1.1.1: Fundamental Particles

All matter is made up of quarks or leptons. Quarks only exist in combination with other quarks; whereas leptons may exist in isolation.


Quarks are points of matter that exist with other quarks as a pair or triplet. They are bound together by gluons, the force carrier particle of the strong force. Collections of quarks are hadrons. A triplet of quarks is known as a baryon.

Quarks may also exist as a pair - one quark, and one anti-quark - known as a meson. The most commonly encountered meson is the pion, denoted by π⁺. Because they are formed by anti-matter, mesons are extremely unstable.Pion.jpg

There are six quarks - up, down, top, bottom, strange and charm; there are also six anti-quarks (anti-up and so on). Quarks possess a partial charge of $\frac{+2}{3}$ or $\frac{-1}{3}$. Anti quarks possess the opposite charge.

The only quarks that constantly exist outside of experimental conditions or high energy particle collisions (e.g. cosmic rays) are the up quark and the down quark The other quarks are unstable and rapidly decay to the stable up and down quarks.


Leptons are point particles that can exist in isolation, unlike quarks. They also have a much lower mass than a quark. They may carry a negative unit charge (-1) or have no charge (0). Much like the quarks, there are six leptons. The three charged leptons are electrons, muons and tau. The three uncharged leptons are neutrinos. To round things off, there are also antimatter anti-leptons for every standard lepton, carrying the opposite charge. The special antimatter antilepton is the positron, which is an anti-electron with a charge of +1.
Charged leptons tend to interact strongly with other particles due to the electromagnetic force. The uncharged neutrinos only interact rarely and are difficult to detect. The electron is the only charged lepton in existence, but the three neutrinos and antineutrinos exist in some quantity.

Makeup of the Universe

All known matter in the universe is made up of:

  • The up and down quark
  • The electron lepton
  • The three uncharged neutrinos and the three uncharged anti-neutrinos

All interactions in the universe are made up of forces that act between these particles. These are discussed in the Fundamental Forces topic.