Ordinary matter is composed ofatoms, themselves once thought to be indivisible elementary particles. The nameatom comes from the Ancient Greek wordἄτομος (atomos) which meansindivisible oruncuttable. Despite thetheories about atoms that had existed forthousands of years, the factual existence of atoms remained controversial until 1905. In that year,Albert Einstein publishedhis paper onBrownian motion, putting to rest theories that had regardedmolecules as mathematical illusions. Einstein subsequently identified matter as ultimately composed of various concentrations ofenergy.[1][3]
Subatomic constituents of the atom were first identified toward the end of the19th century, beginning with theelectron, followed by theproton in 1919, thephoton in the 1920s, and theneutron in 1932.[1] By that time, the advent ofquantum mechanics hadradically altered the definition of a "particle" by putting forward an understanding in which they carried out a simultaneous existence asmatter waves.[4][5]
Many theoretical elaborations upon, andbeyond, the Standard Model have been made since itscodification in the 1970s. These include notions ofsupersymmetry, which double the number of elementary particles by hypothesizing that each known particle associates with a "shadow" partner far more massive.[6][7] However, like anadditional elementary boson mediating gravitation, suchsuperpartners remain undiscovered as of 2025.[8][9][1][needs update]
Notes: [†] An anti-electron (e+ ) is conventionally called a "positron". [‡] The known force carrier bosons all have spin = 1. The hypothetical graviton has spin = 2; it is unknown whether it is a gauge boson as well.
In theStandard Model, elementary particles are represented forpredictive utility aspoint particles. Though extremely successful, the Standard Model is limited by its omission ofgravitation and has some parameters arbitrarily added but unexplained.[10]
According to the current models ofBig 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 allbaryons, in turn consist of up quarks and down quarks.[citation needed]
Some estimates imply that there are roughly 1080 baryons (almost entirely protons and neutrons) in the observable universe.[11]
In terms of number of particles, some estimates imply that nearly all the matter, excludingdark matter, occurs in neutrinos, which constitute the majority of the roughly 1086 elementary particles of matter that exist in the visible universe.[12] Other estimates imply that roughly 1097 elementary particles exist in the visible universe (not includingdark matter), mostly photons and other massless force carriers.[12]
The Standard Model of particle physics contains 12 flavors of elementaryfermions, plus their correspondingantiparticles, as well as elementary bosons that mediate the forces and theHiggs boson, which was reported on July 4, 2012, as having been likely detected by the two main experiments at theLarge Hadron Collider (ATLAS andCMS).[1] 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 withEinstein'sgeneral relativity. There may be hypothetical elementary particles not described by the Standard Model, such as thegraviton, the particle that would carry thegravitational force, andsparticles,supersymmetric partners of the ordinary particles.[13]
The 12 fundamental fermions are divided into 3 generations of 4 particles each. Half of the fermions areleptons, three of which have an electric charge of −1 e, called the electron (e− ), themuon (μ− ), and thetau (τ− ); the other three leptons areneutrinos (ν e,ν μ,ν τ), which are the only elementary fermions with neither electric norcolor charge. The remaining six particles arequarks (discussed below).
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 (t) at172.7 GeV/c2, estimated using theon-shell scheme.
Estimates of the values of quark masses depend on the version ofquantum chromodynamics used to describe quark interactions. Quarks are always confined in an envelope ofgluons that confer vastly greater mass to themesons andbaryons 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.[citation needed]
There are also 12 fundamental fermionic antiparticles that correspond to these 12 particles. For example, theantielectron (positron)e+ is the electron's antiparticle and has an electric charge of +1 e.
Isolated quarks and antiquarks have never been detected, a fact explained byconfinement. Every quark carries one of threecolor charges of thestrong interaction; antiquarks similarly carry anticolor. Color-charged particles interact viagluon exchange in the same way that charged particles interact viaphoton exchange. Gluons are themselves color-charged, however, resulting in an amplification of the strong force as color-charged particles are separated. Unlike theelectromagnetic force, which diminishes as charged particles separate, color-charged particles feel increasing force.[15]
Nonetheless, color-charged particles may combine to form color neutralcomposite particles calledhadrons. 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 neutralmeson. Alternatively, three quarks can exist together, one quark being "red", another "blue", another "green". These three colored quarks together form a color-neutralbaryon. Symmetrically, three antiquarks with the colors "antired", "antiblue" and "antigreen" can form a color-neutralantibaryon.
Quarks also carry fractionalelectric 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++2/3e or−+1/3e, whereas antiquarks have corresponding electric charges of either−+2/3e or ++1/3e.
Evidence for the existence of quarks comes fromdeep inelastic scattering: firingelectrons atnuclei to determine the distribution of charge withinnucleons (which are baryons). If the charge is uniform, theelectric 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 ajet of particles is emitted. This inelastic scattering suggests that the charge in the proton is not uniform but split among smaller charged particles: quarks.
In the Standard Model, vector (spin-1) bosons (gluons,photons, and theW and Z bosons) mediate forces, whereas theHiggs boson (spin-0) is responsible for the intrinsicmass 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, likemesons. The spin of bosons are integers instead of half integers.
Gluons mediate thestrong interaction, which join quarks and thereby formhadrons, which are eitherbaryons (three quarks) ormesons (one quark and one antiquark). Protons and neutrons are baryons, joined by gluons to form theatomic nucleus. Like quarks, gluons exhibitcolor and anticolor – unrelated to the concept of visual color and rather the particles' strong interactions – sometimes in combinations, altogether eight variations of gluons.
There are threeweak gauge bosons: W+, W−, and Z0; these mediate theweak 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 masslessphoton mediates theelectromagnetic interaction. These four gauge bosons form the electroweak interaction among elementary particles.
Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a singleelectroweak force at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at theHERA collider atDESY. The differences at low energies is a consequence of the high masses of the W and Z bosons, which in turn are a consequence of theHiggs mechanism. Through the process ofspontaneous symmetry breaking, the Higgs selects a special direction in electroweak space that causes three electroweak particles to become very heavy (the weak bosons) and one to remain with an undefined rest mass as it is always in motion (the photon). On 4 July 2012, after many years of experimentally searching for evidence of its existence, theHiggs boson was announced to have been observed at CERN's Large Hadron Collider.Peter Higgs who first posited the existence of the Higgs boson was present at the announcement.[16] The Higgs boson is believed to have a mass of approximately125 GeV/c2.[17] Thestatistical significance of this discovery was reported as 5 sigma, which implies a certainty of roughly 99.99994%. In particle physics, this is the level of significance required to officially label experimental observations as adiscovery. Research into the properties of the newly discovered particle continues.
Thegraviton is a hypothetical elementary spin-2 particle proposed to mediate gravitation. While it remains undiscovered due tothe difficulty inherent in its detection, it is sometimes included in tables of elementary particles.[1] The conventional graviton is massless, although some models containing massiveKaluza–Klein gravitons exist.[18]
Although experimental evidence overwhelmingly confirms the predictions derived from theStandard Model, some of its parameters were added arbitrarily, not determined by a particular explanation, which remain mysterious, for instance thehierarchy problem. Theoriesbeyond the Standard Model attempt to resolve these shortcomings.
One extension of the Standard Model attempts to combine theelectroweak interaction with thestrong interaction into a single 'grand unified theory' (GUT). Such a force would bespontaneously broken into the three forces by aHiggs-like mechanism. This breakdown is theorized to occur at high energies, making it difficult to observe unification in a laboratory. The most dramatic prediction of grand unification is the existence ofX and Y bosons, which causeproton decay. The non-observation of proton decay at theSuper-Kamiokande neutrino observatory rules out the simplest GUTs, however, including SU(5) and SO(10).
Supersymmetry extends the Standard Model by adding another class of symmetries to theLagrangian. These symmetries exchangefermionic particles withbosonic ones. Such a symmetry predicts the existence ofsupersymmetric particles, abbreviated assparticles, which include thesleptons,squarks,neutralinos, andcharginos. Each particle in the Standard Model would have a superpartner whosespin differs by1⁄2 from the ordinary particle. Due to thebreaking of supersymmetry, the sparticles are much heavier than their ordinary counterparts; they are so heavy that existingparticle colliders would not be powerful enough to produce them. Some physicists believe that sparticles will be detected by theLarge Hadron Collider atCERN.
String theory is a model of physics whereby all "particles" that make upmatter are composed of strings (measuring at the Planck length) that exist in an 11-dimensional (according toM-theory, the leading version) or 12-dimensional (according toF-theory[19]) universe. These strings vibrate at different frequencies that determine mass, electric charge, color charge, and spin. A "string" can be open (a line) or closed in a loop (a one-dimensional sphere, that is, a circle). As a string moves through space it sweeps out something called aworld sheet. String theory predicts 1- to 10-branes (a 1-brane being a string and a 10-brane being a 10-dimensional object) that prevent tears in the "fabric" of space using theuncertainty principle (e.g., the electron orbiting a hydrogen atom has the probability, albeit small, that it could be anywhere else in the universe at any given moment).
String theory proposes that our universe is merely a 4-brane, inside which exist the three space dimensions and the one time dimension that we observe. The remaining 7 theoretical dimensions either are very tiny and curled up (and too small to be macroscopically accessible) or simply do not/cannot exist in our universe (because they exist in a grander scheme called the "multiverse" outside our known universe).
Some predictions of the string theory include existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string and existence of a massless spin-2 particle behaving like thegraviton.
Technicolor theories try to modify the Standard Model in a minimal way by introducing a new QCD-like interaction. This means one adds a new theory of so-called Techniquarks, interacting via so called Technigluons. The main idea is that the Higgs boson is not an elementary particle but a bound state of these objects.
According to preon theory there are one or more orders of particles more fundamental than those (or most of those) found in the Standard Model. The most fundamental of these are normally called preons, which is derived from "pre-quarks". In essence, preon theory tries to do for the Standard Model what the Standard Model did for theparticle zoo that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to six more fundamental particles and the rules that govern their interactions. Interest in preons has waned since the simplest models were experimentally ruled out in the 1980s.
In this theory, neutrinos are influenced by a new force resulting from their interactions with accelerons, leading to dark energy. Dark energy results as the universe tries to pull neutrinos apart.[20] Accelerons are thought to interact with matter more infrequently than they do with neutrinos.[21]
Gribbin, John; Gribbin, Mary; Gribbin, Jonathan (1998).Q is for quantum: an encyclopedia of particle physics. New York, NY: Free Press.ISBN978-0-684-85578-3.
Oerter, Robert (2006).The theory of almost everything: the Standard Model, the unsung triumph of modern physics. New York, NY: Pi Press.ISBN978-0-452-28786-0.
Schumm, Bruce A. (2004).Deep down things: the breathtaking beauty of particle physics. Baltimore: Johns Hopkins University Press.ISBN978-0-8018-7971-5.
Coughlan, Guy D.; Dodd, James Edmund (1994).The ideas of particle physics: an introduction for scientists (2., reprint ed.). Cambridge: Cambridge Univ. Press.ISBN978-0-521-38677-7. An undergraduate text for those not majoring in physics.
Griffiths, David Jeffrey (1987).Introduction to elementary particles. New York Chichester Brisbane [etc.]: J. Wiley and sons.ISBN978-0-471-60386-3.
The most important address about the current experimental and theoretical knowledge about elementary particle physics is theParticle Data Group, where different international institutions collect all experimental data and give short reviews over the contemporary theoretical understanding.
