Laser polarized muonic helium

We have formed polarized muonic ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ by stopping unpolarized negative muons in a gaseous ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ target which was polarized by spin exchange with laser optically pumped Rb vapor. We measured ${\mathit{P}}_{\mathrm{\mu }}^{\mathit{N}}$, the resulting muon polarization normalized to the nuclear polarization of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. In addition to being a property that characterizes the muon cascade in the μ${\mathrm{-}}^3$He system, ${\mathit{P}}_{\mathrm{\mu }}^{\mathit{N}}$ is of practical value for future experiments on nuclear muon capture. Our result, ${\mathit{P}}_{\mathrm{\mu }}^{\mathit{N}}$=0.072±0.008, is a factor of 2 smaller than predicted by simple atomic cascade theory.