Decay of negative muons bound in $^{27}$Al

TL;DR

This study measures the energy spectrum of electrons from negative muon decay in aluminum atoms, providing the most precise data to date and highlighting the need for improved theoretical radiative corrections.

Contribution

First measurement of the electron energy spectrum from muon decay in $^{27}$Al, with high precision and comparison to theoretical models.

Findings

Muon lifetime measured as 864.6 ns, consistent with previous data.

Decay spectrum asymmetry indicates complete depolarization of muons.

Observed spectrum differences suggest the need for radiative correction improvements.

Abstract

We present the first measurement of the energy spectrum up to 70 MeV of electrons from the decay of negative muons after they become bound in $^{27}$Al atoms. The data were taken with the TWIST apparatus at TRIUMF. We find a muon lifetime of (864.6 $\pm$ 1.2) ns, in agreement with earlier measurements. The asymmetry of the decay spectrum is consistent with zero, indicating that the atomic capture has completely depolarised the muons. The measured momentum spectrum is in reasonable agreement with theoretical predictions at the higher energies, but differences around the peak of the spectrum indicate the need for \oalpha radiative corrections to the calculations. The present measurement is the most precise measurement of the decay spectrum of muons bound to any nucleus.