B. Ananthanarayan, Johan Bijnens, Samuel Friot, Shayan Ghosh
An analytic representation of FK/Fπ


We present an analytic representation of FK/Fπ as calculated in three-flavoured two-loop chiral perturbation theory, and use it to extract values of the low energy constants Lr5, Cr14+Cr15 and Cr15+2Cr17 by means of fitting with data from lattice simulations. Although for the calculation of the two-loop multi-scale integrals involved we have derived exact results using Mellin-Barnes theory, the representation presented in this letter is, for practical purposes, an approximation whose accuracy may be improved to any desired level without a significant increase in its complexity.

LU TP 17-40