Polarization of Collidinge+e−Beams at SPEAR II

We have studied the angular distribution for the reactions $e^{+}{e}^{-}\to e^{+}{e}^{-}$ and $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ at beam energies of 1.55, 3.1, and 3.7 GeV at SPEAR II. At a beam energy of 3.7 GeV a significant azimuthal asymmetry was observed indicating that the electron and positron beams are strongly polarized. The angular distribution of the $\mu$ pairs was found to be in good agreement with the predictions of quantum electrodynamics. The equilibrium value of the polarization and the polarization time constant are found to be $P_0=0.76\mathrm{}\pm 0.05$ and $\tau ={10}_{-5\mathrm{}}^{+20\mathrm{}}$ min.