A proton is made of exactly three quarks

What actually happened

The three-quark picture was always a simplification, and physicists knew it. A proton is a boiling interior in which the strong force constantly conjures short-lived 'sea' quark-antiquark pairs; the two-up-one-down 'valence' quarks are only the persistent surplus. What was genuinely open was whether the proton contains charm quarks as an intrinsic, built-in part of its wavefunction, rather than merely as fleeting pairs that flicker in and out.

Charm quarks are heavier than the proton itself, so an intrinsic charm component sounds paradoxical, but quantum mechanics permits it: the proton is a superposition of states, and a small, permanent five-quark configuration including a charm-anticharm pair is allowed. The question had been argued since the 1980s.

In 2022, a machine-learning analysis of decades of collider data by the NNPDF collaboration found intrinsic charm at the three-sigma level, carrying roughly a percent of the proton's momentum, with a distribution matching the old prediction, and cross-checked against fresh LHCb data. The clean schoolbook proton gained a permanent, heavier ingredient it was never supposed to have.