A search for two body muon decay signals

TL;DR

This study searches for two-body muon decays involving potential new bosons, setting limits on their production and decay asymmetries, and is the first to evaluate anisotropic decay limits.

Contribution

It provides the first explicit limits on anisotropic two-body muon decays involving new bosons with mass and asymmetry considerations.

Findings

Branching ratio limits of order 10^{-5} for bosons with 13-80 MeV/c^2 mass.

Stronger asymmetry dependence for bosons with mass less than 13 MeV/c^2.

First study to evaluate limits for anisotropic muon decays.

Abstract

Lepton family number violation is tested by searching for $\mu^+\to e^+X^0$ decays among the 5.8$\times 10^8$ positive muon decay events analyzed by the TWIST collaboration. Limits are set on the production of both massless and massive $X^0$ bosons. The large angular acceptance of this experiment allows limits to be placed on anisotropic $\mu^+\to e^+X^0$ decays, which can arise from interactions violating both lepton flavor and parity conservation. Branching ratio limits of order $10^{-5}$ are obtained for bosons with masses of 13 - 80 MeV/c$^2$ and with different decay asymmetries. For bosons with masses less than 13 MeV/c$^{2}$ the asymmetry dependence is much stronger and the 90% limit on the branching ratio varies up to $5.8 \times 10^{-5}$. This is the first study that explicitly evaluates the limits for anisotropic two body muon decays.