elementary particle

In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic particles of the universe from which all larger particles are made. In the modern theory of particle physics, the Standard Model, the quarks, leptons, and gauge bosons are elementary particles. Historically, the hadrons (mesons and baryons such as the proton and neutron) and even whole atoms were once regarded as elementary particles.

Elementary particles are grouped according to spin: 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.

• Fermions:

Quarks — up, down, strange, charm, bottom, top

Leptons — electron, muon, tau, electron neutrino, muon neutrino, tau neutrino

• Bosons:

Gauge bosons – gluon, W and Z bosons, photon

Other bosons — Higgs boson, graviton

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

The 12 fundamental fermionic flavours are divided into three generations of four particles each. Six of the particles are quarks. The remaining six are leptons, three of which are neutrinos, and the remaining three of which have an electric charge of −1: the electron and its two cousins, the muon and the tau lepton.

Particle Generations

First generation electron: e− electron-neutrino: νe up quark: u down quark: d

Second generation muon: μ− muon-neutrino: νμ charm quark: c strange quark: s

Third generation tau lepton: τ− tau-neutrino: ντ top quark: t bottom quark: b

Antiparticles

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:

Antiparticles

First generation positron: e+ electron-antineutrino: <math> \bar{\nu}_e </math> up antiquark: <math> \bar{u} </math> down antiquark: <math> \bar{d} </math>

Second generation positive muon: μ+ muon-antineutrino: <math> \bar{\nu}_\mu </math> charm antiquark: <math> \bar{c} </math> strange antiquark: <math> \bar{s} </math>

Third generation positive tau lepton: τ+ tau-antineutrino: <math> \bar{\nu}_\tau </math> top antiquark: <math> \bar{t} </math> bottom antiquark: <math> \bar{b} </math>

Quarks

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 can never separate from one another.

However, color charged particles may combine to form color neutral composite particles called hadrons. A quark may pair up to 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. Or three quarks can exist together: one quark is "red", another "blue", another "green". These three colored quarks together form a color neutral baryon. Or three antiquarks can exist together: one antiquark is "antired", another "antiblue", another "antigreen". These three anticolored antiquarks 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 +2/3 or −1/3, whereas antiquarks have corresponding electric charges of either −2/3 or +1/3.

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, while the Higgs boson (spin-0) is responsible for particles having intrinsic mass.

Gluons

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

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

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 about 200 GeV. 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

One extension of the Standard Model attempts to combine the electroweak interaction with the strong interaction into a single 'grand unified theory' (GUT). Such a force would be spontaneously broken into the three forces by a Higgs-like mechanism. The most dramatic prediction of grand unification is the existence of X bosons, which cause proton decay. However, the non-observation of proton decay at Super-Kamiokande rules out the simplest GUTs, including SU(5) and SO(10).

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

According to string theorists, each kind of fundamental particle corresponds to a different pattern of fundamental string. All strings are essentially the same, although they may be open (lines) or closed (loops). Different particles differ in the coordination of their strings. Modern string theories include supersymmetry, making them superstring theories.

One particular 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 of string theory is the existence of a massless spin-2 particle behaving like the graviton. By predicting gravity, string theory unifies quantum mechanics with general relativity, making it the first consistent theory of quantum gravity.

One problem with string theory is that it predicts that the number of dimensions for spacetime much greater than 4 (the number of observed dimensions). These extra dimensions are supposedly compactified or rolled-up. Other related theories such as brane theories contain extended extra dimensions, which are hidden from us by our confinement to a brane.

Preon theory

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 the particle zoo that came before it. Most models assume that almost everything in the Standard Model can be explained in terms of three to half a dozen 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 1980's.

See also

• Subatomic particle
• Particle physics
• List of particles

References

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

External links

• Greene, Brian, "Elementary particles". The Elegant Universe, NOVA (PBS)
• particleadventure.org: The Standard Model, *Unsolved Mysteries. Beyond The Standard Model, *What is the World Made of? The Naming of Quarks
• University of California: Particle Data Group
• particleadventure.org: Particle chart
• CERNCourier: Season of Higgs and melodrama
• Pentaquark information page

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