The down quark is one of the most abundant quarks in the universe: together with the up quark, they form protons and neutrons.
There is a plethora of particles that are bound states of quarks and gluons. These particles are called hadrons, and are classified in baryons, such as protons and neutrons, that are composed of three quarks, and mesons, made up of a quark and an antiquark. The lightest meson is the pion, which consists of an up quark and a down quark. It exists in two charged forms, π+ and π-, and a neutral form, the π0. Pions occur as the lightest mesons in many processes.
In the 1960s, many new particles were discovered in particle physics. The vast majority decays often after very short lifetimes. To bring order to this particle chaos, M. Gell-Mann and G. Zweig postulated the existence of up and down quarks in 1964, suggesting that particles are bound states of quarks and formalising a categorisation of the new particles. Five years later, the lightest quarks (up, down and strange) were discovered at the particle physics centre SLAC in California.
The name “quark” comes from the passage “Three Quarks for Muster Mark!” from a novel by J. Joyce.