Hadron BrawlerStrong Nuclear Force / Quantum Chromodynamics

Hadron Brawler cannot lose a quark. He has tried.

Inside a proton the strong force behaves unlike any other. The formula on this page is the Cornell potential, fitted in 1975 by Estia Eichten and colleagues at Cornell to charmonium, a newly discovered pairing of a charm quark with its antiquark. It has two terms. At short range the first dominates: a pull shaped like electric attraction, but weak, because the strong coupling αs shrinks at short distances. That shrinking is asymptotic freedom, and it earned David Gross, David Politzer and Frank Wilczek the 2004 Nobel Prize. Up close, quarks rattle around almost freely.

Pull them apart and the second term takes over: σr, energy rising in a straight line with distance. The gluon field between the quarks does not spread out the way an electric field does. It narrows into a tube, and the tube costs about the same energy for every extra femtometre, roughly a giga-electronvolt per femtometre. That is a steady tension of around sixteen tonnes-force, acting across something smaller than a proton. A pull that never weakens means the quark never gets out.

What you get instead is a brawl. Once the tube holds enough energy, it becomes cheaper for the field to create a new quark and antiquark than to keep stretching. The tube snaps, and each broken end takes one of the new quarks. Hit a proton hard enough in a collider and this happens again and again, and what comes out is not free quarks but tight sprays of ordinary particles, called jets.

Colour charge, the property behind all of this, was proposed in 1964–65; quantum chromodynamics took its modern form in 1973. Nobody has ever isolated a single quark.

Hadron Brawler swings. The rope breaks into more rope.