Much of a proton’s mass comes from the energy of the particles inside it

Author: Emily Conover / Source: Science News for Students

A proton’s mass is more than just the sum of its parts.

At last, scientists have figured out what accounts for this subatomic particle’s heft.

Protons are made up of even smaller particles known as quarks. It might seem reasonable that simply adding up the quarks’ masses would give you a proton’s mass. Yet it doesn’t. That sum is far too small to explain the proton’s bulk. New, detailed calculations show that only 9 percent of a proton’s heft comes from the mass of its quarks. The rest comes from complicated effects occurring inside the particle.

Quarks get their masses from a process connected to the Higgs boson. That’s an elementary particle first detected in 2012. But “the quark masses are tiny,” says theoretical physicist Keh-Fei Liu. A coauthor of the new study, he works at the University of Kentucky in Lexington. So for protons, he notes, the Higgs explanation falls short.

Instead, most of the proton’s 938 million electron volts of mass comes from something known as QCD. It’s short for quantum chromodynamics (KWON-tum Kroh-moh-dy-NAM-iks). QCD is a theory that accounts for…

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