The Fundamental Particles: Quarks

An atom is composed of electrons and a nucleus. Inside the nucleus, one can find the proton(s) and neutron(s). Just like the atom is composed of a substructure and likewise for the nucleus, mesons and baryons have a substructure, the subparticles called quarks.

In 1962, Gell-Mann searched for a substructure, the building blocks of particles. At the time, he only postulated the existence of quarks inside the group of baryons and mesons called haydrons. But physicists discovered these quarks in the years to come, the last quark in 1994-5 at Fermilab.

A quark can only exist when it is combined with another quark. Thus, mesons are composed of a quark and an antiquark while baryons have three quarks. Leptons (like the electron), however, do not have any subparticles since they are pointlike particles.

So, the question you must be asking, "If a quark is so small that it cannot exist alone, how do you measure its mass?"

The mass of a quark is not measured in terms of weight measure on a scale. You can not place a quark on a scale and take its mass. You could imagine the size of that scale !

The mass of a quark is related to the mass from the equation F = ma (Force is equal to mass times accleration). This equation tells how an object will behave when a force is applied. The equation of particle physics includes, for example, calcuations of what happens to a quark when struck by a high energy photon.

That is why the mass of a quark can be measured either in MeV (Mega or million electron Volts) or GeV (Giga or billion electron Volts).

The following chart illustrates the six quarks discovered since 1995: