Boson

For other uses, seeBoson (disambiguation).

Bosons form one of the two fundamental classes ofsubatomic particle,the other beingfermions.All subatomic particles must be one or the other. A composite particle (hadron) may fall into either class depending on its composition

Inparticle physics,aboson(/ˈboʊzɒn//ˈboʊsɒn/) is asubatomic particlewhosespin quantum numberhas an integer value (0, 1, 2,...). Bosons form one of the two fundamental classes of subatomic particle, the other beingfermions,which have odd half-integer spin (1⁄2,3⁄2,5⁄2,...). Every observed subatomic particle is either a boson or a fermion.Paul Diraccoined the namebosonto commemorate the contribution ofSatyendra Nath Bose,anIndianphysicist.

Some bosons areelementary particlesoccupying 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 asforce carriers,which give rise to forces between other particles, while one (theHiggs boson) contributes to the phenomenon ofmass.Other bosons, such asmesons,are composite particles made up of smaller constituents.

Outside the realm of particle physics, multiple identical composite bosons (in this context sometimes known as 'bose particles') behave at high densities or low temperatures in a characteristic manner described byBose–Einstein statistics:for example a gas ofhelium-4atoms becomes asuperfluidat temperatures close to absolute zero. Similarly,superconductivityarises because somequasiparticles,such asCooper pairs,behave in the same way.

Name

The namebosonwas coined byPaul Diracto commemorate the contribution ofSatyendra Nath Bose,anIndianphysicist. When Bose was a reader (later professor) at theUniversity of Dhaka,Bengal(now inBangladesh),he andAlbert Einsteindeveloped the theory characterising such particles, now known asBose–Einstein statisticsandBose–Einstein condensate.

Elementary bosons

See also:List of particles § Bosons

All observedelementary particlesare either bosons (with integer spin) orfermions(with odd half-integer spin).Whereas the elementary particles that make up ordinary matter (leptonsandquarks) are fermions, elementary bosons occupy a special role in particle physics. They act either asforce carrierswhich give rise to forces between other particles, or in one case give rise to the phenomenon ofmass.

According to theStandard Model of Particle Physicsthere are five elementary bosons:

• Onescalar boson(spin = 0)
  • 
    H⁰
    Higgs boson– the particle that contributes to the phenomenon ofmassvia theHiggs mechanism
• Fourvector bosons(spin = 1) that act asforce carriers.These are thegauge bosons:
  • 
    γ
    Photon– the force carrier of theelectromagnetic field
  • 
    g
    Gluons(eight different types) – force carriers that mediate thestrong force
  • 
    Z
    Neutral weak boson– the force carrier that mediates theweak force
  • 
    W^±
    Charged weak bosons(two types) – also force carriers that mediate the weak force

• Asecond order tensor boson(spin = 2) called thegraviton(G) has been hypothesised as the force carrier forgravity,but so far all attempts to incorporate gravity into the Standard Model have failed.

Composite bosons

See also:List of particles § Composite particles

Composite particles (such ashadrons,nuclei,andatoms) can be bosons or fermions depending on their constituents. Since bosons have integerspinand fermions odd half-integer spin, any composite particle made up of an even number of fermions is a boson.

Composite bosons include:

• Allmesonsof every type
• Stable nuclei with evenmass numberssuch asdeuterium,helium-4(theAlpha particle),carbon-12,lead-208,and many others.

Asquantum particles,the behaviour of multiple indistinguishable bosons at high densities is described byBose–Einstein statistics.One characteristic which becomes important insuperfluidityand other applications ofBose–Einstein condensatesis that there is no restriction on the number of bosons that may occupy the samequantum state.As a consequence, when for example a gas ofhelium-4atoms is cooled to temperatures very close toabsolute zeroand thekinetic energyof the particles becomes negligible, it condenses into a low-energy state and becomes asuperfluid.

Quasiparticles

Certainquasiparticlesare observed to behave as bosons and to followBose–Einstein statistics,includingCooper pairs,plasmonsandphonons.

See also

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