Hadron

A hadron, in particle physics, is any subatomic (smaller than the atom) particle or antiparticle which is made of quarks.

How hadrons fit with the two other classes of sub atomic particles, bosons and fermions.

Quarks are fundamental particles which among other properties have an electrical charge and a name (e.g. 'up', 'down', 'strange'). The electrical charge of a single quark is always a fraction (e.g. 2/3).^[1] Quarks combine to make a hadron of integer charge, for example, the charges of two 'down' quarks and one 'up' quark combine like this -1/3 -1/3 +2/3 = 0 to make a neutrally charged neutron. Quarks do not appear alone,^[2] they exist in groups and are held together by the strong nuclear force. The word to describe the different types of quarks is flavours. According to the current standard model there are 6 flavours of quarks.^[1] There are two known ways quarks can combine, to give the two categories of hadrons:

• baryons, which are made of three quarks.
• mesons, which are made of one quark and one anti-quark.

Hadrons are also characterized by being affected by the strong nuclear force, which holds them tightly together over a short distance. At the center of an atom is a nucleus (a cluster of protons and neutrons).

List of hadrons

Known to exist

Baryons

• Proton, with quark structure uud ('up up down', referring to the three quarks making up the proton.)
• Neutron, with quark structure udd ('up down down')

Mesons

• Pion
• Kaon

These are the most commonly discussed hadrons. There are many more: complete list of baryons, complete list of mesons.

Not yet known to exist

• Tetraquark
• Pentaquark

These are predictions for different ways quarks may be put together. There is not any solid evidence for them existing yet.

References

1. "Quarks". hyperphysics.phy-astr.gsu.edu.
2. "The Particle Adventure - What is the world made of? - Hadrons, Baryons, and Mesons". www.particleadventure.org.