A Schematic Model of Baryons and Mesons
Hundreds of "elementary" particles are really made of just three quarks.
Everything around you is built from atoms — but the heart of every atom is built from something smaller still, arriving in threes.
The big idea
In the 1950s physicists kept discovering new subatomic particles until there were hundreds of them — far too many for all of them to be "elementary." In 1964 Murray Gell-Mann found the hidden order: nearly all of them are made from just a few tiny building blocks he named quarks.
A proton is three quarks; so is a neutron — just a different mix. Lighter particles, called mesons, are a quark paired with its mirror-twin, an antiquark. The quarks come in types — Gell-Mann's first three were "up," "down" and "strange" — and combining them in different ways builds the whole crowd, the way a few kinds of brick build every wall.
How it came about
Gell-Mann had already tidied the particle chaos once, sorting the particles into neat families by a scheme he wittily named the Eightfold Way, after the Buddhist path. One family had a gap — a missing particle — and when it was found at Brookhaven in 1964, exactly where the pattern said it should be, the families clearly meant something real. Quarks were his answer to why.
He was not alone. At CERN that same year, a young physicist named George Zweig had the very same idea, calling the pieces "aces" — but his write-up was never accepted by a journal, and for years he received far less credit. Gell-Mann took the odd word "quark" from a line in James Joyce's novel Finnegans Wake: "Three quarks for Muster Mark."
Why it mattered
The quark idea turned a bewildering zoo into a simple system — and it was almost too strange to believe, because quarks carry fractional electric charge, one-third and two-thirds of the charge on an electron, something never seen on its own. Gell-Mann was so unsure they were real that he half-dared experimenters to prove they weren't. They turned out to be the bedrock of all matter: the deepest layer of "stuff" we have ever found.
A way to picture it
Think of the hundreds of particles as words, and quarks as a tiny alphabet. You don't need a separate symbol for every word — a few letters, arranged in different orders, spell them all. Three quark "letters" spell every baryon; a quark and an antiquark spell every meson. And just as letters in a real text are never found scattered loose across the page, quarks are never found alone — they only ever appear locked together inside a particle.
Where it sits
The quark model grew out of decades of probing the atom — from Thomson's electron and Rutherford's nucleus, elsewhere in this Library, through the bewildering particle discoveries of the mid-century. It runs forward to today's Standard Model and to the Higgs boson, also here, which gives those quarks their mass. The proton you are made of is, at bottom, three of Gell-Mann's quarks held together — and held so tightly that no one has ever pulled a single one out on its own.
We then refer to the members u(2/3), d(−1/3) and s(−1/3) of the triplet as "quarks".
A search for stable quarks of charge −1/3 or +2/3 and/or stable di-quarks of charge −2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks.