Zweig rule violation in the scalar sector and values of low-energy   constants
  We discuss the role of the Zweig rule violation in the scalar channel for the determination of low-energy constants and condensates arising in the effective chiral Lagrangian of QCD. The analysis of the Goldstone boson masses and decay constants shows that the three-flavor condensate and some low-energy constants are very sensitive to the value of the Zweig Rule violating constant L_6. A similar study is performed in the case of the decay constants. A chiral sum rule based on experimental data in the scalar channel is used to constrain L_6, indicating a significant decrease between the two- and the three-flavor condensates. The analysis of the scalar form factors of the pion at zero momentum suggests that the pseudoscalar decay constant could also be suppressed from N_f=2 to 3. 
S. Descotes