Hidden new physics in meson decays.

A model-independent analysis of a non-standard pseudoscalar contribution to leptonic meson decays ($\pi$,$K$,$D$,$D_s$ and $B$) is presented. As also seen from similar analyses in the literature, we find that two distinct regions in the parameter space arise, a region where the standard model contribution is predominant and a region where the new physics terms cancel precisely. So far, the latter has been regarded as a fine-tuning and for this reason it was neglected from the very beginning. This paper argues that this cancellation appears naturally in a class of models, most notably those that realize the Glashow-Weinberg-Paschos mechanism for avoiding flavor-changing neutral currents, in particular in the lepton sector. Thus, such a region that allows for larger Yukawa couplings ($10^{-4}\lesssim (G_\eta^{P}/G_F)U^{-1}m_l^{-1} \lesssim 10^{-3}$ MeV$^{-1}$) cannot be readily ruled out solely by the meson leptonic decays and is degenerated with the Standard Model prediction in these decays.