A note on isospin violation in Pl2(γ) decays

We discuss the size of isospin violating effects of both electromagnetic (e not equal 0) and strong (m(u) not equal m(d)) origin in P-l2(gamma) decays. Our results are relevant for the extraction of vertical bar V-us/V-ud vertical bar from the measured ratio of the K-mu 2(gamma) and pi(mu 2(gamma)) decay widths combined with F-K/F-pi obtained in lattice calculations in the isospin limit. We point out that strong isospin-breaking corrections, neglected in previous studies, enter at the same level as the electromagnetic effects. The updated value for the ratio of CKM elements reads vertical bar V-us/V-ud vertical bar = 0.2316(12). (C) 2011 Elsevier B.V. All rights reserved.