The Quark Model
All hadrons are made up of a combination of a fundamental particle called quarks. There are six different types and quark, with each quark having a corresponding antiquark (there are also six different types of lepton). Quarks are subject to both strong and weak forces, with the strong force holding the quarks together in a proton for example, and the weak force changing quarks into other quarks.
The charge and mass properties of quarks are given in the table below.
Electric Charge | Generation | ||
| u or up Mass=5 MeV | c or charm Mass=1500 MeV | t or top Mass=174000 MeV |
| d or down Mass=10 MeV | s or strange Mass=200 MeV | b or bottom Mass=4700 MeV |
Isolated quarks cannot exist. They must combine to form particles, either in particle antiparticle pairs called mesons or in combinations of three quarks called baryons. For example
a proton is made up of two up quarks and a down quark,
The charge on this combination of quarks is
a neutron is made up of an up quark and two down quarks,
The charge on this combination of quarks is
a
meson is made up of a down quark and an anti – up quark,
The charge on this pair is
a
meson is made up of a anti - down quark and an up quark,
The charge on this pair is
Quarks exert the strong force on each other, and baryons exert the strong force on each other, being made up of quarks. The full theory of the strong force interaction is called quantum chromodynamics or QCD. The 'chromo' label originates in that each quark is labelled with a colour – red, green or blue. When quarks combine to form a particle, they must do so on a way that makes the 'colour' of the particle white e.g. red+blue+green=white as in the ordinary light spectrum.
