The Irish novelist James Joyce (1882-1941) had a drunken character in Finnegan’s Wake raise a quart of dark beer to toast a man named Finnegan who had just died. He mistakenly said “raise a quark for muster Mark”. The American physicist, Nobel laureate Murray Gell-Mann (1929-2019), who was one of the scientists who proposed the existence of the quark in 1964, thought it was so funny that he named this sub-particle after the drunken host. The Russian-American physicist, George Zweig, also independently proposed the existence of the quark that same year.
A quark is a type of elementary particle that is a fundamental constituent of matter. Quarks combine to create composite particles called hadrons. Hadrons are subatomic particles of a type that includes protons and neutrons, which can take part in the strong interaction that holds atomic nuclei together. Indeed, the most stable hadrons are protons and neutrons–the components that form the nuclei of atoms. Because of a phenomenon termed color confinement, quarks have not been directly observed or found in isolation. For this reason, they have been found only within hadrons. Because of this, a great deal of what scientists have learned about quarks has been derived from studying hadrons.
Quarks also show certain intrinsic properties, including mass, color, electric charge, and spin. They are the only known elementary particle in the Standard Model of Particle Physics to display all four fundamental interactions–also termed fundamental forces–the strong interaction, the weak interaction, gravitation, and electromagnetism. Quarks are also the only known elementary particles whose electric charges are not integer multiples of the elementary charge.
The types of quarks are referred to as flavors: up, down, strange, charm, bottom, and top. The heavier quarks quickly experience a metamorphosis into up and down quarks as the result of a process called particle decay. Particle decay refers to the transformation from a higher mass state to lower mass states. For this reason, up and down quarks are stable, as well as the most abundant in the Universe. In contrast, strange, charm, bottom, and top quarks can only be churned out in high energy collisions–such as those involving cosmic rays or particle accelerators. For every quark flavor there is a corresponding antiquark. The antiquark antiparticle differs from the quark only in certain properties, such as electric charge. The antiquark antiparticles have equal magnitude but an opposite sign.
There was little evidence for the physical existence of quarks until deep inelastic scattering experiments were conducted at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for the existence of all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.