Anti-quarks

Quarks are not found individually, but are found with other quarks arranged to form composites known as hadrons. There are two basic types of hadrons baryons, composed of three quarks, and mesons, composed of a quark and an anti-quark. Examples of baryons are the neutron and the proton. Neutrons are made of two down quarks and one up quark. Protons are made of two up quarks and one down quark. An example of the meson is the pion. This particle is made of an up quark and a down anti-quark. Such particles are unstable and tend to decay rapidly. The anti-particle of the proton is the anti-proton. The exception to the rule is the electron, whose anti-particle is the positron. 

Hadrons are also made up of even tinier particles called quarks. Six kinds of quarks have been described. They are up, down, charm, strange, top, and bottom. Each has a different charge and there are also anti-quarks with the opposite charges of their twin. 

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