Duality violations and spectral sum rules
  We study the issue of duality violations in the VV-AA vacuum polarization function in the chiral limit. This is done with the help of a model with an expansion in inverse powers of the number of colors, Nc, allowing us to consider resonances with a finite width. Due to these duality violations, the Operator Product Expansion (OPE) and the moments of the spectral function (e.g. the Weinberg sum rules) do not match at finite momentum, and we analyze this difference in detail. We also perform a comparative study of many of the different methods proposed in the literature for the extraction of the OPE parameters and find that, when applied to our model, they all fare quite similarly. In fact, the model strongly suggests that a significant improvement in precision can only be expected after duality violations are included. To this end, we propose a method to parameterize these duality violations. The method works quite well for the model, and we hope that it may also be useful in future determinations of OPE parameters in QCD. 
O. Cata (IFAE-UAB), M. Golterman (San Francisco State U.) and S. Peris   (IFAE-UAB)