|Murray Gell-Mann won the 1969 Nobel Prize in Physics.Credit: Santa Fe Institute|
Topics: Nobel Laureate, Nobel Prize, Particle Physics, Quarks, Standard Model, Theoretical Physics
The Nobel Prize in Physics 1969 was awarded to Murray Gell-Mann "for his contributions and discoveries concerning the classification of elementary particles and their interactions."
The Nobel Prize in Physics 1969. NobelPrize.org. Nobel Media AB 2019. Wed. 29 May 2019. < https://www.nobelprize.org/prizes/physics/1969/summary/ >
Murray Gell-Mann, one of the founders of modern particle physics, died on 24 May, aged 89. Gell-Mann’s most influential contribution was to propose the theory of quarks — fundamental particles that make up most ordinary matter.
To bring order to a plethora of recently discovered subatomic particles, in 1961 Gell-Mann proposed a set of rules based on symmetries in the fundamental forces of nature. The rules classified subatomic particles called hadrons into eight groups, a scheme he named the eightfold way in a reference to Buddhist philosophy.
In 1964, he realized that such rules would naturally arise if the particles were composed of two, three or more fundamental particles, held together by the strong nuclear force. (US–Russian physicist George Zweig came to the same conclusion independently in the same year.) Protons and neutrons, for example, would be made up of three of these more fundamental particles, which Gell-Man named quarks, inspired by a quote — “Three quarks for Muster Mark!” — from James Joyce’s 1939 novel Finnegan's Wake. 
Quarks and Leptons are the building blocks which build up matter, i.e., they are seen as the "elementary particles". In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (over 200). The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. Quarks are observed to occur only in combinations of two quarks (mesons), three quarks (baryons). There was a recent claim of observation of particles with five quarks (pentaquark), but further experimentation has not borne it out.