Probing new physics throughμ−euniversality inK→ℓν

The recent NA48/2 improvement on $R_K=\Gamma (K\to e\nu )/\Gamma (K\to \mu \nu )$ emphasizes the role of $K_{l2}$ decays in probing the $\mu -e$ universality. Supersymmetric (SUSY) extensions of the Standard Model can exhibit $\mu -e$ nonuniversal contributions. Their origin is twofold: those deriving from lepton flavor conserving couplings are subdominant with respect to those arising from lepton flavor violating (LFV) sources. We show that $\mu -e$ nonuniversality in $K_{\ell 2}$ is quite effective in constraining relevant regions of SUSY models with LFV (for instance, supergravities with a see-saw mechanism for neutrino masses). A comparison with analogous bounds coming from $\tau$ LFV decays proves the relevance of the measurement of $R_K$ to probe LFV in SUSY.