B. Dehning

First observation of transverse beam polarization in LEP

Abstract The results of beam polarization measurements performed in 1990 at the CERN Large Electron Positron storage ring (LEP) are reported. A significant asymmetry was observed in the Compton back-scattered photon distribution when illuminating the LEP electron beam with circularly polarized laser light. The corresponding polarization level is estimated to be 9.1% ± 0.3% (statistical) ± 1.8% (systematic). The validity of the polarization signal was assessed by varying the laser light polarization and by applying to the electron beam known depolarizing resonances. The measurement is consistent with the predicted polarization degree.

Measurement of LEP beam energy by resonant spin depolarization

Abstract A transverse beam polarization of around 10% reproducibly observed in LEP in 1991. Resonant spin depolarization was performed at four occasions, from September 16 to November 11, providing measurements of the beam energy with a precision of ± 1.5 × 10−5. Several cross-checks were performed to ascertain that the observed resonance corresponds to the fundamental spin precession frequency. The variability of the results, ± 6 × 10−5, is consistent with the expected stability and reproducibility of the machine.

Scattering of high energy electrons off thermal photons

We have measured the scattering of the LEP beam from the thermal photon background in the beam pipe. The results agree with the predicted rate for a Planck distribution and set a limit ωc<0.04 eV on the cut-off frequency appearing in models of stochastic electrodynamics.

First Evidence of Transverse Polarization in LEP

Results on first experimental evidence of transverse polarization in LEP at E = 46.5 GeV (ν s = 105.55) are reported. The polarization level associated to the significant asymmetry observed in the Compton back-scattered photon distribution when illuminating the electron beam with circularly polarized laser light is estimated to be 9% ± 2% (systematic) ±2% (statistical). The validity of the polarization signal was assessed by exciting a depolarizing resonance. The measurement is consistent with the predicted polarization degree with the especially prepared polarization optics.