What evidence showed that protons and neutrons contain smaller particles?

Evidence that protons and neutrons contain smaller particles, known as quarks, was primarily established through high-energy scattering experiments conducted in the late 1960s.123

In these experiments, physicists used particle accelerators to fire beams of leptons—specifically electrons—at target materials containing protons and neutrons, such as hydrogen and deuterium. Researchers observed that some of the electrons were deflected at large angles during these collisions. Similar to the historical gold-foil experiment performed by Ernest Rutherford, this large-angle scattering provided direct evidence that protons and neutrons are not elementary, structureless particles, but rather complex structures containing smaller, pointlike charged objects.231

These internal components were initially termed partons before being identified as quarks. This discovery confirmed the quark model proposed in 1964, which posited that hadrons like protons and neutrons are bound states of these fractionally charged particles.214