Protons contain intrinsic charm quarks, a new study suggests

The massive quarks — counterintuitively heavier than the proton itself — might carry about 0.6 percent of a proton’s momentum.

Protons and Neutrons: The Massive Pandemonium in Matter – Of Particular Significance

Gluons and sea quarks in the proton at low scales

Physicists discover a mind-bending puzzle about protons at the quantum level

How are protons and neutrons spherical when they're made up of three quarks? - Quora

Evidence for intrinsic charm quarks in the proton. - Abstract - Europe PMC

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Physicists may be a step closer to solving the mystery of proton size

Quark, Definition, Flavors, & Colors

Protons Contain a Particle That's Heavier Than the Proton Itself