Boson

In particle physics, a boson (/ˈboʊzɒn/^[1] /ˈboʊsɒn/^[2]) is a subatomic particle whose spin quantum number has an integer value (0, 1, 2, ...). The class of bosons is one of the two fundamental classes of subatomic particle, the other being fermions, which have half odd-integer spin (1/2, 3/2, 5/2, ...). Every observed subatomic particle is either a boson or a fermion. Paul Dirac coined the term boson to classify the fundamental particles that obey Bose–Einstein statistics, the quantum framework pioneered by Satyendra Nath Bose.

Some bosons are elementary particles occupying a special role in particle physics, distinct from the role of fermions (which are sometimes described as the constituents of "ordinary matter"). Certain elementary bosons (e.g. gluons) act as force carriers, which give rise to forces between other particles, while one (the Higgs boson) contributes to the phenomenon of mass. Other bosons, such as mesons, are composite particles made up of smaller constituents.

Outside the realm of particle physics, multiple identical composite bosons behave at high densities or low temperatures in a characteristic manner described by Bose–Einstein statistics: for example, a gas of helium-4 atoms becomes a superfluid at temperatures close to absolute zero. Similarly, superconductivity arises because some quasiparticles, such as Cooper pairs, behave in this characteristic manner.

The term boson was coined by Paul Dirac^[3][4] to classify the fundamental particles that obey Bose–Einstein statistics, a quantum framework pioneered by the Indian physicist Satyendra Nath Bose.^[5] While serving as a reader (later professor) at the University of Dhaka in Bengal (now in Bangladesh),^[6] Bose achieved a fundamental breakthrough: by treating photons as identical and indistinguishable particles, he independently derived Planck's law without relying on classical physics.^[7] He sent this foundational manuscript to Albert Einstein, whose subsequent translation and endorsement of the work led to the formal development of the statistics and the theoretical prediction of the Bose–Einstein condensate.^[8]

Standard Model of particle physics
Elementary particles of the Standard Model
Background Particle physics Standard Model Quantum field theory Gauge theory Spontaneous symmetry breaking Higgs mechanism
Constituents Electroweak interaction Quantum chromodynamics CKM matrix Standard Model mathematics
Limitations Strong CP problem Hierarchy problem Neutrino oscillations Physics beyond the Standard Model
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All observed elementary particles are either bosons (with integer spin) or fermions (with odd half-integer spin).^[9] Whereas the elementary particles that make up ordinary matter (leptons and quarks) are fermions, elementary bosons occupy a special role in particle physics. They act either as force carriers which give rise to forces between other particles, or in one case give rise to the phenomenon of mass.

According to the Standard Model of Particle Physics there are five elementary bosons:

• One scalar boson (spin = 0)
  • H⁰ Higgs boson – the particle that contributes to the phenomenon of mass via the Higgs mechanism
• Four vector bosons (spin = 1) that act as force carriers. These are the gauge bosons:
  • photon photon – the force carrier of the electromagnetic field
  • gluon gluons (eight different types) – force carriers that mediate the strong force
  • Z boson neutral weak boson – the force carrier that mediates the weak force
  • W^± charged weak bosons (two types) – also force carriers that mediate the weak force

A second-order tensor boson (spin = 2) called the graviton (G) has been hypothesised as the force carrier for gravity, but so far all attempts to incorporate gravity into the Standard Model have failed.^{[lower-alpha 1]}

Composite particles (such as hadrons, nuclei, and atoms) can be bosons or fermions depending on their constituents. Since bosons have integer spin and fermions half odd-integer spin, any composite particle made up of an even number of fermions is a boson (e.g., 1/2 + 1/2 + 1/2 + 1/2 = 2 for the three quarks and an electron in a hydrogen atom).

Composite bosons include:

• All mesons of every type
• Stable nuclei with even mass numbers such as deuterium, helium-4 (the alpha particle),^[10] carbon-12, lead-208, and many others.^{[lower-alpha 2]}

As quantum particles, the behaviour of multiple indistinguishable bosons at high densities is described by Bose–Einstein statistics. One characteristic which becomes important in superfluidity and other applications of Bose–Einstein condensates is that there is no restriction on the number of bosons that may occupy the same quantum state. As a consequence, when for example a gas of helium-4 atoms is cooled to temperatures very close to absolute zero and the kinetic energy of the particles becomes negligible, it condenses into a low-energy state and becomes a superfluid.

Other examples in condensed matter systems include Cooper pairs in superconductors and excitons in semiconductors.^[11]

Certain quasiparticles are observed to behave as bosons and to follow Bose–Einstein statistics, including Cooper pairs, plasmons and phonons.^[12]: 130

• Anyon – Type of two-dimensional quasiparticle
• Physics:Bose gas – State of matter of many bosons
• Parastatistics – Notion in statistical mechanics

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