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The quantity R_K = Gamma(K -> e nu)/Gamma(K -> mu nu) studied by theexperiment NA62 at CERN is known to probe lepton-flavour violating (LFV)parameters of the Minimal Supersymmetric Standard Model (MSSM). A non-zeroparameter delta_{RR}^{13} can open the decay channel K -> e nu_tau and enhanceR_K over its Standard-Model value. In the region of the parameter space probedby NA62 the contribution from a bino-stau loop diagram is numerically dominantand the mixing between left-handed and right-handed staus is important. Forlarge values of the stau mixing angle theta_tau the commonly adopted massinsertion approximation is not accurate. We therefore express thesupersymmetric contribution to R_K in terms of the mass of the lightest staueigenstate, the mixing angle theta_tau, and other relevant MSSM parameter suchas tan(beta) and the charged-Higgs boson mass M_H and plot the parameterregions constrained by R_K. We further study to which extent R_K can bedepleted through MSSM contributions interfering destructively with the SMamplitude for K -> e nu_e. This lepton-flavour conserving (LFC) mechanisminvolves the parameter combination |delta_{LL}^{13} delta_{RR}^{13}|, which canbe constrained with a naturalness consideration for the electron mass or withthe measurement of the anomalous magnetic moment of the electron. The LFCeffect on R_K is marginal, an NA62 measurement of R_K significantly below theStandard Model expectation would indicate physics beyond the MSSM.

Gamma(K -> e nu)/Gamma(K -> mu nu) in the Minimal Supersymmetric Standard Model