The Breaking of Isospin and the ρ–ω–System

Although there are no doubts that all observed strong interaction phenomena can be described within the theory of QCD, a quantitative description of the strong interaction phenomena in the low energy sector is still lacking, although some features of the low energy phenomena have been partially understood by the lattice gauge theory approach. The low energy sector of the physics of the strong interactions is dominated by the low–lying pseudoscalar mesons (π,K, η, η′) and the low–lying vector mesons (ρ, ω,K∗, φ). It is well–known that the structures of the quark wave functions of the pseudoscalar mesons (0−+) and of the vector mesons (1−−) differ substantially. In the vector meson channel there is a strong mixing between the eights component of the SU(3) octet (wave function: (ūu+ d̄d−2s̄s)/( √ 6)) and of the SU(3) singlet (wave function: (ūu+d̄d+s̄s)/( √ 3)). The mixing strength is such that the mass eigenstates are nearly the state (ūu+ d̄d)/( √ 2)), the ω–meson, and the state s̄s, the φ–meson. While this feature looks peculiar, when viewed upon from the platform of the underlying SU(3) symmetry, it finds a simple interpretation, if one takes into account the Zweig rule [1], which states that the mixing must take place in such a way that quark lines are neither destroyed nor created. On the other hand the pseudoscalar mesons follow the pattern prescribed by the SU(3) symmetry in the absence of singlet–octet mixing. The neutral