Muonium in superfluid helium

We present the muon-spin-relaxation studies in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the temperature range 0.5–1.5 K. At temperatures below 1 K we observed muonium spin precession. From the magnetic-field dependence of the amplitude of the muonium component we estimate a muonium formation rate of about 5×${10}^7$ ${\mathrm{s}}^{\mathrm{-}1}$ at 0.5 K. We find that external electric fields, preventing the convergence of muon and electron, reduce the amplitude of the muonium precession signal.