Evolving images of the proton: Hadron physics over the past 40 years

TL;DR

Over the past 40 years, our understanding of proton structure has evolved from simple quark models to complex phenomena arising from strong coupling QCD, highlighting the interplay of experiment and theory in hadron physics.

Contribution

This paper provides a personal overview of the significant developments in hadron physics and the understanding of proton structure over four decades.

Findings

Proton structure is more complex than the simple three-quark model.

Strong coupling QCD explains phenomena like confinement and mass generation.

Experiment and theory together are essential to understand hadron properties.

Abstract

Once upon a time, the world was simple: the proton contained three quarks, two {\it ups} and a {\it down}. How these give the proton its mass and its spin seemed obvious. Over the past forty years the proton has become more complicated, and how even these most obvious of its properties is explained in a universe of quarks, antiquarks and gluons remains a challenge. That this should be so should come as no surprise. Quantum Chromodynamics, the theory of the strong interaction, is seemingly simple, and its consequences are straightforward in the domain of hard scattering where perturbation theory applies. However, the beauty of the hadron world is its diversity. The existence of hadrons, their properties, and their binding into nuclei do not appear in the Lagrangian of QCD. They all emerge as a result of its strong coupling. Strong coupling QCD creates complex phenomena, much richer than known 40 years ago: a richness that ensures colour confinement and accounts for more than 95\% of the mass of the visible Universe. How strong coupling QCD really works requires a synergy between experiment and theory. A very personal view of these fascinating developments in cold QCD is presented.