In the Standard Model of particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. The Standard Model recognizes seventeen distinct particles—twelve fermions and five bosons. As a consequence of flavor and color combinations and antimatter, the fermions and bosons are known to have 48 and 13 variations, respectively. These 61 elementary particles include electrons and other leptons, quarks, and the fundamental bosons. Subatomic particles such as protons or neutrons, which contain two or more elementary particles, are known as composite particles.
History The concept of an elementary particle is not based on measurements but instead depends on a theoretical framework. Ordinary matter is composed of atoms, themselves once thought to be indivisible elementary particles. The name atom comes from the Ancient Greek word ἄτομος (atomos) which means indivisible or uncuttable. Despite the theories about atoms that had existed for thousands of years, their factual existence remained controversial until 1905. In that year, Albert Einstein published his paper on Brownian motion, putting to rest theories that had regarded molecules as mathematical illusions. Einstein subsequently identified matter as ultimately composed of various concentrations of energy.
Subatomic constituents of the atom were first identified toward the end of the 19th century, beginning with the electron, followed by the proton in 1919, the photon in the 1920s, and the neutron in 1932. By that time, the advent of quantum mechanics had radically altered the definition of a "particle" by putting forward an understanding in which they carried out a simultaneous existence as matter waves.
Many theoretical elaborations upon, and beyond, the Standard Model have been made since its codification in the 1970s. These include notions of supersymmetry, which double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive. However, like an additional elementary boson mediating gravitation, such superpartners remain undiscovered as of 2026.
Overview
All elementary particles are either fermions or bosons. These classes are distinguished by their quantum statistics: fermions obey Fermi–Dirac statistics and bosons obey Bose–Einstein statistics. Their spin is differentiated via the spin–statistics theorem: it is half-integer for fermions, and integer for bosons.
In the Standard Model, elementary particles are represented for predictive utility as point particles. Though extremely successful, the Standard Model is limited by its omission of gravitation and has some parameters arbitrarily added but unexplained.
Cosmic abundance of elementary particles
According to the current models of Big Bang nucleosynthesis, the primordial composition of visible matter of the universe should be about 75% hydrogen and 25% helium-4 (in mass). Neutrons are made of one up and two down quarks, while protons are made of two up and one down quark. Since the other common elementary particles (such as electrons, neutrinos, or weak bosons) are so light or so rare when compared to atomic nuclei, we can neglect their mass contribution to the observable universe's total mass. Therefore, one can conclude that most of the visible mass of the universe consists of protons and neutrons, which, like all baryons, in turn consist of up quarks and down quarks.
Some estimates imply that there are roughly baryons (almost entirely protons and neutrons) in the observable universe.
The number of protons in the observable universe is called the Eddington number.
In terms of number of particles, some estimates imply that nearly all the matter, excluding dark matter, occurs in neutrinos, which constitute the majority of the roughly elementary particles of matter that exist in the visible universe. Other estimates imply that roughly elementary particles exist in the visible universe (not including dark matter), mostly photons and other massless force carriers.
Standard Model
The Standard Model of particle physics contains 12 flavors of elementary fermions, plus their corresponding antiparticles, as well as elementary bosons that mediate the forces and the Higgs boson, which was reported on July 4, 2012, as having been likely detected by the two main experiments at the Large Hadron Collider (ATLAS and CMS). The Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, however, since it is not known if it is compatible with Einstein's general relativity. There may be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force, and sparticles, supersymmetric partners of the ordinary particles.
Fundamental fermions
The 12 fundamental fermions are divided into 3 generations of 4 particles each. Half of the fermions are leptons, three of which have an electric charge of −1 e, called the electron (), the muon (), and the tau (); the other three leptons are neutrinos (, , ), which are the only elementary fermions with neither electric nor color charge. The remaining six particles are quarks (discussed below).
Generations + Particle generations Leptons First generation Second generation Third generationName Symbol Name Symbol Name Symbol electron muon tau electron neutrino muon neutrino tau neutrino Quarks First generation Second generation Third generation up quark charm quark c top quark down quark strange quark bottom quark
Mass The following table lists current measured masses and mass estimates for all the fermions, using the same scale of measure: millions of electron-volts relative to square of light speed (MeV/c2). For example, the most accurately known quark mass is of the top quark () at , estimated using the on-shell scheme.
+ Current values for elementary fermion masses Particle symbol Particle name Mass value Quark mass estimation scheme (point) , , Neutrino(any type) < electron up quark MSbar scheme ( = ) down quark MSbar scheme ( = ) strange quark MSbar scheme ( = ) muon(mu lepton) charm quark MSbar scheme ( = c) tauon (tau lepton) bottom quark MSbar scheme ( = b) top quark On-shell scheme
Estimates of the values of quark masses depend on the version of quantum chromodynamics used to describe quark interactions. Quarks are always confined in an envelope of gluons that confer vastly greater mass to the mesons and baryons where quarks occur, so values for quark masses cannot be measured directly. Since their masses are so small compared to the effective mass of the surrounding gluons, slight differences in the calculation make large differences in the masses.
Antiparticles
There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. For example, the antielectron (positron) is the electron's antiparticle and has an electric charge of +1 e.
+ Particle generations Antileptons First generation Second generation Third generationName Symbol Name Symbol Name Symbol positron antimuon antitau electron antineutrino muon antineutrino tau antineutrino Antiquarks First generation Second generation Third generation up antiquark charm antiquark top antiquark down antiquark strange antiquark bottom antiquark
Quarks
Isolated quarks and antiquarks have never been detected, a fact explained by confinement. Every quark carries one of three color charges of the strong interaction; antiquarks similarly carry anticolor. Color-charged particles interact via gluon exchange in the same way that charged particles interact via photon exchange. Gluons are themselves color-charged, however, resulting in an amplification of the strong force as color-charged particles are separated. Unlike the electromagnetic force, which diminishes as charged particles separate, color-charged particles feel increasing force.
Nonetheless, color-charged particles may combine to form color neutral composite particles called hadrons. A quark may pair up with an antiquark: the quark has a color and the antiquark has the corresponding anticolor. The color and anticolor cancel out, forming a color neutral meson. Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form a color-neutral baryon. Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutral antibaryon.
Quarks also carry fractional electric charges, but, since they are confined within hadrons whose charges are all integral, fractional charges have never been isolated. Note that quarks have electric charges of either e or e, whereas antiquarks have corresponding electric charges of either e or e.
Evidence for the existence of quarks comes from deep inelastic scattering: firing electrons at nuclei to determine the distribution of charge within nucleons (which are baryons). If the charge is uniform, the electric field around the proton should be uniform and the electron should scatter elastically. Low-energy electrons do scatter in this way, but, above a particular energy, the protons deflect some electrons through large angles. The recoiling electron has much less energy and a jet of particles is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks.
Fundamental bosons
In the Standard Model, vector (spin-1) bosons (gluons, photons, and the W and Z bosons) mediate forces, whereas the Higgs boson (spin-0) is responsible for the intrinsic mass of particles. Bosons differ from fermions in the fact that multiple bosons can occupy the same quantum state (Pauli exclusion principle). Also, bosons can be either elementary, like photons, or a combination, like mesons. The spin of bosons are integers instead of half integers.
Gluons
Gluons mediate the strong interaction, which join quarks and thereby form hadrons, which are either baryons (three quarks) or mesons (one quark and one antiquark). Protons and neutrons are baryons, joined by gluons to form the atomic nucleus. Like quarks, gluons exhibit color and anticolor – unrelated to the concept of visual color and rather the particles' strong interactions – sometimes in combinations, altogether eight variations of gluons.
Electroweak bosons
There are three weak gauge bosons: W+, W−, and Z0; these mediate the weak interaction. The W bosons are known for their mediation in nuclear decay: The W− converts a neutron into a proton then decays into an electron and electron-antineutrino pair. The Z0 does not convert particle flavor or charges, but rather changes momentum; it is the only mechanism for elastically scattering neutrinos. The weak gauge bosons were discovered due to momentum change in electrons from neutrino-Z exchange. The massless photon mediates the electromagnetic interaction. These four gauge bosons form the electroweak interaction among elementary particles.
Higgs boson