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Elementary particle

In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which all other particles are made. In the Standard Model, the quarks, leptons, and gauge bosons are elementary particles.[1][2]

Historically, the hadrons (mesons and baryons such as the proton and neutron) and even whole atoms were once regarded as elementary particles. A central feature in elementary particle theory is the early 20th century idea of "quanta", which revolutionised the understanding of electromagnetic radiation and brought about quantum mechanics.




All elementary particles are either bosons or fermions (depending on their spin). The spin-statistics theorem identifies the resulting quantum statistics that differentiates fermions from bosons. According to this methodology: particles normally associated with matter are fermions, having half-integer spin; they are divided into twelve flavours. Particles associated with fundamental forces are bosons, having integer spin.[3]

Quarksup, down, strange, charm, bottom, top
Gauge bosonsgluon, W and Z bosons, photon
Other bosons — Higgs boson, graviton

Standard Model

Main article: Standard Model

The Standard Model of particle physics contains 12 flavours of elementary fermions, plus their corresponding antiparticles, as well as elementary bosons that mediate the forces and the still undiscovered Higgs boson. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, since it is fundamentally incompatible with Einstein's general relativity. There are likely to be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force or the sparticles, supersymmetric partners of the ordinary particles.

Fundamental fermions

Main article: fermion


Particle Generations
First generation Second generation Third generation


Main article: antimatter

There are also 12 fundamental fermionic antiparticles which correspond to these 12 particles. The positron e+ corresponds to the electron and has an electric charge of +1 and so on:

First generation
  • positron: e+
  • electron-antineutrino: \bar{\nu}_e
  • up antiquark: \bar{u}
  • down antiquark: \bar{d}
Second generation
  • positive muon: μ+
  • muon-antineutrino: \bar{\nu}_\mu
  • charm antiquark: \bar{c}
  • strange antiquark: \bar{s}
Third generation
  • positive tau: τ+
  • tau-antineutrino: \bar{\nu}_\tau
  • top antiquark: \bar{t}
  • bottom antiquark: \bar{b}


Main article: quark

Quarks and antiquarks have never been detected to be isolated, 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. However, gluons are themselves color charged, 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; effectively, they very rarely separate from one another (and when they do they create an energy carrier particle which later converts to two new quarks of different type).




Fundamental bosons

Main article: boson

In the Standard Model, vector (spin-1) bosons (gluons, photons, and the W and Z bosons) mediate forces, while the Higgs boson (spin-0) is responsible for particles having intrinsic mass.


Main article: gluon

Gluons are the mediators of the strong interaction and carry both colour and anticolour. Although gluons are massless, they are never observed in detectors due to colour confinement; rather, they produce jets of hadrons, similar to single quarks. The first evidence for gluons came from annihilations of electrons and positrons at high energies which sometimes produced three jets — a quark, an antiquark, and a gluon.

Electroweak bosons

Main article: W and Z bosons

There are three weak gauge bosons: W+, W, and Z0; these mediate the weak interaction. The massless photon mediates the electromagnetic interaction.

Higgs boson

Main article: Higgs boson

Although the weak and electromagnetic forces appear quite different to us at everyday energies, the two forces are theorized to unify as a single electroweak force at high energies. This prediction was clearly confirmed by measurements of cross-sections for high-energy electron-proton scattering at the HERA collider at DESY. 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 the Higgs mechanism. Through the process of spontaneous 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 massless (the photon). Although the Higgs mechanism has become an accepted part of the Standard Model, the Higgs boson itself has not yet been observed in detectors. Indirect evidence for the Higgs boson suggests its mass lies below 200-250 GeV.[4] In this case, the LHC experiments will be able to discover this last missing piece of the Standard Model.

Beyond the Standard Model

Although all experimental evidence confirms the predictions of the Standard Model, many physicists find this model to be unsatisfactory due to its many undetermined parameters, many fundamental particles, the non-observation of the Higgs boson and other more theoretical considerations such as the hierarchy problem. There are many speculative theories beyond the Standard Model which attempt to rectify these deficiencies.

Grand unification

Main article: grand unification theory



Main article: supersymmetry

Supersymmetry extends the Standard Model by adding an additional class of symmetries to the Lagrangian. These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence of supersymmetric particles, abbreviated as sparticles, which include the sleptons, squarks, neutralinos and charginos. Each particle in the Standard Model would have a superpartner whose spin differs by 1/2 from the ordinary particle. Due to the breaking of supersymmetry, the sparticles are much heavier than their ordinary counterparts; they are so heavy that existing particle colliders would not be powerful enough to produce them. However, some physicists believe that sparticles will be detected when the Large Hadron Collider at CERN begins running.

String theory

Main article: string theory

String Theory is a theory of physics where all "particles" that make up matter and energy are comprised of strings (measuring at the Planck length) that exist in an 11-dimensional (according to M-theory, the leading version) universe. These strings vibrate at different frequencies which determine mass, electric charge, color charge, and spin. A string can be open (a line) or closed in a loop (a one-dimensional sphere, like a circle). As a string moves through space it sweeps out something called a world sheet. String theory predicts 1- to 10-branes (a 1-brane being a string and a 10-brane being a 10-dimensional object) which prevent tears in the "fabric" of space using the uncertainty 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).

As it relates to our own existence, string theory posits that our universe is merely a 4-brane, inside which exist the 3 space dimensions and the 1 time dimension that we observe. The remaining 6 theoretical dimensions are either very tiny and curled up (and too small to affect our universe in any way) or simply do not/cannot exist in our universe (because they exist in a grander scheme called the "multiverse" outside our known universe).

One particularly interesting prediction of string theory is the existence of extremely massive counterparts of ordinary particles due to vibrational excitations of the fundamental string. Another important prediction is the existence of a massless spin-2 particle behaving like the graviton.

Preon theory

Main article: preon


See also


  1. ^ Gribbin, John (2000). Q is for Quantum - An Encyclopedia of Particle Physics. Simon & Schuster. ISBN 0-684-85578-X. 
  4. ^ Quark experiment predicts heavier Higgs

Greene, Brian (1999). The Elegant Universe. W.W.Norton & Company. ISBN 0-393-05858-1. 

Further reading

Feynman, R.P. & Weinberg, S. (1987). Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures, New York: Cambridge University Press.

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Elementary_particle". A list of authors is available in Wikipedia.