Measurement of polarization transfer in Møller scattering of relativistic electrons

A dedicated Mott polarimeter was designed to study the final spin state in Møller scattering of transversely polarized electron beams in the relativistic energy range. The polarization transfer was measured for two incident-beam polarization orientations with respect to the Møller scattering plane, at 3 MeV incident-beam energy. The results are found to be in agreement with the predictions of relativistic quantum mechanics. Estimates for the measurement of quantum spin correlations in Møller scattering, based on a simultaneous determination of the spin projections for both electrons in the final state, are also presented.

Figure 1

Polarization transfer from a transversely polarized 3 MeV beam electron to the secondary Møller electrons, (a) beam polarization in the Møller scattering plane, (b) beam polarization perpendicular to the Møller scattering plane. The polarization transfer (length of the transverse polarization vector component divided by the initial beam polarization) dependence on the scattering angle $\theta$ (measured in the laboratory frame of reference) is plotted with the solid line. The corresponding polarization of the second electron is plotted with a dashed line. Equal polarization sharing occurs for the symmetric scattering configuration ($\theta =26.75°$ for incident electrons with 3 MeV kinetic energy, which corresponds to 90° in the center-of-mass frame).

Figure 2

Drawing of the experimental setup in configuration A (vertical Mott scattering plane, see text for details): a, Møller (Be) target; b, Møller scattering chamber; c, Mott scattering chamber; T, tagging counter; L and R, detectors in the Mott polarimeter. The shielding and the full length of the dump pipes are not shown.

Figure 3

Schematic illustration of both experimental configurations (cross section in the gold-target plane, perpendicular to the direction of the Møller-electrons stream); L and R denote the locations of the detectors in the Mott polarimeter.

Figure 4

Signal amplitude spectrum in the tagging counter pointing directly at the beryllium target. The higher peak consists of Mott-scattered beam electrons, and the lower peak of Møller electrons. The black vertical lines indicate the cuts applied to select the Møller scattering events.

Figure 5

Typical signal amplitude spectrum, recorded in the polarimeter with the beam trigger, before (a) and after (b) offline background subtraction. Histograms for opposite beam polarizations are plotted with different colors. The peak corresponds to beam electrons Mott-scattered off the beryllium and gold targets. The black vertical line indicates the low-amplitude cut used to calculate the asymmetry.

Figure 6

Histogram of the time distance between the rising edge position in the polarimeter and the tagging detector, after empty-target background subtraction, for events with signal amplitude within $\pm 2\sigma$ of the Møller peak. The distribution is fitted with a sum of a constant (background) and a Gaussian (signal), shown with a solid line. The background interpolation under the signal peak is marked with a dashed line.

Figure 7

Typical signal amplitude spectrum, recorded in the polarimeter with the Møller (coincidence) trigger, before offline background subtraction (a), after background subtraction (b), after background subtraction and energy cuts in the tagging detector (c) and after background subtraction and energy and timing selections (d). Histograms for opposite beam polarizations are plotted with different colors. The peak around 1.5 MeV corresponds to Møller electrons backscattered off the gold target and the peak at higher energies to doubly Mott-scattered beam electrons (false coincidences). The black vertical lines mark the part of the spectrum used to calculate the asymmetry.

Figure 8

Polarization transfer (length of the transverse polarization vector component in the final state of symmetric Møller scattering divided by the initial beam polarization), plotted as a function of the angle between the beam polarization vector and the Møller scattering (horizontal) plane. Solid line, theoretical predictions [14]; points, experimental data.