Limit on the electric charge-nonconserving $\mu^+ \to invisible$ decay

TL;DR

This paper establishes the first experimental limit on the charge-nonconserving invisible muon decay and explores a new experimental approach to improve sensitivity to such rare processes, which could indicate physics beyond the Standard Model.

Contribution

It provides the first upper limit on the branching ratio of $ ext{Br}(oldsymbol{ ext{muon} o ext{invisible})}$ and proposes a new experimental method to detect this decay at unprecedented sensitivity levels.

Findings

First limit on $ ext{Br}( ext{muon} o ext{invisible})$ is $< 5.2 imes 10^{-3}$.

Feasibility of a new experiment to reach sensitivity of a few parts in 10^{11}.

Discussion of constraints on $ au o ext{invisible}$ decay.

Abstract

The first limit on the branching ratio of the electric charge-nonconserving invisible muon decay $Br(\mu^+ \to invisible) < 5.2 \times 10^{-3}$ is obtained from the recently reported results on new determination of the Fermi constant from muon decays. The results of a feasibility study of a new proposed experiment for a sensitive search for this decay mode at the level of a few parts in 10^{11} are presented. Constrains on the $\tau \to invisible$ decay rate are discussed. These leptonic charge-nonconserving processes may hold in four-dimensional world in models with infinite extra dimensions, thus making their searches complementary to collider experiments probing new physics.