Kℓ3 decays in chiral perturbation theory Original Research Article

The process Kℓ3 is calculated to two-loop order (p6) in chiral perturbation theory (ChPT) in the isospin conserved case. We present expressions suitable for use with previous work in two-loop ChPT where the order p4 parameters (Lir) were determined from experiment. We point out that all the order p6 parameters (Cir) that appear in the value of f+(0) relevant for the determination of |Vus| can be determined from Kℓ3 measurements via the slope and the curvature of the scalar form-factor. As by product we update the value of the CKM matrix element |Vus|.