Short description: Subatomic particle having no known substructure

An elementary particle or fundamental particle is a subatomic particle that is not known to be composed of smaller particles.^[1] In the Standard Model, elementary particles include fermions, bosons, quarks, leptons, gauge bosons, and the Higgs boson.

Sketch overview of elementary particles, interactions, antimatter, and composite particle structure within the Standard Model.

Description

The Standard Model recognizes seventeen distinct elementary particles: twelve fermions and five bosons. Because of flavour, color charge, and antimatter combinations, these particles appear in a larger number of physical variations.^[2]

Elementary particles are distinguished from composite particles. For example, protons and neutrons are not elementary, because they are made of quarks. By contrast, electrons, quarks, photons, gluons, and neutrinos are treated as elementary in the Standard Model.

The concept of an elementary particle depends on the theoretical framework used.^[3]

History

Atoms were once regarded as indivisible elementary particles. The word atom comes from the Greek atomos, meaning indivisible or uncuttable. The physical reality of atoms remained debated until the early twentieth century, when Einstein’s analysis of Brownian motion supported the atomic interpretation of matter.^[1]^[4]

Subatomic constituents were identified beginning with the electron near the end of the nineteenth century, followed by the proton, photon, and neutron.^[1] Quantum mechanics then changed the meaning of the word particle by showing that particles also behave as matter waves.^[5]^[6]

Since the Standard Model was developed in the 1970s, many extensions have been proposed. Supersymmetry, for example, predicts heavier partner particles for known elementary particles, but such superpartners have not been discovered.^[7]^[8]^[9]^[10]^[1]

Standard Model overview

All elementary particles are either fermions or bosons. Fermions obey Fermi–Dirac statistics and have half-integer spin. Bosons obey Bose–Einstein statistics and have integer spin.^[1]

In the Standard Model, elementary particles are represented as point particles for predictive calculations. The model is highly successful but incomplete, because it does not include gravity and contains parameters that are measured rather than explained from deeper principles.^[11]

Fundamental fermions

The twelve fundamental fermions are divided into three generations. Each generation contains two quarks and two leptons.

Particle generations
Leptons
First generation		Second generation		Third generation
Electron	Electron neutrino	Muon	Muon neutrino	Tau	Tau neutrino
Quarks
First generation		Second generation		Third generation
Up quark	Down quark	Charm quark	Strange quark	Top quark	Bottom quark

Half of the fundamental fermions are leptons. The charged leptons are the electron, muon, and tau. The neutral leptons are the electron neutrino, muon neutrino, and tau neutrino.

The remaining six fermions are quarks. Quarks carry color charge and fractional electric charge.

Fermion masses

The measured masses of elementary fermions vary greatly. Neutrino masses are extremely small, while the top quark is the most massive known elementary fermion.^[12]

Quark masses cannot be measured in isolation because quarks are confined inside hadrons. Their quoted masses therefore depend on the quantum chromodynamics scheme used.

Antiparticles

Each fundamental fermion has a corresponding antiparticle. The electron’s antiparticle is the positron, which has the same mass as the electron but opposite electric charge.

Antimatter particles have opposite quantum numbers from their corresponding matter particles. When a particle and its antiparticle meet, they may annihilate into other particles or photons.

Quarks

Quarks and antiquarks have never been observed as isolated particles, a fact explained by color confinement. Quarks carry one of three color charges, while antiquarks carry corresponding anticolors.

Color-charged particles interact through gluon exchange. Unlike electromagnetism, the strong force does not weaken in a simple way as quarks separate; instead, increasing separation produces stronger confinement effects.^[13]

Quarks combine into color-neutral composite particles called hadrons. A quark and antiquark form a meson. Three quarks form a baryon, such as a proton or neutron.

Fundamental bosons

Fundamental bosons include gauge bosons and the Higgs boson. Gauge bosons mediate interactions, while the Higgs boson is associated with the origin of mass through the Higgs mechanism.

Gluons

Gluons mediate the strong interaction. They bind quarks into hadrons, including baryons and mesons. Gluons themselves carry color and anticolor charge, producing eight gluon variations in the Standard Model.

Electroweak bosons

The electroweak bosons are the photon, W⁺, W⁻, and Z⁰. The photon mediates electromagnetism. The W and Z bosons mediate the weak interaction.

The weak interaction is responsible for processes such as beta decay. The Z boson can mediate neutral-current interactions, including elastic scattering of neutrinos.

Higgs boson

The Higgs boson is a spin-0 boson associated with the Higgs field. The Higgs mechanism explains why the W and Z bosons are massive while the photon remains massless.

On 4 July 2012, CERN announced observation of a new particle consistent with the Higgs boson.^[14] It has a mass of about 125 GeV/c².^[15]

Cosmic abundance

Most visible mass in the universe is contained in protons and neutrons. These are baryons made mainly of up and down quarks. Some estimates suggest that the observable universe contains roughly 10⁸⁰ baryons.^[16]

In terms of particle number, neutrinos and photons are extremely abundant in the visible universe.^[17]

Beyond the Standard Model

The Standard Model does not explain every feature of nature. It does not include gravity, does not explain the hierarchy between weak and gravitational forces, and leaves some parameters unexplained.

Graviton

The graviton is a hypothetical spin-2 boson proposed to mediate gravity. It has not been detected.^[1] Some models include massive Kaluza–Klein gravitons.^[18]

Grand unification

Grand unified theories attempt to combine the strong, weak, and electromagnetic interactions into a single interaction at very high energy. Some simple grand unified models predict proton decay, which has not been observed.

Supersymmetry

Supersymmetry proposes a symmetry between fermions and bosons. It predicts supersymmetric partner particles such as sleptons, squarks, neutralinos, and charginos. These particles have not been experimentally confirmed.

String theory

String theory proposes that particles are excitations of tiny one-dimensional strings. Different vibrational modes correspond to different particle properties such as mass, charge, and spin. Some versions of string theory require extra dimensions and predict a massless spin-2 particle resembling the graviton.^[19]

Technicolor and preons

Technicolor theories propose new strong interactions in which the Higgs boson may be composite rather than elementary. Preon theories suggest that some particles currently considered elementary may themselves be made of more fundamental constituents.

Accelerons

Accelerons are hypothetical particles proposed in models linking neutrino mass to dark energy and the accelerating expansion of the universe.^[20]^[21]

Physical interpretation

Elementary particles are the smallest known excitations of quantum fields in the Standard Model. Their interactions, masses, charges, and quantum numbers determine the structure of atoms, nuclei, radiation, and ordinary matter.