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After nearly four decades of theoretical dead ends, physicists have finally pinned down the source of the proton's spin, a breakthrough that could reshape our understanding of matter's fundamental building blocks. The discovery, made possible by the Swiss supercomputer Piz Daint, resolves the so-called 'proton spin crisis' that has stumped researchers since 1987.

The puzzle began when experiments at CERN, SLAC, and DESY revealed that quarks—the particles once thought to solely determine a nucleon's spin—account for only 30% of it. The remaining 70% was attributed to relativistic quantum effects, but the exact mechanics remained elusive, leaving a gap in the Standard Model of particle physics.

Now, a team led by Constantia Alexandrou, a professor of physics at the University of Cyprus and the Cyprus Institute, has used the Piz Daint supercomputer to calculate these quantum effects with unprecedented precision. Their work, which also involved Karl Jansen, lead scientist at DESY-Zeuthen, breaks down the spin contributions from gluons, quarks, and sea quarks—the fleeting quark-antiquark pairs that flicker in and out of existence inside nucleons.

To achieve this, the researchers had to overcome a significant hurdle: determining the true physical mass of quarks, which cannot be isolated due to the strong force that binds them. They solved this by anchoring the mass of up and down quarks to the mass of the pion, a meson composed of one up and one down antiquark. This allowed them to accurately simulate the quarks' spin behavior.

The calculations, which also required reducing statistical errors and converting dimensionless simulation data into measurable physical values, were only feasible thanks to the computational power of Piz Daint. The supercomputer's graphics processors were essential, and the team optimized their algorithms to maximize efficiency.

Why This Matters for Physics

Understanding nucleon spin is not just an academic exercise; it underpins the behavior of all matter. The spin of quarks and gluons determines how materials conduct electricity, respond to temperature changes, and exhibit various physical properties. By resolving the spin crisis, this research provides a more complete picture of the forces that govern the universe at its most fundamental level.

The team's findings, published in a peer-reviewed journal, mark a significant step forward. As Jansen noted, the availability of powerful supercomputers like Piz Daint across Europe is crucial for such advanced simulations, and Alexandrou emphasized that the optimization of algorithms was key to leveraging the machine's capabilities.

While the mystery of proton spin may be solved, the implications of this work extend beyond particle physics. It offers a foundation for future research into quantum chromodynamics and could inform technologies that rely on spin-based phenomena, such as spintronics. For now, the decades-old puzzle has finally been cracked, thanks to a combination of theoretical insight and computational muscle.