The dipion mass spectrum in e+e− annihilation and τ decay: Isospin Symmetry breaking effects from the (ρ, ω, ϕ) mixing

A way to explain the puzzling difference between the pion form factor as measured in e+e− annihilations and in τ decays is discussed. We show that isospin symmetry breaking, beside the already identified effects, produces also a full mixing between the ρ0, ω and ϕ mesons which generates an isospin 0 component inside the ρ0 meson. This effect, not accounted for in current treatments of the problem, seems able to account for the apparent mismatch between e+e− and τ data below the ϕ mass.