Boson

In particle physics, bosons are particles with an integer spin, as opposed to fermions which have half-integer spin. From a behaviour point of view, fermions are particles that obey the Fermi-Dirac statistics while bosons are particles that obey the Bose-Einstein statistics. They may be either elementary, like the photon, or composite, as mesons. All force carrier particles are bosons. They are named after Satyendra Nath Bose. In contrast to fermions, several bosons can occupy the same quantum state. Thus, bosons with the same energy can occupy the same place in space.

While most bosons are composite particles, four bosons (the gauge bosons) are elementary particles not known to be composed of other particles. The only boson in the Standard Model that is yet to be discovered experimentally is the Higgs boson.

Basic properties
All elementary and composite particles in 3 dimensional space are either bosons or fermions, depending on their spin. Particles with half-integer spin are fermions; particles with integer spin are bosons. The spin-statistics theorem identifies the resulting quantum statistics that differentiate fermions and bosons. Bosons obey Bose–Einstein statistics. Fermions, on the other hand, cannot occupy the same quantum state as each other; they obey the Fermi-Dirac statistics and the Pauli exclusion principle. They "resist" being placed close to each other. So, fermions possess "rigidness" and thus sometimes are considered to be "particles of matter". The properties of lasers and masers, superfluid helium-4 and Bose–Einstein condensates are all consequences of statistics of bosons. Another result is that the spectrum of a photon gas in thermal equilibrium is a Planck spectrum, one example of which is black-body radiation; another is the thermal radiation of the opaque early Universe seen today as microwave background radiation. Interaction of virtual bosons with real fermions are called fundamental interactions, and these result in all forces we know. The bosons involved in these interactions are called gauge bosons. These include the W and Z bosons of the weak force, the gluons of the strong force, the photons of the electromagnetic force, and, in quantum gravity, the yet to be verified graviton of the gravitational force.

In large systems, the difference between bosonic and fermionic statistics is only apparent at large densities—when their wave functions overlap. At low densities, both types of statistics are well approximated by Maxwell-Boltzmann statistics, which is described by classical mechanics.

Composite bosons
Particles composed of a number of other particles (such as protons, neutrons or nuclei) can be either fermions or bosons, depending on their total spin. Hence, many nuclei are bosons. For instance, consider 3He. It is made of 2 protons, a neutron and 2 electrons. Since the spins of these five fermions must add to a half integer, 3He is a fermion. On the other hand 4He, which is made of six fermions, is a boson. Likewise, the deuteron (2H+), which is composed of a proton and a neutron, is a boson, however the neutral deuterium atom, which also has an electron, is a fermion.

Composite bosons exhibit bosonic behavior only at distances large compared to their structure size. At a small distance they behave according to properties of their constituent particles. For example, despite the fact that an alpha particle is a boson, at high energy it interacts with another alpha particle not as a boson but as an ensemble of fermions.

Examples of bosons

 * Photons, which mediate the electromagnetic force
 * W and Z bosons, which mediate the weak nuclear force
 * Gluons, which mediate the strong nuclear force
 * Higgs bosons
 * Phonons
 * Cooper pairs