$\mu - e$ Conversion With Four Generations

TL;DR

This paper investigates how a hypothetical fourth generation of neutrinos could significantly increase the rate of muon-to-electron conversion, providing new constraints and implications for upcoming experiments.

Contribution

It introduces the impact of a fourth generation neutrino on $$ conversion rates and compares constraints from different experimental bounds, highlighting the most stringent current limits.

Findings

Fourth generation neutrino can enhance $$ conversion rates by orders of magnitude.

Current $$ conversion bounds from gold nuclei impose the strongest constraints.

The flavor-changing parameter $$ is constrained to be less than $1.6 imes 10^{-5}$ for neutrino masses above 100 GeV.

Abstract

We study $\mu - e$ conversion with sequential four generations. A large mass for the fourth generation neutrino can enhance the conversion rate by orders of magnitude. We compare constraints obtained from $\mu - e$ conversion using experimental bounds on various nuclei with those from $\mu \to e \gamma$ and $\mu \to e\bar e e$. We find that the current bound from $\mu - e$ conversion with Au puts the most stringent constraint in this model. The relevant flavor changing parameter $\lambda_{\mu e} = V^*_{\mu 4}V_{e4}^{}$ is constrained to be less than $1.6\times 10^{-5}$ for the fourth generation neutrino mass larger than 100 GeV. Implications for future $\mu -e$ conversion, $\mu \to e\gamma$ and $\mu \to e\bar e e$ experiments are discussed.