Decay Constants of Pseudoscalar Mesons to Two Loops in Three-Flavor   Partially Quenched $\chi$PT
  This paper presents a first study of the decay constants of the charged, or flavor-off-diagonal, pseudoscalar mesons to two loops for three flavors of sea quarks, in Partially Quenched Chiral Perturbation Theory (PQ$\chi$PT). Explicit analytical expressions up to ${\cal O}(p^6)$ in the momentum expansion are given. The calculations have been performed within the supersymmetric formulation of PQ$\chi$PT. We also present some numerical results to indicate the size of the corrections. 
Johan Bijnens and Timo A. Lahde