong CP violating phase. A massless up quark has long been proposed as one potential solution. A lattice calculation of the constants of the chiral Lagrangian essential for the determination of the up quark mass, 2 alpha_8 - alpha_5, is presented. We find 2 alpha_8 - alpha_5 = 0.29 +/- 0.18, which corresponds to m_u / m_d = 0.410 +/- 0.036. This is the first such calculation using a physical number of dynamical light quarks, N_f = 3.4 pages, 5 figures, submitted to Phys. Rev. Lett., corrected small normalization error in f_pi (conclusions were unaffected), improved lattice spacing analysis, improved finite volume analysisHigh Energy Physics - LatticeHigh Energy Physics - Phenomenologytext