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As our knowledge of physics grows, so we discover smaller and smaller particles. Can this continue forever, or are there fundamental particles, which we can't break down any further? Our current understanding of physics leads us to believe that there do indeed exist these particles, from which we can create the structures we know about. They are Leptons and Quarks.
These are particles such as muons and electrons, there are 6 leptons in total, each with their anti-lepton counterpart. For the electron, muon and taon (which are referred to as different flavours of the lepton) there is a corresponding neutrino (a lepton) associated with it. Leptons do not participate in the strong interaction and are generally not seen within the nucleus.
The term 'quark' was introduced by Murray Gell-Mann, the word originating from the book 'Finnegan's Wake' by James Joyce in which the quotation 'Three quarks for Muster Mark' appears. We now know there are are six quarks (or called flavours of quarks), which are grouped into 3 pairs (or generations); up & down, charmed & strange and top and bottom. It is these fundamental particles which form neutrons, protons etc, which are collectively known as hadrons, (it is mainly the up and down which form the world around us). The quarks are peculiar as they posses a charge which is a fraction of that for the electron. Take for example the proton, it has charge +1 and is formed from up and down quarks so the only combination available is 2 up quarks and a down.
There are two types of hadron, the Baryon which is a system of three quarks (e.g. the proton) or Mesons, a two quark system containing a quark - antiquark pair (e.g. the pion or pi-meson). Baryons are usually confined within nuclei as are unstable and decay if isolated, for example a neutron has a lifetime of about fifteen minutes if not inside the nucleus. The exception to this is the proton which is essentially stable in free space.