Generations of matter
|neutral||electron neutrino||muon neutrino||tau neutrino|
Fermions and bosons
Those particles with half-integer spins, such as 1/2, 3/2, 5/2, are known as fermions, while those particles with integer spins, such as 0, 1, 2, are known as bosons. The two families of particles obey different rules and broadly have different roles in the world around us. A key distinction between the two families is that fermions obey the Pauli exclusion principle; that is, there cannot be two identical fermions simultaneously having the same quantum numbers (meaning, roughly, having the same position, velocity and spin direction). In contrast, bosons obey the rules of Bose–Einstein statistics and have no such restriction, so they may “bunch together” even if in identical states. Also, composite particles can have spins different from their component particles. For example, a helium atom in the ground state has spin 0 and behaves like a boson, even though the quarks and electrons which make it up are all fermions.
This has profound consequences:
- Quarks and leptons (including electrons and neutrinos), which make up what is classically known as matter, are all fermions with spin 1/2. The common idea that “matter takes up space” actually comes from the Pauli exclusion principle acting on these particles to prevent the fermions that make up matter from being in the same quantum state. Further compaction would require electrons to occupy the same energy states, and therefore a kind of pressure (sometimes known as degeneracy pressure of electrons) acts to resist the fermions being overly close.
- Elementary fermions with other spins (3/2, 5/2, etc.) are not known to exist.
- Elementary particles which are thought of as carrying forces are all bosons with spin 1. They include the photon which carries the electromagnetic force, the gluon (strong force), and the W and Z bosons (weak force). The ability of bosons to occupy the same quantum state is used in the laser, which aligns many photons having the same quantum number (the same direction and frequency), superfluid liquid helium resulting from helium-4 atoms being bosons, and superconductivity where pairs of electrons (which individually are fermions) act as single composite bosons.
- Elementary bosons with other spins (0, 2, 3 etc.) were not historically known to exist, although they have received considerable theoretical treatment and are well established within their respective mainstream theories. In particular, theoreticians have proposed the graviton (predicted to exist by some quantum gravity theories) with spin 2, and the Higgs boson (explaining electroweak symmetry breaking) with spin 0. Since 2013, the Higgs boson with spin 0 has been considered proven to exist. It is the first scalar elementary particle (spin 0) known to exist in nature.
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Szokásos és egzotikus hadronok
A részecskefizikában hadronnak nevezzük az olyan összetett szubatomi részecskéket, amelyeknek összetevői kvarkok és gluonok
A „hagyományos” hadronok a Gell-Mann kvarkmodelljének megfelelő, azaz 3 kvarkból vagy kvark-antikvark párból álló hadronok.
1/ A barionok három kvarkból (az antibarionok pedig három antikvarkból) álló feles spinű részecskék, azaz fermionok.
Fő példái a nukleonok: a proton és a neutron
2/A mezonok egy kvarkból és egy antikvarkból állnak, mint a pionok, kaonok és egy csomó más részecske. Egyes spinű részecskék, azaz bozonok.
Egyfajta sematikus ábrázolásuk is elérhető volt. L. alább: