Quarks are one of the three tiniest basic, or elementary, particles from which every substance is made.
Quarks are too small for their size to be measured, but their mass can. The biggest quark, called a top quark, is as heavy as an atom of gold. The smallest, called an up quark, is 35,000 times lighter.
There are six kinds, or flavors, of quark: up (u), down (d), bottom (b), top (t), strange (s) and charm (c).
Down, bottom and strange quarks carry one-third of the negative charge of electrons; up, top and charm ones carry two-thirds of the positive charge of protons.
Quarks never exist separately but in combination with one or two other quarks. Combinations of two or three quarks are called hadrons.
Three-quark hadrons are called baryons and include protons and neutrons. Rare two-quark hadrons are mesons.
A proton is made from two up quarks (two lots of +2/3 of a charge) and one down quark (-1/3) and has a positive charge of 1.
A neutron is made from two down quarks (two lots of –1/3 of a charge) and an up quark (+2/3). The charges cancel each other out, giving a neutron no charge.
The theory of quarks was first proposed by Murray Gell-Mann and Georg Zweig in 1964.
Quarks are named after a famous passage in James Joyce’s hook Ulysses: ‘Three quarks for Muster Mark!’