Spin
Spin is the name for the angular momentum carried by a particle.
For composite particles, the spin is made up from the combination of the spins of the constituents plus the angular momentum of their motion around one-another.
For fundamental particles spin is an intrinsic and inherently quantum property, it cannot be understood in terms of motions internal to the object.
The intrinsic spin must be included in applications of conservation of angular momentum.
Quantum Units
In quantum theories all angular momenta due to motion of one object around another are given as integer multiples of Planck's constant divided by (2pi). This quantity is called h-bar and is 6.58 x 10-25 GeV seconds.
Fermions and Bosons
Particles can be divided into two classes on the basis of their spins.
- Bosons are particles with integer spin in h-bar units.
- Fermions are particles with and odd number of half-integer h-bar units of spin. (1/2, 3/2, 5/2 ...)
Fermions obey a Pauli Exclusion Principle -- no two
of the same type can exist in the same state at the same place
and time.
You may be familiar with this type of rule for electrons in
atoms.
Bosons do not follow such a rule and, in a certain sense, favor states with many particles in the same state. For example, a laser beam is formed from a "coherent state" of many identical photons.
Matter particles -- quarks and leptons -- are fermions with 1/2 unit of spin. Baryons are made from odd numbers of quarks and so are fermions.
Force carriers -- gluons, W's, Z's, and photons are bosons with 1 unit of spin. Mesons also are bosons.
