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.
Some bosons areelementary 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 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 behave at high densities or low temperatures in a characteristic manner described byBose–Einstein statistics: for example, a gas ofhelium-4 atoms becomes asuperfluid at temperatures close to absolute zero. Similarly,superconductivity arises because somequasiparticles, such asCooper pairs, behave in this characteristic manner.
All observedelementary particles are either bosons (with integer spin) orfermions (with odd half-integer spin).[8] Whereas the elementary particles that make up ordinary matter (leptons andquarks) are fermions, elementary bosons occupy a special role in particle physics. They act either asforce carriers which give rise to forces between other particles, or in one case give rise to the phenomenon ofmass.
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.[a]
Composite particles (such ashadrons,nuclei, andatoms) can be bosons or fermions depending on their constituents. Since bosons have integerspin and fermions half odd-integer spin, any composite particle made up of aneven 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).
Asquantum particles, the behaviour of multiple indistinguishable bosons at high densities is described by Bose–Einstein statistics. One characteristic which becomes important insuperfluidity and other applications ofBose–Einstein condensates is 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-4 atoms is cooled to temperatures very close toabsolute zero and thekinetic energy of the particles becomes negligible, it condenses into a low-energy state and becomes asuperfluid.
Other examples in condensed matter systems includeCooper pairs in superconductors andexcitons in semiconductors.[10]
^Despite being the carrier of the gravitational force which interacts with mass, most attempts atquantum gravity have expected the graviton to have no mass, just like the photon has no electric charge, and theW and Z bosons have no"flavour".
^Even-mass-number nuclides comprise 153 / 254 = 60% of all stable nuclides. They are bosons, i.e. they have integer spin, and almost all of them (148 of the 153) are even-proton / even-neutron (EE) nuclides. The EE nuclides necessarily have spin 0 because of pairing. The remaining 5 stable bosonic nuclides are odd-proton / odd-neutron (OO) stable nuclides (seeEven and odd atomic nuclei § Odd proton, odd neutron). The five odd–odd bosonic nuclides are:
^Carroll, Sean (2007).Guidebook. Dark Matter, Dark Energy: The dark side of the universe. The Teaching Company. Part 2, p. 43.ISBN978-1598033502.... boson: A force-carrying particle, as opposed to a matter particle (fermion). Bosons can be piled on top of each other without limit. Examples are photons, gluons, gravitons, weak bosons, and the Higgs boson. The spin of a boson is always an integer: 0, 1, 2, and so on ...
^Monique Combescot and Shiue-Yuan Shiau, "Excitons and Cooper Pairs: Two Composite Bosons in Many-Body Physics", Oxford University Press (ISBN9780198753735).
