The Compton polarimeter’s laser system, used to measure the parallel spin of electrons, is aligned during the Calcium Radius Experiment at Jefferson Lab. Credit: Jefferson Lab photo/Dave Gaskell
“No one has measured the polarization of an electron beam to this precision at any lab, anywhere in the world,” said Dave Gaskell, an experimental nuclear physicist at Jefferson Lab and a co-author on the paper. “That’s the headline here. This isn’t just a benchmark for Compton polarimetry, but for any electron polarization measurement technique.”
The Compton polarimeter’s laser resonates inside a locked optical cavity during the running of the CREX experiment. Credit: Jefferson Lab photo/Dave GaskellCalcium Radius Experiment , conducted in tandem with the Lead Radius Experiment “The CREX and PREX-II collaboration cared about knowing the polarization well enough that we dedicated the beam time to make a high-quality measurement,” Gaskell said. “And we made full use of that time.”
In many experiments involving CEBAF, the dominant source of systematic uncertainty is knowledge of the electron beam’s polarization. The CREX team used the Compton polarimeter to bring that unknown to the lowest level ever reported. The difference between the number of hits when the electrons are flipped from a forward longitudinal state to a backward one is proportional to the beam’s polarization. This assumes the polarization of the laser is constant.
“It doesn’t explain what dark matter is. It doesn’t explain where CP violation comes from, or why there’s mostly matter in the universe and not antimatter,” Dalton continued. The EIC will collide electrons with protons or heavier atomic nuclei to probe their inner workings and gain insight on the forces that bind them.