Subatomic particle
A subatomic particle is a particle smaller than an atom: it may be elementary or composite. Particle physics and nuclear physics concern themselves with the study of these particles, their interactions, and matter made up of them which do not aggregate into atoms.
These particles include atomic constituents such as electrons, protons, and neutrons (protons and neutrons are composite particles, made up of quarks), as well as other particles such as photons and neutrinos which are produced copiously in the sun. However, most of the particles that have been discovered and studied are not encountered under normal earth conditions; they are produced in cosmic rays and during scattering processes in particle accelerators.


Helium atom (schematic)
Showing two protons (red), two neutrons (green) and two electrons (yellow).
Dividing an atom
The study of electrochemistry led G. Johnstone Stoney to postulate the existence of the electron (denoted e−) in 1874 as a constituent of the atom. It was observed in 1897 by J. J. Thomson. Subsequent speculation about the structure of atoms was severely constrained by the 1907 experiment of Ernest Rutherford which showed that the atom was mostly empty space, and almost all its mass was concentrated into the (relatively) tiny atomic nucleus. The development of the quantum theory led to the understanding of chemistry in terms of the arrangement of electrons in the mostly empty volume of atoms. Protons (p+) were known to be the nucleus of the hydrogen atom. Neutrons (n) were postulated by Rutherford and discovered by James Chadwick in 1932. The word nucleon denotes both the neutron and the proton.
Electrons, which are negatively charged, have a mass of 1/1836 of a hydrogen atom, the remainder of the atom's mass coming from the positively charged proton. The atomic number of an element counts the number of protons. Neutrons are neutral particles with a mass almost equal to that of the proton. Different isotopes of the same nucleus contain the same number of protons but differing numbers of neutrons. The mass number of a nucleus counts the total number of nucleons.
Chemistry concerns itself with the arrangement of electrons in atoms and molecules, and nuclear physics with the arrangement of protons and neutrons in a nucleus. The study of subatomic particles, atoms and molecules, their structure and interactions, involves quantum mechanics and quantum field theory (when dealing with processes that change the number of particles). The study of subatomic particles per se is called particle physics. Since many particles need to be created in high energy particle accelerators or cosmic rays, sometimes particle physics is also called high energy physics.
Classification of subatomic particles
Symmetries play a very important role in the physics of subatomic particles by providing intrinsic quantum numbers which are used to classify particles. Poincaré symmetry, which is the full symmetry of special relativity, is enjoyed by any Hamiltonian which describes these particles. Hence all particles have the following quantum numbers —
• the mass (m) of the particle,
• its spin (J): all particles with integer values of spin are called bosons, those with half-integer spins are called fermions.
• its intrinsic parity (P), which is a multiplicative quantum number.
In addition, some particles may have a definite C-parity (C). Particles may also carry other quantum numbers related to internal symmetries, such as charges and flavour quantum numbers.
Corresponding to every particle there exists an antiparticle. Every additive quantum number of a particle is reversed in sign for the antiparticle. Equality of the masses and lifetimes of particle and antiparticle follows in local quantum field theories through CPT symmetry, and hence tests of these equalities constitute important tests of this symmetry.
Elementary particles
A full classification of subatomic particles involves understanding the fundamental forces that they are subject to: the electromagnetic, weak and strong forces. In the modern unified quantum field theory of these three forces, called the standard model, the elementary particles are
• spin J = 1 particles called gauge bosons. These include
o photons, which are carriers of the electromagnetic force,
o W bosons and Z bosons which mediate the weak forces, and
o gluons, which carry the strong force.
• spin J = 1/2 fermions which constitute all matter in the universe and come in two varieties—
o leptons such as the electron, muon, tau lepton, the three corresponding neutrinos (these are called six flavours of leptons), and their antiparticles. These are affected essentially only by the weak and electromagnetic forces. The former allow flavour changes (for example, from a muon to an electron)
o quarks which come in six other flavours, and are affected by all three forces unified into the standard model. The weak interactions cause flavour changes.
• spin J = 0 (and P = +1) Higgs boson which is responsible for the masses of the quarks, leptons, W and Z bosons. This remains to be actually seen in experiments; a major purpose of the Large Hadron Collider (LHC) is to search for this particle.
Conjectures and predictions
Further structures beyond the standard model are often invoked. In particular, there is a search for a theory that unifies the standard model with gravity. There is strong evidence that when such a theory is found it will include gravitons (constrained to have spin J = 2), to mediate this fourth fundamental interaction. A further structure called supersymmetry is often invoked, although direct experimental evidence for it is lacking. Supersymmetric extensions of the standard model would contain a bosonic partner for each of the fermions described above (called selectrons, smuons, staus, sneutrinos, squarks), and a fermionic partner for each boson (called gauginos and Higgsinos). Supersymmetric extensions which include a theory of gravity (called supergravity) also involve a partner of the graviton, called the gravitino, which has spin J = 3/2. In many versions of these theories there are extra bosons called axions with J = 0 and P = −1. Relic particles are postulated to be remnants of the early cosmological expansion of the Big Bang.
There were attempts to build theories which posited that the elementary particles in the standard model are actually composites built out of really elementary particles variously called preons, rishons or quinks. However, these theories are so strongly constrained by experimental data now that they are almost ruled out. Extended supersymmetric theories have also been postulated; these allow particles such as leptoquarks, which transmute leptons into quarks.
Composite particles
All observed subatomic composite particles are called hadrons. All bosonic hadrons are called mesons and all fermionic hadrons are baryons. The most well-known baryons are the constituents of atomic nuclei called protons and neutrons, and collectively named nucleons. The quark model of hadrons states that mesons are built out of a quark and an antiquark, whereas a baryon is made up of three quarks. As of 2005, searches for exotic hadrons are currently under way.
History
J. J. Thomson discovered electrons in 1897. In 1905 Albert Einstein demonstrated the physical reality of the photons which were postulated by Max Planck in order to solve the problem of black body radiation in thermodynamics. Ernest Rutherford discovered in 1907 in the gold foil experiment that the atom is mainly empty space, and that it contains a heavy but small atomic nucleus. The early successes of the quantum theory involved explaining properties of atoms in terms of their electronic structure. The proton was soon identified as the nucleus of hydrogen. The neutron was postulated by Rutherford following his discovery of the nucleus, but was discovered by James Chadwick much later, in 1932. Neutrinos were postulated in 1931 by Wolfgang Pauli (and named by Enrico Fermi) to be produced in beta decays (the weak interaction) of neutrons, but were not discovered till 1956. Pions were postulated by Hideki Yukawa as mediators of the strong force which binds the nucleus together. The muon was discovered in 1936 by Carl D. Anderson, and initially mistaken for the pion. In the 1950s the first kaons were discovered in cosmic rays.
The development of new particle accelerators and particle detectors in the 1950s led to the discovery of a huge variety of hadrons, prompting Wolfgang Pauli's remark: "Had I foreseen this, I would have gone into botany". The classification of hadrons through the quark model in 1961 was the beginning of the golden age of modern particle physics, which culminated in the completion of the unified theory called the standard model in the 1970s. The discovery of the weak gauge bosons through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics. Among the standard model particles the existence of the Higgs boson remains to be verified— this is seen as the primary physics goal of the accelerator called the Large Hadron Collider in CERN. All currently known particles fit into the standard model.


The future
Particle physicists internationally agree on the most important goals of particle physics research in the near and intermediate future. The overarching goal, which is pursued in several distinct ways, is to find and understand what physics may lie beyond the standard model. There are several powerful experimental reasons to expect new physics, including dark matter and neutrino mass. There are also theoretical hints that this new physics should be found at accessible energy scales. Most importantly, though, there may be unexpected and unpredicted surprises which will give us the most opportunity to learn about nature.
Much of the efforts to find this new physics are focused on new collider experiments. A (relatively) near term goal is the completion of the Large Hadron Collider (LHC) in 2007 which will continue the search for the Higgs boson, supersymmetric particles, and other new physics. An intermediate goal is the construction of the International Linear Collider (ILC) which will complement the LHC by allowing more precise measurements of the properties of newly found particles. A decision for the technology of the ILC has been taken in August 2004, but the site has still to be agreed upon.
Additionally, there are important non-collider experiments which also attempt to find and understand physics beyond the standard model. One important non-collider effort is the determination of the neutrino masses since these masses may arise from neutrinos mixing with very heavy particles. In addition, cosmological observations provide many useful constraints on the dark matter, although it may be impossible to determine the exact nature of the dark matter without the colliders. Finally, lower bounds on the very long life time of the proton put constraints on Grand Unification Theories at energy scales much higher than collider experiments will be able to probe any time soon.

Standard Model
The Standard Model of particle physics is our current understanding of the physics of elementary particles. All Standard Model particles except the Higgs boson have been observed.
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Fermions (half-integer spin)
Quark structure proton: 2 up quarks and 1 down quark.
Fermions have half-integer spin; for all known elementary fermions this is ½. Each fermion has its own distinct antiparticle. Fermions are the basic building blocks of all matter. They are classified according to whether they interact via the colour force or not. According to the Standard Model, there are 12 flavors of elementary fermions: six quarks and six leptons.
• Quarks interact via the color force. Their respective antiparticles are known as antiquarks. Quarks exist in six flavors:
Generation Name/Flavor Electric charge (e)
Mass (MeV)
Antiquark
1 Up
(u) +2/3 1.5 to 4 antiup quark

Down
(d) −1/3 4 to 8 antidown quark

2 Strange
(s) −1/3 80 to 130 antistrange quark

Charm
(c) +2/3 1,150 to 1,350 anticharm quark

3 Bottom
(b) −1/3 4,100 to 4,400 antibottom quark

Top
(t) +2/3 171,400 ± 2,100 antitop quark

• Leptons do not interact via the color force. Their respective antiparticles are known as antileptons (although the antiparticle of the electron is called the positron for historical reasons). Leptons also exist in six flavors:
Charged lepton / antiparticle Neutrino / antineutrino
Name Symbol Electric charge (e)
Mass (MeV)
Name Symbol Electric charge (e)
Mass (MeV)

Electron / Positron

−1 / +1 0.511 Electron neutrino / Electron antineutrino

0 < 0.0000022 [1]

Muon

−1 / +1 105.7 Muon neutrino / Muon antineutrino
0 < 0.17 [1]

Tau lepton

−1 / +1 1,777 Tau neutrino / Tau antineutrino
0 < 15.5 [1]

Note that the neutrino masses are known to be non-zero because of neutrino oscillation, but their masses are sufficiently light that they have not been measured directly as of 2006.
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Bosons (integer spin)
Bosons have whole number spins. The fundamental forces of nature are mediated by gauge bosons, and mass is hypothesized to be created by the Higgs boson. According to the Standard Model the elementary bosons are:
Name Charge (e)
Spin Mass (GeV)
Force mediated
Photon
0 1 0 Electromagnetism

W±
±1 1 80.4 Weak nuclear

Z0
0 1 91.2 Weak nuclear

Gluon
0 1 0 Strong nuclear

Higgs
0 0 >112 See below
The Higgs boson (spin-0) is predicted by electroweak theory, and is the only Standard Model particle not yet observed. In the Higgs mechanism of the Standard Model, the massive Higgs boson is created by spontaneous symmetry breaking of the Higgs field. The intrinsic masses of the elementary particles (particularly the massive W± and Z0 bosons) would be explained by their interactions with this field. Many physicists expect the Higgs to be discovered at the Large Hadron Collider (LHC) particle accelerator now under construction at CERN.
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Hypothetical particles
Supersymmetric theories predict the existence of more particles, none of which have been confirmed experimentally as of 2006.
• The neutralino (spin-½) is a superposition of the superpartners of several neutral Standard Model particles. It is a leading candidate for dark matter. The partners of charged bosons are called charginos.
• The photino (spin-½) is the superpartner of the photon.
• The gravitino (spin-3⁄2) is the superpartner of the graviton boson in supergravity theories.
• Sleptons and squarks (spin-0) are the supersymmetric partners of the Standard Model fermions. The stop squark (superpartner of the top quark) is thought to be light and is often the subject of experimental searches.
Other theories predict the existence of additional bosons.
• The graviton (spin-2) has been proposed to mediate gravity in theories of quantum gravity.
• The graviscalar (spin-0) and graviphoton (spin-1).
• The axion (spin-0) is a pseudoscalar particle introduced in Peccei-Quinn theory to solve the strong-CP problem.
• The saxion (spin-0, scalar, R parity=1) and the axino (spin-1/2, R parity = -1) form together with the axion a supermultiplet in supersymmetric extensions of Peccei-Quinn theory.
• The X boson and the Y boson are predicted by GUT theories to be heavier equivalents of the W and Z.
• The magnetic photon.
• Sterile neutrinos are introduced by many extensions to the Standard Model, and may be needed to explain the LSND results.
Magnetic monopole is a generic name for particles with non-zero magnetic charge. They are predicted by some GUT theories.
A tachyon is a hypothetical particle that travels faster than the speed of light and has an imaginary rest mass.
The preon was a suggested substructure for both quarks and leptons, but modern collider experiments have all but disproven their existence.
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Composite particles
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Hadrons
Hadrons are defined as strongly interacting composite particles. Hadrons are either:
• Fermions, in which case they are called baryons.
• Bosons, in which case they are called mesons.
Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks "aces"), describe the known hadrons as composed of valence quarks and/or antiquarks, tightly bound by the color force, which is mediated by gluons. A "sea" of virtual quark-antiquark pairs is also present in each hadron.
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Baryons (fermions)


A combination of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet.
For a detailed list, see List of baryons.
Ordinary baryons (fermions) contain three valence quarks or three valence antiquarks each.
• Nucleons are the fermionic constituents of normal atomic nuclei:
o Protons
o Neutrons
• Hyperons such as the Λ, Σ, Ξ, and Ω particles, which contain one or more strange quarks, are short-lived and heavier than nucleons. Although not normally present in atomic nuclei, they can appear in short-lived hypernuclei.
• A number of charmed and bottom baryons have also been observed.
Some hints at the existence of exotic baryons have been found recently; however, negative results have also been reported. Their existence is uncertain.
• Pentaquarks consist of four valence quarks and one valence antiquark.
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Mesons (bosons)


Mesons of spin 0 form a nonet
For a detailed list, see List of mesons.
Ordinary mesons (bosons) contain a valence quark and a valence antiquark, and include the pion, kaon, the J/ψ, and many other types of mesons. In quantum hadrodynamic models, the strong force between nucleons is mediated by mesons.
Exotic mesons may also exist. Positive signatures have been reported for all of these particles at some time, but their existence is still somewhat uncertain.
• Tetraquarks consist of two valence quarks and two valence antiquarks.
• Glueballs are bound states of gluons with no valence quarks.
• Hybrids consist of one or more valence quark-antiquark pairs and one or more real gluons.
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Atomic nuclei
Atomic nuclei consist of protons and neutrons. Each type of nucleus contains a specific number of protons and a specific number of neutrons, and is called a nuclide or isotope. Nuclear reactions can change one nuclide into another. See Isotope table (complete) for a list of isotopes.
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Atoms
Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. Each type of atom corresponds to a specific chemical element, of which 111 have been officially named. Refer to the periodic table for an overview.
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Molecules
Molecules are the smallest particles into which a non-elemental substance can be divided while maintaining the physical properties of the substance. Each type of molecule corresponds to a specific chemical compound. Molecules are composites of one or more atoms. See list of compounds for a list of molecules.
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Condensed matter
The field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As a result, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are a selection of field excitations, called quasi-particles, that can be created and explored. These include:
• Phonons are vibrational modes in a crystal lattice.
• Excitons are bound states of an electron and a hole.
• Plasmons are coherent excitations of a plasma.
• Polaritons are mixtures of photons with other quasi-particles.
• Polarons are moving, charged (quasi-) particles that are surrounded by ions in a material.
• Magnons are coherent excitations of electron spins in a material.
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Other
• A WIMP (weakly interacting massive particle) is any one of a number of particles that might explain dark matter (such as the neutralino or the axion).
• The pomeron, used to explain the elastic scattering of hadrons and the location of Regge poles in Regge theory.
• The skyrmion, a topological soliton of the pion field, used to model the low-energy properties of the nucleon, such as the axial vector current coupling and the mass.
• A goldstone boson is a massless excitation of a field that has been spontaneously broken. The pions are quasi-Goldstone bosons (quasi- because they are not exactly massless) of the broken chiral isospin symmetry of quantum chromodynamics.
• A goldstino is a Goldstone fermion produced by the spontaneous breaking of supersymmetry.
• An instanton is a field configuration which is a local minimum of the Euclidean action. Instantons are used in nonperturbative calculations of tunneling rates.
• An Oh-My-God particle is an ultra-high energy cosmic ray (probably a proton) falling well beyond the GZK cutoff, the energy limit beyond which virtually no cosmic rays should be detected.
• A spurion is the name given to a "particle" inserted mathematically into an isospin-violating decay in order to analyze it as though it conserved isospin.
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Classification by speed
• A tardyon or bradyon travels slower than light and has a non-zero rest mass.
• A luxon travels at the speed of light and has no rest mass.
• A tachyon (mentioned above) is a hypothetical particle that travels faster than the speed of light and has an imaginary rest mass.