Accurate Electron Spin Optical Polarimetry

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This award funds work that will help improve the accuracy of electron spin measurements done at large electron accelerators both in the United States and internationally. Spin is an intrinsic property of the electron that determines much of its behavior. Until the middle of the 20th century, scientists believed that nature would work in exactly the same way in both a laboratory and a perfect mirror image of that laboratory. In other words, by doing any physics experiment that one could imagine, one wouldn't be able to tell whether he or she was in one laboratory or its "looking glass" equivalent. This idea was shown to be incorrect by experiments that "violated parity," producing only electrons that were 'left-handed' with their velocities and spins pointing in opposite directions. Today, these experiments have become increasingly sophisticated, such that the average spin of the electrons, analogous to the spin of toy tops, must be determined with exquisite sensitivity, both at low energy, when they are produced, and at high energy after they have been accelerated to speeds very close to that of light. The goal of this work is to measure the spins at low energy in a new way, using the method of Accurate Electron Spin Optical Polarimetry (AESOP). The AESOP method has been proven to accuracies of about 1.5%, and will now be refined to provide accuracies better that 0.5%. As such, it will greatly increase the quality of the information being provided by a new generation of parity violation experiments. This in turn will yield an even better understanding of the electroweak force, one of the three fundamental forces of nature. The AESOP technique, based on the measurement of light polarization following the fluorescence of atoms excited by the electrons whose average spin is to be determined, was invented and developed by this research team in previous work funded by the NSF. It provides an absolute measurement of electron spin polarization that does not require theoretical calculations of dynamic processes for calibration. It does, however, have potential systematic errors, especially if the electron beam being studied has a broad energy spread with a polarization that varies across this width. This technique has previously been used to measure electron beam polarization with an accuracy of between 1% and 1.5%. The research team now proposes to design and build, as a proof of concept, an AESOP system that will allow for a detailed study of the method's potential systematic errors and show how to eliminate or minimize such errors. The research team expects to demonstrate repeated and reliable electron optical polarimetric measurements with better than 0.5% accuracy, which is needed to calibrate Mott polarimeters used at accelerator injectors to an accuracy of 0.5%. Such Mott accuracy will be required for the next generation of electro-weak parity violation experiments. As part of this project, a protocol will be developed with the injector groups at the Thomas Jefferson National Accelerator Facility (JLab) and the Mainzer Mikrotron (MAMI) for installing an AESOP facility at one or both accelerators.