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The Compton polarimeter’s laser system, used to measure the parallel spin of electrons, is aligned during the Calcium Radius Experiment at Jefferson Lab. Photo Credit: Jefferson Lab /Dave Gaskell |
Scientists are getting a more detailed look than ever before at the electrons they use in precision experiments.
Nuclear physicists with the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility have shattered a nearly 30-year-old record for the measurement of parallel spin within an electron beam – or electron beam polarimetry, for short. The achievement sets the stage for high-profile experiments at Jefferson Lab that could open the door to new physics discoveries.
In a peer-reviewed paper published in the journal Physical Review C, a collaboration of Jefferson Lab researchers and scientific users reported a measurement more precise than a benchmark achieved during the 1994-95 run of the SLAC Large Detector (SLD) experiment at the SLAC National Accelerator Laboratory in Menlo Park, California.
“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.”
Compton polarimetry involves detecting photons – particles of light – scattered by charged particles, such as electrons. That scattering, aka the Compton effect, can be achieved by sending laser light and an electron beam on a collision course.
Electrons – and photons – carry a property called spin (which physicists measure as angular momentum). Like mass or electric charge, spin is an intrinsic property of the electron. When particles spin in the same direction at a given time, the quantity is known as polarization. And for physicists probing the heart of matter on the tiniest scales, knowledge of that polarization is crucial.
“Think of the electron beam as a tool that you're using to measure something, like a ruler,” said Mark Macrae Dalton, another Jefferson Lab physicist and co-author on the paper. “Is it in inches or is it in millimeters? You have to understand the ruler in order to understand any measurement. Otherwise, you can’t measure anything.”