Who discovered down quarks?

Answer 1

Quarks were theorized in 1964 and experiementally verified in 1968. To say "invented" would be a misnomer.

This next part is a direct quote from the Wikipedia article titled "Quark" [ http://en.wikipedia.org/wiki/Quarks ]:

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweigin 1964. There was little evidence for this model until 1968, when electron-proton scattering experiments indicated that the electrons were scattering off three point-like constituents inside the proton. All six flavors of quark have been observed, following the discovery of the top quark at Fermilab in 1995.

Answer 2

The quark's existence was proposed in 1964. In 1968, experiments verified candidates for quarks in protons but were initially reluctant to declare they were actually quarks. The final quark flavor proposed in the current standard model was discovered in 1995.

Answer 3

In 1964 George Zweig and Murray Gell-Mann formulated a theory proposing the existence of quarks.

Answer 4

it was postulated by Murray Gell-Mann