Vertical Beam Polarization at MAMI

Abstract

For the first time a vertically polarized electron beam has been used for physics experiments at MAMI in the energy range between 180 and 855 MeV. The beam-normal single-spin asymmetry A n , which is a direct probe of higherorder photon exchange beyond the first Born approximation, has been measured in the reaction 12 C( e, e ) 12 C. Vertical polarization orientation was necessary to measure this asymmetry with the existing experimental setup. In this paper we describe the procedure to orient the electron polarization vector vertically, and the concept of determining both its magnitude and orientation with the available setup. A sophisticated method has been developed to overcome the lack of a polarimeter setup sensitive to the vertical polarization component.

Comparative study of eV to GeV electrons and positrons scattering elastically from neutral atoms

“…The spin asymmetry which is associated with leptons spinpolarized perpendicular to the scattering plane (the so-called Sherman function) is of particular interest since it is most easily accessible to experiment and so serves to measure the beam polarization [7,8]. Moreover, its knowledge is important in parity violation studies [9].…”

Transverse Single Spin Asymmetries in Electron Scattering on Hydrogen Targets

“…The polarized electron source [13] produces longitudinally polarized electrons by illuminating a GaAs crystal with a circularly polarized laser. A Wien filter and a solenoid are installed in the injection beam line, so that the beam spin can be rotated into both the transverse and vertical orientations [14]. To minimize systematic uncertainties arising from possible time instabilities, the beam polarization was flipped every 20.08 ms by changing the Pockels cell voltage in the laser optics.…”

Measurement of the Beam Normal Single-Spin Asymmetry of $^{12}$C

“…At the Jefferson-Lab the PREX-Collaboration together with HAPPEX performed a measurement of A n for several nuclei ( 1 H, 4 He, 12 C, 208 Pb) in the Q 2 range between 0.01 and 0.1 GeV 2 /c 2 . While they found an agreement between their measurement and the theoretical calculations from [5] for light nuclei, for lead the theory predicted a value of A (th) n ≈ −8 ppm, but the measurement resulted in a value of A (ex) n = 0.28 ± 0.25 ppm.…”

“…The goal of this experiment was to remeasure the beam normal spin asymmetry of 12 C with a high precision in the Q 2 range between 0.02 and 0.05 GeV 2 /c 2 . This requires an accelerator with a high intensity transverse polarised electron beam of high quality as well as a high rate capable detector system.…”

“…Since the depolarisation along the beam line is negligible [11], the previously measured absolute polarisation is in this configuration directed vertically. The procedure of this measurement is described in more detail in [12].…”