Elementary particle

In particle physics, an elementary particle is a particle of which other, larger particles are composed. For example, atoms are made up of smaller particles known as electrons, protons, and neutrons. The proton and neutron, in turn, are composed of more elementary particles known as quarks. One of the outstanding problems of particle physics is to find the most elementary particles — or the so-called fundamental particles — which make up all the other particles found in Nature, and are not themselves made up of smaller particles.

Standard Model

The Standard Model of particle physics contains 12 flavours of elementary fermions (" matter particles"), 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: ν_e up quark: u down quark: d	Second generation muon: μ⁻ muon-neutrino: ν_μ charm quark: c strange quark: s	Third generation tau lepton: τ⁻ tau-neutrino: ν_τ 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: $\bar{\nu}_e$ 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 lepton: τ⁺ tau-antineutrino: $\bar{\nu}_\tau$ top antiquark: $\bar{t}$ bottom antiquark: $\bar{b}$

Quarks

Main article: quark

Quarks and antiquarks have never been detected to be isolated, a fact explained by confinement. Every quark carries one of three colour 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 colour charged, resulting in an amplification of the strong force as colour charged particles are separated. Unlike the electromagnetic force which diminishes as charged particles separate, colour charged particles feel increasing force; effectively, they can never separate from one another.

However, colour charged particles may combine to form colour neutral composite particles called hadrons. A quark may pair up to an antiquark: the quark has a colour and the antiquark has the corresponding anticolor. The colour and anticolor cancel out, forming a colour neutral meson. Or three quarks can exist together: one quark is "red", another "blue", another "green". These three colored quarks together form a colour neutral baryon. Or three antiquarks can exist together: one antiquark is "antired", another "antiblue", another "antigreen". These three anticolored antiquarks form a colour 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 is responsible for particles having intrinsic mass.

Gluons

Gluons are the mediators of the strong interaction and carry both colour and anticolor. Although gluons are massless, they are never observed in detectors due to confinement; rather, they produce jets of hadrons, similar to single quarks.

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 be unified as a single electroweak force at high energies. The reason for this difference at low energies is thought to be due to the existence of the Higgs field. 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 electromagnetic photon). Although the Higgs mechanism has become an accepted part of the Standard Model, the Higgs boson itself has never been observed in detectors. This is thought to be due to the particle's great mass, but its continuing absence is a major cause of concern for particle physicists.

Beyond the Standard Model

Although there is no experimental evidence that does not agree with the Standard Model, many physicists find it to be unsatisfactory due to its many free parameters, many fundamental particles, the non-observation of the Higgs boson and other problems such as the hierarchy problem. As a result, there are many speculative theories beyond the Standard Model.

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 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

Main article: string theory

According to string theorists, each kind of fundamental particle corresponds to a different patterns 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.