nLab fundamental particle

Contents

Context

Physics

1. Idea
2. Examples
3. Related concepts
4. Scales
5. References

1. Idea

In physics, by a fundamental or elementary particle one means a particle which is not a bound state of other objects, hence which is “indivisible”.

In computational practice this means that elementary particles are those that are really point-like, while non-elementary particles have a finite extension (if only in some averaged sense).

In the standard model of particle physics elementary particles include among the fermions the electrons and quarks, and among the bosons the photon and the Higgs boson. In contrast, all hadrons, hence all baryons, hence in particular the proton and neutron, are bound states of quarks, and then all nuclei are bound states of protons and neutrons, and all atoms are bound states of nuclei and electrons.

But of course, “atoms” get their name from a time when they were thought to be indivisible elementary particles, which highlights that the notion of what counts as “elementary” can change over time.

Various “beyond standard” models postulate that at least some of the currently understood elementary particles are in fact not so. For instance in “composite Higgs models” such as in technicolor models, the Higgs boson (which appears to be an elementary particle according to available measurements) is postulated to be a bound state and hence non-elementary.

In a slightly different manner, in usual string theory phenomenology none of the currently observed elementary particles would in fact be elementary, but all of them would arise as massless “states of strings”.

2. Examples

fields and particles in particle physics

and in the standard model of particle physics:

force field gauge bosons

photon - electromagnetic field (abelian Yang-Mills field)
W, Z, B-boson - electroweak field (Yang-Mills field)
gluon - strong nuclear force (Yang-Mills field)
graviton - field of gravity
infraparticle

scalar bosons

Higgs boson, inflaton (scalar field)

matter field fermions (spinors, Dirac fields)

flavors of fundamental fermions in the standard model of particle physics:
generation of fermions	1st generation	2nd generation	3d generation
quarks ( $q$ )
up-type	up quark ( $u$ )	charm quark ( $c$ )	top quark ( $t$ )
down-type	down quark ( $d$ )	strange quark ( $s$ )	bottom quark ( $b$ )
leptons
charged	electron	muon	tauon
neutral	electron neutrino	muon neutrino	tau neutrino

bound states:
mesons	light mesons: pion ( $u d$ ) ρ-meson ( $u d$ ) ω-meson ( $u d$ ) f1-meson a1-meson	strange-mesons: ϕ-meson ( $s \bar s$ ), kaon, K-meson ( $u s$ , $d s$ ) eta-meson ( $u u + d d + s s$ ) charmed heavy mesons*: D-meson ( $u c$ , $d c$ , $s c$ ) J/ψ-meson ( $c \bar c$ )	bottom heavy mesons: B-meson ( $q b$ ) ϒ-meson ( $b \bar b$ )
baryons	nucleons: proton $(u u d)$ neutron $(u d d)$