Tests of the naturalness of the coupling constants in ChPT at order p6

We derive constraints on combinations of O(p⁶) chiral coupling constants by matching a recent two-loop calculation of the πK scattering amplitude with a set of sum rules. We examine the validity of the natural expectation that the values of the chiral couplings can be associated with physics properties of the light resonance sector. We focus, in particular, on flavor symmetry breaking of vector resonances. A resonance chiral Lagrangian is constructed which incorporates flavor symmetry breaking more completely than was done before. We use πK unsubtracted sum rules as tests of the modelling of the resonance contributions to the chiral couplings. In some cases the O(p⁶) couplings are found not to be dominated by the resonance contributions.