What gives the proton its spin? This seemingly simple question touches on one of the most fundamental and unresolved mysteries in modern physics—and has far-reaching implications for our understanding of the universe.
In the July 2025 issue of New Scientist, Prof. Juan Rojo, head of the Physics and Astronomy Department at VU Amsterdam, delves into this deep scientific puzzle. His article, "Het mysterie van de protondans", takes readers inside the dynamic quantum world that exists at the heart of the proton.
Protons, the positively charged building blocks of atomic nuclei, are themselves made up of even smaller particles—quarks and gluons. These particles interact under the rules of quantum chromodynamics (QCD), a theory where classical physics no longer applies. One major question that continues to intrigue physicists is: where does the proton's spin come from?
Initially, scientists believed the proton’s spin was simply the sum of the spins of its three constituent quarks. However, experimental findings in the 1980s revealed that these quarks account for only a small fraction of the total spin. This unexpected result sparked what became known as the “proton spin crisis.”
Together with collaborators at Nikhef, Prof. Rojo and other researchers are using cutting-edge particle accelerators and neural network models to probe the proton’s internal dynamics. Their findings are contributing to a more complete picture of how spin emerges from the complex interplay between quarks and gluons—bringing us closer to solving one of the key challenges in high-energy physics.
Read the full article here