Origin of the Proton Mass

TL;DR

This paper reviews forty years of research in quantum chromodynamics (QCD) to explain the origin of the proton mass, a fundamental question in particle physics, and discusses experiments to validate the current understanding.

Contribution

It summarizes the development of QCD theories over forty years that shed light on the proton mass origin and proposes experimental tests for validation.

Findings

QCD explains the proton mass beyond Higgs mechanism

Scale invariance in QCD is broken to generate mass

Experiments can test the QCD-based mass generation hypothesis

Abstract

Atomic nuclei lie at the core of everything visible; and at the first level of approximation, their atomic weights are simply the sum of the masses of all the neutrons and protons (nucleons) they contain. Each nucleon has a mass $m_N \approx 1\,{\rm GeV}\approx 2000$-times the electron mass. The Higgs boson -- discovered at the large hadron collider in 2012, a decade ago -- produces the latter, but what generates the nucleon mass? This is a pivotal question. The answer is widely supposed to lie within quantum chromodynamics (QCD), the strong-interaction piece of the Standard Model. Yet, it is far from obvious. In fact, removing Higgs-boson couplings into QCD, one arrives at a scale invariant theory, which, classically, can't support any masses at all. This contribution sketches forty years of developments in QCD, which suggest a solution to the puzzle, and highlight some of the experiments that can validate the picture.