The proton’s weak side is just as feeble as physicists thought

Protons are no one-trick ponies. Although famous for their positive electric charge, the particles also carry an analogous, lesser-known charge, called the weak charge. Now, physicists have made the most precise measurement of the proton’s underdog attribute, members of the Q-weak experiment report in the May 10 Nature.

The weak charge dictates how the proton responds to the weak nuclear force. Best known for its role in certain types of radioactive decay, the weak force is named for its feebleness compared with other forces such as electromagnetism (SN: 3/3/18, p. 11).

In the experiment, located at the Thomas Jefferson National Accelerator Facility in Newport News, Va., scientists scattered beams of electrons off of protons in liquid hydrogen. The beams came in two different configurations, depending on a quantum property of the electrons called spin. In one setup, the electrons behaved as if spinning clockwise; in the other, counterclockwise. Observing the difference between how protons interacted with electrons of the two spin orientations let the team calculate the proton’s weak charge.

The resulting charge is approximately 0.0719, a unitless number that agrees with predictions of the standard model of particle physics, the theory that describes how particles interact. That agreement partially rules out the existence of certain new types of particles, which is one reason scientists have been so interested in better understanding the proton’s weak side.