Boson
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In particle physics, bosons, named after Satyendra Nath Bose, are particles having integer spin. Most bosons are composite particles, but four bosons (the so-called gauge bosons) are elementary particles which are not known to be composed of other particles.
[edit] Boson properties
All elementary particles and composite particles are either bosons or fermions (depending on their spin). The spin-statistics theorem identifies the resulting quantum statistics that differentiate fermions and bosons. Informally speaking, fermions are "stiff" and thus considered to be particles of matter.
Due to their integer spin, bosons obey Bose–Einstein statistics, one consequence of which is the Bose–Einstein condensation of particles—in which any number of bosons can share the same quantum state. This allows masers and lasers to operate—all photons in these devices are in the same quantum state.
Interaction of virtual bosons with real fermions are called fundamental interactions. Momentum conservation in these interactions mathematically results in all forces we know. The bosons involved in these interactions are called gauge bosons—such as the W vector bosons of the weak force, the gluons of the strong force, the photons of the electromagnetic force, and (in theory) the graviton of the gravitational force.
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 in fact bosons. So even though the main three massive subatomic particles i.e. the proton, neutron, and electron are all fermions, it is possible for a single element such as helium to have some isotopes that are fermions (e.g. 3He) and other isotopes that are bosons (e.g. 4He). (3He) is composed of one neutron and two protons [PNP]. Likewise, the deuteron (2H), which is composed of one proton plus one neutron [NP] is a boson, while the triton (3H), which is composed of two neutrons plus one proton [NPN] is a fermion. The deuterium atom composed of three fermions (proton+neutron+electron)is a fermion, while its nucleus [NP] when separated from the electron is a boson.
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 constituting 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.
While fermions obey the Pauli exclusion principle: "no more than one fermion can occupy a single quantum state", there is no exclusion property for bosons, which can occupy the same quantum state. The result is that the spectrum of photon gas of certain equilibrium temperature is a Planck spectrum (one example of which is black-body radiation; another is the thermal radiation of the early Universe seen today as microwave background radiation). Operation of lasers, the properties of superfluid helium-4 and recent formation of Bose–Einstein condensates of atoms are all consequences of statistics of bosons.
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 reduce to Maxwell-Boltzmann statistics, so both the boson and fermion particles behave as classical particles.
Examples of bosons:
- Photons, which mediate the electromagnetic force
- W and Z bosons, which mediate the weak nuclear force
- Gluons
- Higgs bosons
- Phonons
- Cooper pairs
[edit] See also
- Bosonic field
- Bose gas
- Fermions
- Identical particles
- List of particles
- Parastatistics
- Tonks-Girardeau gas
- Standard model
- Superconductivity
[edit] References
- Sakurai, J.J. (1994). Modern Quantum Mechanics (Revised Edition), pp 361-363. Addison-Wesley Publishing Company. ISBN 0-201-53929-2.
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| Fermions: Quarks: (Up · Down · Strange · Charm · Bottom · Top) | Leptons: (Electron · Muon · Tau · Neutrinos) | |
| Gauge bosons: Photon | W and Z bosons | Gluons | |
| Not yet observed: Higgs boson | Graviton | Other hypothetical particles | |
