Revisiting the $\Gamma(K \to e \nu)/\Gamma(K \to \mu \nu)$ ratio in supersymmetric unified models

TL;DR

This paper investigates how supersymmetric models can affect the ratio of kaon decay rates to electrons versus muons, analyzing one-loop corrections and constraints from other observables to assess potential deviations from the Standard Model.

Contribution

It provides a comprehensive calculation of supersymmetric effects on $R_K$, considering various models and experimental constraints, and concludes these effects are generally too small to reach current experimental sensitivity.

Findings

Supersymmetric models typically yield small contributions to $R_K$.

Constraints from $B$-meson decays limit the size of possible $R_K$ deviations.

Unconstrained MSSM can produce larger effects but still cannot saturate experimental bounds.

Abstract

It has been pointed out that supersymmetric extensions of the Standard Model can induce significant changes to the theoretical prediction of the ratio $\Gamma(K\rightarrow e\nu)/\Gamma (K\rightarrow\mu\nu)\equiv R_{K}$, through lepton flavour violating couplings. In this work we carry out a full computation of all one-loop corrections to the relevant $\nu\ell H^{+}$ vertex, and discuss the new contributions to $R_{K}$ arising in the context of different constrained (minimal supergravity inspired) models which succeed in accounting for neutrino data, further considering the possibility of accommodating a near future observation of a $\mu\to e\gamma$ transition. We also re-evaluate the prospects for $R_{K}$ in the framework of unconstrained supersymmetric models. In all cases, we address the question of whether it is possible to saturate the current experimental sensitivity on $R_{K}$ while in agreement with the recent limits on $B$-meson decay observables (in particular BR($B_{s}\to\mu^+\mu^-$) and R($B_{u}\to\tau\nu$)), as well as BR($\tau\to e \gamma$) and available collider constraints. Our findings reveal that in view of the recent bounds, and even when enhanced by effective sources of flavour violation in the right-handed $\tilde{e}-\tilde{\tau}$ sector, constrained supersymmetric (seesaw) models typically provide excessively small contributions to $R_{K}$. Larger contributions can be found in more general settings, where the charged Higgs mass can be effectively lowered, and even further enhanced in the unconstrained MSSM. However, our analysis clearly shows that even in this last case SUSY contributions to $R_{K}$ are still unable to saturate the current experimental bounds on this observable, especially due to a strong tension with the $B_{u}\to\tau\nu$ bound.