Abstract
Polarized electron beams have played an important role in scattering experiments at moderate to high beam energies. Historically, these experiments have been primarily targeted at studying hadronic structure — from the quark contribution to the spin structure of protons and neutrons, to nucleon elastic form factors, as well as contributions to these elastic form factors from (strange) sea quarks. Other experiments have aimed to place constraints on new physics beyond the Standard Model. For most experiments, knowledge of the magnitude of the electron beam polarization has not been a limiting systematic uncertainty, with only moderately precise beam polarimetry requirements. However, a new generation of experiments will require extremely precise measurements of the beam polarization, significantly better than 1%. This paper will review standard electron beam polarimetry techniques and possible future technologies, with an emphasis on the ever-improving precision that is being driven by the requirements of electron scattering experiments.
Highlights
The use of spin-polarized electron beams has been an important tool for illuminating the fundamental nature of electron interactions and for the study of the atomic nucleus
The earliest productive studies using a polarized electron beam took place at the Stanford Linear Accelerator in the early 1970s, with a polarized source based on photoionization from a polarized atomic beam providing tens of nanoamps with ∼80% polarization. This beam was first used for high-energy polarized electron scattering with a magnetized ferromagnetic foil target,[1] demonstrating the utility of this target for electron beam polarimetry. This was followed by measurements using a polarized proton solid target to measure the proton elastic form-factors[2] and pursue the first study of spin structure functions[3] in deep inelastic scattering, topics which have remained prominent in electromagnetic spin physics
The use of pure iron foils polarized out-of-plane by large applied magnetic fields has allowed a significant reduction in the overall systematic uncertainty, resulting
Summary
The use of spin-polarized electron beams has been an important tool for illuminating the fundamental nature of electron interactions and for the study of the atomic nucleus. The first measurement of parity violation in electron scattering (PVeS) was made at SLAC in 1978 in order to help to establish the Weinberg–Salam–Glashow model of electroweak unification.[7] This experiment utilized a polarized source based on photoemission from a semiconducting GaAs cathode, which produced several microamps of beam current with a polarization of around 40%. In addition to those measurements at fixed target facilities, the parity-violating ALR measurement from electron–positron collisions with SLD at SLAC, which used Compton polarimetry near the interaction point, is listed[17] for comparison These measurements have made use of three techniques in order to determine the absolute longitudinal beam polarization. The planned electromagnetic studies require electron polarimetry at the level of 1–2%, but a program of PVeS study for partonic spinstructure functions has been proposed which will require a 0.5% precision on the electron polarization in the challenging collider environment
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