A quark is currently the smallest known unit of matter, and they are regarded as the "building blocks- of matter. Because of their size, quarks are often regarded as "elementary particles."".
Quarks make up all forms of matter, including protons and neutrons. But there are other kinds of matter that quarks are part of. For example, they account for all known mesons and baryons, which fall under the category hadrons. .
When two or more quarks bond together, they form a hadron.
BARYONS AND MESONS.
Baryons are particles that are composed of an arrangement of three quarks. Neutrons and protons are baryons. Other baryons include the lambda, sigma, xi, and omega particles. .
Mesons on the other hand are composed of only two quarks. They are composed of a quark-antiquark pair. These include the pion, kaon, and rho particles. .
In the same way that antiparticles and antineutrinos exist, so too do antiquarks. Baryons can be formed by three antiquarks together as well as they can be formed by three quarks. .
For example, a proton can have two "up- quarks and one "down- quark (uud), or it can have two "up- antiquarks and one "down- antiquark (uud).
Mesons have a quark and an antiquark. For example, the πo meson has either a uu or dd combination. .
HISTORY OF THE QUARK.
In 1935, a Japanese theorist, Hideki Yukawa, predicted the existence of another type of particle. At this point in time, most believed that the electron, neutron and proton were the only types of particles that existed.
Shortly after World War II, many physicists returned from military laboratories to their own work, and began to investigate Yukawa's claims. They were surprised to discover that collisions caused in high-energy accelerators produced a large number of unidentified particle types.
In 1963, Murray Gell-man and George Zweig suggested a possible relationship between all these different types of particles. They suggested that all of these particles were composed of still smaller particles, which they called "quarks.