Masses and Decay Constants of Pseudoscalar Mesons to Two Loops in   Two-Flavor Partially Quenched Chiral Perturbation Theory
  This paper presents a first study of the masses and decay constants of the charged, or flavor-off-diagonal, pseudoscalar mesons to two loops for two 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. A numerical analysis is done to indicate the size of the corrections. 
Johan Bijnens and Timo A. L\"ahde (Lund)