Production of single W bosons at GeV and measurement of WWγ gauge couplings

Single W boson production in electron-positron collisions is studied with the L3 detector at LEP. The data sample collected at a centre-of-mass energy of $\sqrt{s} = 188.7 \mathrm{ Ge\kern -0.1em V}{}$ corresponds to an integrated luminosity of $176.4\,\mbox{pb$^{-1}$}$. Events with a single energetic lepton or two acoplanar hadronic jets are selected. Within phase-space cuts, the total cross-section is measured to be $0.53 \pm 0.12 \pm 0.03 \rm{pb}$, consistent with the Standard Model expectation. Including our single W boson results obtained at lower $\sqrt{s}$, the WW$\gamma$ gauge couplings $\kappa_\gamma$ and $\lambda_\gamma$ are determined to be \mbox{$\kappa_\gamma = 0.93 \pm 0.16 \pm 0.09$} and \mbox{$\lambda_\gamma = -0.31^{+0.68}_{-0.19} \pm 0.13$}.