Revisiting $μ$-$e$ conversion in $R$-parity violating SUSY

TL;DR

This paper systematically reevaluates μ−e conversion in the context of trilinear $R$-parity violating SUSY by embedding the model in an EFT framework that flows from the high-scale RPV theory to SMEFT and then LEFT, with RG running across scales. By deriving matching conditions for $λ$ and $λ′$ couplings and computing the resulting Wilson coefficients, the authors compare μ−e conversion constraints with those from μ→eγ and μ→3e, finding that RG effects are typically modest but crucial for certain combinations. They provide numerical upper limits on 15 $λ′$ and 6 $λ$ combinations, showing that future μ−e conversion experiments (COMET Phase I and Mu2e) can surpass other channels for many couplings and probe tree-level contributions that are invisible to radiative decays. The results demonstrate the complementary power of μ−e conversion to map out the RPV SUSY parameter space and emphasize the importance of RG effects in interpreting upcoming experimental data.

Abstract

The $μ$-$e$ conversion process is one of the most powerful ways to test lepton-flavor-violating (LFV) interactions involving charged leptons. The standard model with massive neutrinos predicts an extremely low rate for $μ$-$e$ conversion, making this process an excellent probe for testing LFV arising from new physics. Among many theoretical models that can induce LFV, the Supersymmetric model with R-parity violating interactions is one of the most studied for $μ$-$e$ conversion. In this work, we revisit trilinear R-parity violating interactions for $μ$-$e$ conversion, considering renormalization group (RG) running effects from high to low energy scales. The $μ$-$e$ conversion, $μ\to e γ$, and $μ\to eee$ experimental data are compared to give upper limits on the relevant 15 combinations of the trilinear $λ^{\prime}$ couplings and 6 combinations of the $λ$ couplings, certain of which are underexplored in previous studies. We find that RG running effects influence the limits by no more than 30\% in most cases, but can improve constraints by $\sim$80\% in certain combinations, which cannot be neglected. In the near future, COMET and Mu2e are expected to begin data-taking and aim to provide the most stringent constraints on $μ$-$e$ conversion. These next-generation $μ$-$e$ experiments have the ability to give much more comprehensive examinations on most trilinear coupling combinations than the $μ\to eγ$ and $μ\to 3e$ decay experiments. The $μ$-$e$ experiments will not only deepen our understanding of LFV but also provide a crucial way to examine the underlying new physics contributions.

Figures (2)

• Figure 1: The upper limits with and without RG effects on the combinations of $\lambda^{\prime}$ couplings from muon cLFV processes $\mu$-$e$ conversion (in blue), $\mu\to e\gamma$ (in orange), and $\mu\to 3 e$ (in green). The units of the upper limits are $[m_{\tilde{q}}^{2} ~(\text{TeV}^{-2})]$.
• Figure 2: The upper limits with and without RG effects on the combinations of $\lambda$ couplings from muon cLFV processes $\mu$-$e$ conversion (in blue), $\mu\to e\gamma$ (in orange), and $\mu\to 3 e$ (in green). The units of the upper limits are $[m_{\tilde{\ell}}^{2} ~(\text{TeV}^{-2})]$.