Enhancing $l_i \to 3 l_j$ with the $Z^0$-penguin

TL;DR

This paper explores how the $Z^0$-penguin diagram can significantly enhance charged lepton flavor violating decays, such as $l_i o 3 l_j$, in various extensions of the MSSM, surpassing previous expectations.

Contribution

It demonstrates that the $Z^0$-penguin can be a dominant contribution to LFV decays in models beyond the MSSM, especially in the inverse seesaw and R-parity violating supersymmetry.

Findings

The $Z^0$-penguin enhances $l_i o 3 l_j$ decays in extended MSSM models.

Numerical analysis shows larger contributions from $Z^0$-penguin in specific models.

The $Z^0$-penguin can dominate over photon penguin contributions in certain scenarios.

Abstract

Lepton flavor violation (LFV) has been observed in neutrino oscillations. For charged lepton FV decays only upper limits are known, but sizable branching ratios are expected in many neutrino mass models. High scale models, such as the classical supersymmetric seesaw, usually predict that decays $l_i \to 3 l_j$ are roughly a factor $\alpha$ maller than the corresponding decays $l_i \to l_j \gamma$. Here we demonstrate that the $Z^0$-penguin diagram can give an enhancement for decays $l_i \to 3 l_j$ in many extensions of the MSSM. We first discuss why the $Z^0$-penguin is not dominant in the MSSM with seesaw and show that much larger contributions from the $Z^0$-penguin are expected in general. We then demonstrate the effect numerically in two example models, namely, the supersymmetric inverse seesaw and R-parity violating supersymmetry.