Quantum Mechanics/Quarks and Leptons

All matter is divided into two broad categories: hadrons and leptons.

All the hadrons are composed of quarks, whereas leptons are material particles not containing quarks. Furthermore, leptons are believed to be point-like fundamental particles.

Quarks come in 6 different types:

• up (u), down (d) (1st generation)
• strange (s), charm (c) (2nd generation)
• bottom (b), top (t) (3rd generation)

In some literature, the bottom quark is also referred to as the "beauty" quark, and the top quark the "truth" quark.

However, quarks are never seen in isolation but always bind together in pairs or triplets, to form protons, neutrons, and many other particles. The up and down quarks form the protons and neutrons that we are familiar with: a proton is a bound state of 2 up quarks and 1 down quark, whilst a neutron is a bound state of 1 up quark and 2 down quarks.

Leptons also come in 6 different varieties: the familiar electron (e), the muon ($\mu$), the tau ($\tau$), and the electron neutrino (${\nu }_e$), mu neutrino (${\nu }_{\mu }$) and tau neutrino (${\nu }_{\tau }$). Until recent years it was thought the neutrino was a massless particle but it has since been found that neutrinos can "oscillate" (change) between different types. This is only possible if they have mass, albeit a very small mass in the case of the electron neutrino.