Laser-Sharp Look at Spinning Electrons Sets the Stage for New Physics Discoveries

A close-up view of a green laser beam inside a reflective cylindrical chamber, which is the Compton polarimeter’s laser system

The Compton polarimeter’s laser system, used to measure electrons’ parallel spin, is aligned during the Calcium Radius Experiment in Hall A of the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Accelerator Facility.

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The Science

Like mass or electric charge, spin is an intrinsic property of the electron. When electrons spin in the same direction at a given time, the quantity is called polarization. Knowledge of that parallel spin is vital for scientists probing the nature of matter on the tiniest scales. In particular, it sheds light on structure of nuclei of heavy atoms such as lead. Now, nuclear physicists have measured the polarization of an electron beam more precisely than ever before. They achieved the record measurement by sending laser light and electrons on a collision course and detecting the photons, or particles of light, that bounce off. This interaction is known as the Compton effect.

The Impact

The Standard Model of particle physics attempts to describe the most basic constituents of atoms, such as quarks and gluons, along with three of the four fundamental forces: the strong force, the weak force, and the electromagnetic force. But it isn’t complete. That’s why scientists are planning a series of novel experiments to test this theory and possibly help reshape their description of the universe. The recent Compton polarization measurement has surpassed the level of precision required for those future studies.

Summary

When weighing experiment against theory, it is crucial that scientists understand the uncertainties these comparisons reveal. Otherwise, those tests would have no scientific value. In many studies involving electron beams, knowledge of the energized particles’ spin is the main source of uncertainty. To reduce this uncertainty, scientists at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) developed a device to measure polarization more precisely than ever.

The researchers used the device, called a Compton polarimeter, in Hall A of the Continuous Electron Beam Accelerator Facility, a Department of Energy Office of Science user facility. The system diverts the electron beam into an optical cavity, where it collides with laser light. The photons that the electron beam knocks out hurtle into a detector that passes their signals to a suite of data collectors. The Compton polarimeter was part of the Calcium Radius Experiment (CREX), which probed the nuclei of medium-weight atoms for insight on their structure. During CREX, the research team reduced the uncertainty of the energized electrons’ spin to 0.36%. This broke a record of 0.5% that was set with much higher beam energy at the SLAC National Accelerator Laboratory in 1995. This new measurement also crossed the 0.4% threshold needed for the flagship MOLLER experiment, which will measure the weak charge on an electron as a test of the Standard Model.

Contact

Mark Macrae Dalton
Thomas Jefferson National Accelerator Facility
dalton@jlab.org

Funding

This work was supported in part by the Department of Energy Office of Science, Office of Nuclear Physics.

Publications

Zec, A.,, et al. (CREX Collaboration), Ultrahigh-precision Compton polarimetry at 2 GeV. Physical Review C 109, 024323 (2024). [DOI: 10.1103/PhysRevC.109.024323]