Historically, when physicists first grappled with understanding the forces within the atomic nucleus in the 1930s, the concept of quarks and gluons was still unknown. To explain the short-range nature of the strong force, a groundbreaking proposal emerged, centered around a particle known as the pion.
In 1935, Hideki Yukawa introduced a theory suggesting that the strong force, much like electromagnetism, could be mediated by a particle. He proposed a potential, described mathematically as $$V(r) = Kfrac{e^{-mu r}}{r} $$, which accounted for the short-range behavior observed in nuclear interactions. This potential’s form in momentum space, $1/(p^2+mu^2)$, mirrors the propagator of a particle with mass $mu$. Yukawa’s theory posited that a spinless boson, the pion, was responsible for mediating the strong force between protons and neutrons, effectively binding the nucleus together. This model provided a compelling explanation for the observed nuclear forces at the time. It’s worth noting that during the 1930s and 40s, early particle physics faced challenges in distinguishing between muons and Pions, leading to some historical confusion and terms like “mesotron” which blurred the lines between these particles.
The pion model remained a relevant framework even as our quantitative understanding of nuclear interactions deepened in the 1960s. Pions are understood as “pseudo-Goldstone bosons,” nearly massless, spinless particles whose existence is linked to a broken symmetry – specifically, the approximate $SU(3)$ symmetry associated with the light quark flavors (up, down, and strange). This symmetry breaking is not perfect, which explains why pions, along with kaons, possess a small mass rather than being truly massless. Despite this mass, they are significantly lighter than protons and neutrons, reinforcing their role as key players in the strong interaction at lower energies.
However, the theory treating pions as fundamental fields encounters limitations. It is not renormalizable, leading to a complex and highly nonlinear Lagrangian. This inherent complexity results in physically nonsensical predictions when applied to very short distances or high energies – scales at or below the size of a proton. Essentially, the pion-mediated force description starts to break down when probing the finer structure of hadrons.
The quest for a more fundamental understanding led to the development of Quantum Chromodynamics (QCD). QCD revolutionized our view by explaining protons, neutrons, pions, kaons, and a multitude of other hadrons as composite particles made up of quarks, gluons, and antiquarks. In QCD, all strong interactions are ultimately attributed to the fundamental QCD Lagrangian, where gluons emerge as the true force carriers.
Therefore, when exploring physics at high energies or with sufficient resolution to peer inside protons and discern quarks, gluons become the essential mediators of the strong force. Pions, in contrast, serve as effective messengers only within approximate theories applicable at lower energies, significantly below the proton mass. This low-energy regime also corresponds to scenarios where the velocities of hadrons are much less than the speed of light. In essence, while pions provided an invaluable early understanding of the strong force, gluons represent the fundamental description at a deeper level.
