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The proton’s weak charge defines the strength of certain interactions between protons and other particles. A precise determination of this quantity provides a stringent test of the standard model of particle physics.
Subatomic particles interact through four fundamental forces. However, only two of these forces have effects on macroscopic scales: gravity keeps us grounded on Earth, and electromagnetism causes lightning on stormy days. We are not directly influenced by the other two forces — the weak and strong forces. Similarly, it is generally known that mass is at the root of gravitational interactions and that electric charges and magnetic moments are central to electromagnetism. But the physical properties that describe the strength of weak and strong interactions, known as weak and colour charges, respectively, are less familiar. In a paper in Nature, the Jefferson Lab Qweak Collaboration1 reports the first high-precision measurement of the weak charge of the proton, which sets tight constraints on physics that cannot be described by current theories.