In particle physics, an elementary particle
or fundamental particle is a subatomic particle
with no substructure, thus not composed of
other particles. Particles currently thought
to be elementary include the fundamental fermions
(quarks, leptons, antiquarks, and antileptons),
which generally are "matter particles" and
"antimatter particles", as well as the fundamental
bosons (gauge bosons and the Higgs boson),
which generally are "force particles" that
mediate interactions among fermions. A particle
containing two or more elementary particles
is a composite particle.
Everyday matter is composed of atoms, once
presumed to be matter's elementary particles—atom
meaning "unable to cut" in Greek—although
the atom's existence remained controversial
until about 1910, as some leading physicists
regarded molecules as mathematical illusions,
and matter as ultimately composed of energy.
Soon, subatomic constituents of the atom were
identified. As the 1930s opened, the electron
and the proton had been observed, along with
the photon, the particle of electromagnetic
radiation. At that time, the recent advent
of quantum mechanics was radically altering
the conception of particles, as a single particle
could seemingly span a field as would a wave,
a paradox still eluding satisfactory explanation.Via
quantum theory, protons and neutrons were
found to contain quarks—up quarks and down
quarks—now considered elementary particles.
And within a molecule, the electron's three
degrees of freedom (charge, spin, orbital)
can separate via the wavefunction into three
quasiparticles (holon, spinon, orbiton). Yet
a free electron—which is not orbiting an
atomic nucleus and lacks orbital motion—appears
unsplittable and remains regarded as an elementary
particle.Around 1980, an elementary particle's
status as indeed elementary—an ultimate
constituent of substance—was mostly discarded
for a more practical outlook, embodied in
particle physics' Standard Model, what's known
as science's most experimentally successful
theory. Many elaborations upon and theories
beyond the Standard Model, including the popular
supersymmetry, double the number of elementary
particles by hypothesizing that each known
particle associates with a "shadow" partner
far more massive, although all such superpartners
remain undiscovered. Meanwhile, an elementary
boson mediating gravitation—the graviton—remains
hypothetical.
== Overview ==
All elementary particles are—depending on
their spin—either bosons or fermions. These
are differentiated via the spin–statistics
theorem of quantum statistics. Particles of
half-integer spin exhibit Fermi–Dirac statistics
and are fermions. Particles of integer spin,
in other words full-integer, exhibit Bose–Einstein
statistics and are bosons.
Notes:
1. The antielectron (e+) is traditionally
called positron.
2. The known force carrier bosons all have
spin = 1 and are therefore vector bosons.
The hypothetical graviton has spin = 2 and
is a tensor boson; whether it is a gauge boson
as well, is unknown.
In the Standard Model, elementary particles
are represented for predictive utility as
point particles. Though extremely successful,
the Standard Model is limited to the microcosm
by its omission of gravitation and has some
parameters arbitrarily added but unexplained.
== Common elementary particles ==
According to the current models of big 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 up 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 all baryons, in turn consist of
up quarks and down quarks.
Some estimates imply that there are roughly
1080 baryons (almost entirely protons and
neutrons) in the observable universe.The number
of protons in the observable universe is called
the Eddington number.
In terms of number of particles, some estimates
imply that nearly all the matter, excluding
dark matter, occurs in neutrinos, and that
roughly 1086 elementary particles of matter
exist in the visible universe, mostly neutrinos.
Other estimates imply that roughly 1097 elementary
particles exist in the visible universe (not
including dark matter), mostly photons and
other massless force carriers.
== Standard Model ==
The Standard Model of particle physics contains
12 flavors of elementary fermions, plus their
corresponding antiparticles, as well as elementary
bosons that mediate the forces and the Higgs
boson, which was reported on July 4, 2012,
as having been likely detected by the two
main experiments at the Large Hadron Collider
(ATLAS and CMS). However, the Standard Model
is widely considered to be a provisional theory
rather than a truly fundamental one, since
it is not known if it is compatible with Einstein's
general relativity. There may be hypothetical
elementary particles not described by the
Standard Model, such as the graviton, the
particle that would carry the gravitational
force, and sparticles, supersymmetric partners
of the ordinary particles.
=== Fundamental fermions ===
The 12 fundamental fermions are divided into
3 generations of 4 particles each. Half of
the fermions are leptons, three of which have
an electric charge of −1, called the electron
(e−), the muon (μ−), and the tau (τ−),
the other three leptons are neutrinos (νe,
νμ, ντ), which are the only elementary
fermions with no electric or color charge.
The remaining six particles are quarks (discussed
below).
==== Generations ====
==== 
Mass ====
The following table lists current measured
masses and mass estimates for all the fermions,
using the same scale of measure: millions
of electron-volts (MeV). For example, the
most accurately known quark mass is of the
top quark (t) at 172.7 GeV/c² or 172 700
MeV/c², estimated using the On-shell scheme.
Estimates of the values of quark masses depend
on the version of quantum chromodynamics used
to describe quark interactions. Quarks are
always confined in an envelope of gluons which
confer vastly greater mass to the mesons and
baryons 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.
==== Antiparticles ====
There are also 12 fundamental fermionic antiparticles
that correspond to these 12 particles. For
example, the antielectron (positron) e+ is
the electron's antiparticle and has an electric
charge of +1.
==== Quarks ====
Isolated quarks and antiquarks have never
been detected, 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.
However, color-charged particles may combine
to form color neutral composite particles
called hadrons. 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
neutral meson. Alternatively, three quarks
can exist together, one quark being "red",
another "blue", another "green". These three
colored quarks together form a color-neutral
baryon. Symmetrically, three antiquarks with
the colors "antired", "antiblue" and "antigreen"
can 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, whereas the Higgs boson (spin-0)
is responsible for the intrinsic mass 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, like mesons. The spin of bosons
are integers instead of half integers.
==== Gluons ====
Gluons mediate the strong interaction, which
join quarks and thereby form hadrons, which
are either baryons (three quarks) or mesons
(one quark and one antiquark). Protons and
neutrons are baryons, joined by gluons to
form the atomic nucleus. Like quarks, gluons
exhibit color and anticolor—unrelated to
the concept of visual color—sometimes in
combinations, altogether eight variations
of gluons.
==== Electroweak bosons ====
There are three weak gauge bosons: W+, W−,
and Z0; these mediate the weak interaction.
The W bosons are known for their mediation
in nuclear decay. The W− converts a neutron
into a proton then decay into an electron
and electron antineutrino pair. The Z0 does
not convert charge but rather changes momentum
and 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 massless
photon mediates the electromagnetic interaction.
These four gauge bosons form the electroweak
interaction among elementary particles.
==== 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). On 4 July 2012, after
many years of experimentally searching for
evidence of its existence, the Higgs 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. The Higgs boson
is believed to have a mass of approximately
125 GeV. The statistical 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 a discovery. Research into
the properties of the newly discovered particle
continues.
==== Graviton ====
The graviton is a hypothetical elementary
spin-2 particle proposed to mediate gravitation.
While it remains undiscovered due to the difficulty
inherent in its detection, it is sometimes
included in tables of elementary particles.
The conventional graviton is massless, although
there exist models containing massive Kaluza–Klein
gravitons.
== Beyond the Standard Model ==
Although experimental evidence overwhelmingly
confirms the predictions derived from the
Standard Model, some of its parameters were
added arbitrarily, not determined by a particular
explanation, which remain mysteries, for instance
the hierarchy problem. Theories beyond the
Standard Model attempt to resolve these shortcomings.
=== 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 and Y bosons, which cause proton decay.
However, the non-observation of proton decay
at the Super-Kamiokande neutrino observatory
rules out the simplest GUTs, including SU(5)
and SO(10).
=== Supersymmetry ===
Supersymmetry extends the Standard Model by
adding another 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 by the Large Hadron Collider
at CERN.
=== String theory ===
String theory is a model of physics where
all "particles" that make up matter are composed
of strings (measuring at the Planck length)
that exist in an 11-dimensional (according
to M-theory, the leading version) or 12-dimensional
(according to F-theory) 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, 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) that 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).
String theory proposes 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 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 the
graviton.
=== Technicolor ===
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.
=== 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 1980s.
=== Acceleron theory ===
Accelerons are the hypothetical subatomic
particles that integrally link the newfound
mass of the neutrino to the dark energy conjectured
to be accelerating the expansion of the universe.In
theory, neutrinos are influenced by a new
force resulting from their interactions with
accelerons. Dark energy results as the universe
tries to pull neutrinos apart.
== See also ==
== 
Notes ==
== 
Further reading ==
=== General readers ===
Feynman, R.P. & Weinberg, S. (1987) Elementary
Particles and the Laws of Physics: The 1986
Dirac Memorial Lectures. Cambridge Univ. Press.
Ford, Kenneth W. (2005) The Quantum World.
Harvard Univ. Press.
Brian Greene (1999). The Elegant Universe.
W.W.Norton & Company. ISBN 978-0-393-05858-1.
John Gribbin (2000) Q is for Quantum – An
Encyclopedia of Particle Physics. Simon & Schuster.
ISBN 0-684-85578-X.
Oerter, Robert (2006) The Theory of Almost
Everything: The Standard Model, the Unsung
Triumph of Modern Physics. Plume.
Schumm, Bruce A. (2004) Deep Down Things:
The Breathtaking Beauty of Particle Physics.
Johns Hopkins University Press. ISBN 0-8018-7971-X.
Martinus Veltman (2003). Facts and Mysteries
in Elementary Particle Physics. World Scientific.
ISBN 978-981-238-149-1.
Frank Close (2004). Particle Physics: A Very
Short Introduction. Oxford: Oxford University
Press. ISBN 978-0-19-280434-1.
Seiden, Abraham (2005). Particle Physics – A
Comprehensive Introduction. Addison Wesley.
ISBN 978-0-8053-8736-0.
=== Textbooks ===
Bettini, Alessandro (2008) Introduction to
Elementary Particle Physics. Cambridge Univ.
Press. ISBN 978-0-521-88021-3
Coughlan, G. D., J. E. Dodd, and B. M. Gripaios
(2006) The Ideas of Particle Physics: An Introduction
for Scientists, 3rd ed. Cambridge Univ. Press.
An undergraduate text for those not majoring
in physics.
Griffiths, David J. (1987) Introduction to
Elementary Particles. John Wiley & Sons. ISBN
0-471-60386-4.
Kane, Gordon L. (1987). Modern Elementary
Particle Physics. Perseus Books. ISBN 978-0-201-11749-3.
Perkins, Donald H. (2000) Introduction to
High Energy Physics, 4th ed. Cambridge Univ.
Press.
== External links ==
The most important address about the current
experimental and theoretical knowledge about
elementary particle physics is the Particle
Data Group, where different international
institutions collect all experimental data
and give short reviews over the contemporary
theoretical understanding.
Particle Data Groupother pages are:
Greene, Brian, "Elementary particles", The
Elegant Universe, NOVA (PBS)
particleadventure.org, a well-made introduction
also for non physicists
CERNCourier: Season of Higgs and melodrama
Pentaquark information page
Interactions.org, particle physics news
Symmetry Magazine, a joint Fermilab/SLAC publication
"Sized Matter: perception of the extreme unseen",
Michigan University project for artistic visualisation
of subatomic particles
Elementary Particles made thinkable, an interactive
visualisation allowing physical properties
to be compared
