pion
/ PIE-on /
Among all the particles built from quarks, the lightest is the pion. It comes in three varieties — positively charged, negatively charged, and neutral — and it is everywhere in particle physics: when a high-energy proton hits anything, the spray of debris is dominated by pions. They are so common that physicists half-jokingly call them the photons of the strong-force world.
A pion is the lightest meson — one of the up or down quarks paired with one of the up or down antiquarks. The positive pion is an up quark and an anti-down; the negative pion is its antiparticle; the neutral pion is a quantum blend of up-antiup and down-antidown. Charged pions are quite long-lived for an unstable hadron, lasting about 26 billionths of a second before decaying into a muon and a neutrino; the neutral pion is far shorter-lived and decays mostly into two photons. The pion was predicted in 1935, before it was found, as the carrier of the force between nucleons.
The pion plays a starring role in nuclear physics: the exchange of pions between protons and neutrons is the original explanation for the force that binds a nucleus together. It is also theoretically special. The pion is unusually light because it is an approximate Goldstone boson — a leftover of a near-symmetry of the strong interaction that is spontaneously broken. That makes the humble pion a sensitive probe of the deepest workings of the strong force.
Charged pion content: positive pion = up + anti-down; negative pion = down + anti-up. The charged pion decays mostly to a muon and a neutrino.
The pion is the lightest hadron and the most copiously produced particle in high-energy collisions.
The pion is light not because its quarks are light in the usual sense, but for a deep symmetry reason. So you should not picture it as just a tiny meson; its lightness is a clue about hidden structure in the strong interaction.