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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 or 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 deuterium (2H), which is composed of one proton plus one neutron [NP] is a boson, while the tritium (3H), which is composed of two neutrons plus one proton [NPN] is a fermion.
Composite bosons exhibit bosonic behavior only at distances large compared to their structure size. At 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 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:
- The nucleus of Deuterium, an isotope of Hydrogen
- Helium-4 atoms
- Sodium-23 atoms
- Any nucleus with integer spin
- Photons, which mediate the electromagnetic force
- W and Z bosons, which mediate the weak nuclear force
- Gluons
- Higgs bosons
- Phonons
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Since gluons themselves carry color charge (again, unlike the photon which is electrically neutral), they participate in strong interactions. These gluon-gluon interactions constrain color fields to string-like objects called "flux tubes", which exert constant force when stretched. Due to this force, quarks are confined within composite particles called hadrons. This effectively limits the range of the strong interaction to 10-15 meters, roughly the size of an atomic nucleus.
Gluons also share this property of being confined within hadrons. One consequence is that gluons are not directly involved in the nuclear forces. The force mediators for these are other hadrons called mesons.
Although in the normal phase of QCD single gluons may not travel freely, it is predicted that there exist hadrons which are formed entirely of gluons — called glueballs. There are also conjectures about other exotic hadrons in which real gluons (as opposed to virtual ones found in ordinary hadrons) would be primary constituents. Beyond the normal phase of QCD (at extreme temperatures and pressures), quark gluon plasma forms. In such a plasma there are no hadrons; quarks and gluons become free particles.
[edit] Experimental observations
The first direct experimental evidence of gluons was found in 1979 when three-jet events were observed at the electron-positron collider called PETRA at DESY in Hamburg. Quantitative studies of deep inelastic scattering at the Stanford Linear Accelerator Center had established their existence a decade before that.
Experimentally, confinement is verified by the failure of free quark searches. Neither free quarks nor free gluons have ever been observed. Although there have been hints of exotic hadrons, no glueball has been observed either. Quark-gluon plasma has been found recently at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratories (BNL).
[edit] See also
he gluon is a vector boson; like the photon, it has a spin of 1. While massive spin-1 particles have three polarization states, massless gauge bosons like the gluon have only two polarization states because gauge invariance requires the polarization to be transverse. In quantum field theory, unbroken gauge invariance requires that gauge bosons have zero mass (experiment limits the gluon's mass to less than a few MeV). The gluon has negative intrinsic parity and zero isospin. It is its own antiparticle.
[edit] Numerology of gluons
Unlike the single photon of QED or the three W and Z bosons of the weak interaction, there are eight independent types of gluon in QCD.
This may be difficult to understand intuitively. Quarks may carry three types of color charge; antiquarks carry three types of anticolor. Gluons may be thought of as carrying both color and anticolor or as describing how quark color changes during interactions.
Technically, QCD is a gauge theory with SU(3) gauge symmetry. Quarks are introduced as spinor fields in Nf flavours, each in the fundamental representation (triplet, denoted 3) of the colour gauge group, SU(3). The gluons are vector fields in the adjoint representation (octets, denoted 8) of colour SU(3). For a general gauge group, the number of force-carriers (like photons or gluons) is always equal to the dimension of the adjoint representation. For the simple case of SU(N), the dimension of this representation is N2−1.
[edit]
In particle physics, gluons are subatomic particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei.
In technical terms, they are vector gauge bosons that mediate strong color charge interactions of quarks in quantum chromodynamics (QCD). Unlike the neutral photon of quantum electrodynamics (QED), gluons themselves participate in strong interactions. The gluon has the ability to do this as it carries the color charge and so interacts with itself, making QCD significantly harder to analyse than QED.
Contents
particle physics, gluons are subatomic particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei. In technical terms, they are vector gauge bosons that mediate strong color charge interactions of quarks in quantum chromodynamics (QCD). Unlike the neutral photon of quantum electrodynamics (QED), gluons themselves participate in strong interactions. The gluon has the ability to do this as it carries the color charge and so interacts with itself, making QCD significantly harder to analyse than QED
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 or 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 deuterium (2H), which is composed of one proton plus one neutron [NP] is a boson, while the tritium (3H), which is composed of two neutrons plus one proton [NPN] is a fermion.
Composite bosons exhibit bosonic behavior only at distances large compared to their structure size. At 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 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:
- The nucleus of Deuterium, an isotope of Hydrogen
- Helium-4 atoms
- Sodium-23 atoms
- Any nucleus with integer spin
- Photons, which mediate the electromagnetic force
- W and Z bosons, which mediate the weak nuclear force
- Gluons
- Higgs bosons
- Phonons
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.
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