Isospin violation in mesons and the constituent-quark masses

Isospin-violating mass differences and mixing angles in mesons, which arise in part from an intrinsic mass difference between up and down quarks, are studied in a relativized quark potential model. Satisfactory results, comparable to those of nonrelativistic calculations, are obtained, but in addition we find that observed isospin breaking leads to tight constraints on the allowed values of constituent-quark masses: 1/2($m_u$+$m_d$) must be between about 200 and 300 MeV.