Pions are not fundamental particles like quarks or electrons but mesons, meaning they’re made of a quark and an antiquark bound together by the strong nuclear force.
First predicted by Hideki Yukawa in 1935 as the carriers of the residual strong force, pions were later discovered in cosmic rays and became the first confirmed mesons. Pions act on a larger scale than gluons, mediating the attraction between nucleons (protons and neutrons) within an atomic nucleus. When a proton or neutron emits a pion, another nucleon quickly absorbs it, keeping the nucleus intact in a balance.
Pions come in 3 varieties: positively charged (π⁺), negatively charged (π⁻), and neutral (π⁰). They are the lightest mesons, which makes them relatively short-lived. Charged pions decay into muons and neutrinos, while neutral pions decay even faster into photons.
Pions also played a major role in the development of quantum chromodynamics (QCD), the modern theory of the strong force. Studying their interactions helped us understand how quarks and gluons work together to form hadrons.