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=                               Hadron                               =
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                            Introduction
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In particle physics, a hadron  (, 'hadrós;' "stout, thick") is a
subatomic composite particle made of two or more quarks held together
by the strong force in a similar way as molecules are held together by
the electromagnetic force. Most of the mass of ordinary matter comes
from two hadrons, the proton and the neutron.

Hadrons are categorized into two families: baryons, made of an odd
number of quarks - usually three quarks - and mesons, made of an even
number of quarks�usually one quark and one antiquark. Protons and
neutrons are examples of baryons; pions are an example of a meson.
"Exotic" hadrons, containing more than three valence quarks, have been
discovered in recent years. A tetraquark state (an exotic meson),
named the Z(4430)�, was discovered in 2007 by the Belle Collaboration
and confirmed as a resonance in 2014 by the LHCb collaboration. Two
pentaquark states (exotic baryons), named  and , were discovered in
2015 by the LHCb collaboration.
There are several more exotic hadron candidates, and other
colour-singlet quark combinations that may also exist.

Almost all "free" hadrons and antihadrons (meaning, in isolation and
not bound within an atomic nucleus) are believed to be unstable and
eventually decay (break down) into other particles. The only known
exception relates to free protons, which are 'possibly' stable, or at
least, take immense amounts of time to decay (order of 1034+ years).
Free neutrons are unstable and decay with a half-life of about 611
seconds. Their respective antiparticles are expected to follow the
same pattern, but they are difficult to capture and study, because
they immediately annihilate on contact with ordinary matter. "Bound"
protons and neutrons, contained within an atomic nucleus, are
generally considered stable. Experimentally, hadron physics is studied
by colliding protons or nuclei of heavy elements such as lead or gold,
and detecting the debris in the produced particle showers. In the
natural environment, mesons such as pions are produced by the
collisions of cosmic rays with the atmosphere.


                             Etymology
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The term "hadron" was introduced by Lev B. Okun in a plenary talk at
the 1962 International Conference on High Energy Physics.
In this talk he said:


                             Properties
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alt=A green and a magenta ("antigreen") arrow canceling out each other
out white, representing a meson; a red, a green, and a blue arrow
canceling out to white, representing a baryon; a yellow ("antiblue"),
a magenta, and a cyan ("antired") arrow canceling out to white,
representing an antibaryon.

According to the quark model,
the properties of hadrons are primarily determined by their so-called
'valence quarks'. For example, a proton is composed of two up quarks
(each with electric charge +, for a total of + together) and one down
quark (with electric charge �). Adding these together yields the
proton charge of +1. Although quarks also carry color charge, hadrons
must have zero total color charge because of a phenomenon called color
confinement. That is, hadrons must be "colorless" or "white". The
simplest ways for this to occur are with a quark of one color and an
antiquark of the corresponding anticolor, or three quarks of different
colors. Hadrons with the first arrangement are a type of meson, and
those with the second arrangement are a type of baryon.

Massless virtual gluons compose the numerical majority of particles
inside hadrons. The strength of the strong force gluons which bind the
quarks together has sufficient energy ('E') to have resonances
composed of massive ('m') quarks ('E > mc2') . One outcome is that
short-lived pairs of virtual quarks and antiquarks are continually
forming and vanishing again inside a hadron. Because the virtual
quarks are not stable wave packets (quanta), but an irregular and
transient phenomenon, it is not meaningful to ask which quark is real
and which virtual; only the small excess is apparent from the outside
in the form of a hadron. Therefore, when a hadron or anti-hadron is
stated to consist of (typically) 2 or 3 quarks, this technically
refers to the constant excess of quarks vs. antiquarks.

Like all subatomic particles, hadrons are assigned quantum numbers
corresponding to the representations of the Poincaré group:
'JPC'('m'), where 'J' is the spin quantum number, 'P' the intrinsic
parity (or P-parity), 'C' the charge conjugation (or C-parity), and
'm' the particle's mass. Note that the mass of a hadron has very
little to do with the mass of its valence quarks; rather, due to
mass-energy equivalence, most of the mass comes from the large amount
of energy associated with the strong interaction. Hadrons may also
carry flavor quantum numbers such as isospin (G parity), and
strangeness. All quarks carry an additive, conserved quantum number
called a baryon number ('B'), which is + for quarks and � for
antiquarks. This means that baryons (composite particles made of
three, five or a larger odd number of quarks) have 'B' = 1 whereas
mesons have 'B' = 0.

Hadrons have excited states known as resonances. Each ground state
hadron may have several excited states; several hundreds of resonances
have been observed in experiments. Resonances decay extremely quickly
(within about 10�24 seconds) via the strong nuclear force.

In other phases of matter the hadrons may disappear.  For example, at
very high temperature and high pressure, unless there are sufficiently
many flavors of quarks, the theory of quantum chromodynamics (QCD)
predicts that quarks and gluons will no longer be confined within
hadrons, "because the strength of the strong interaction diminishes
with energy". This property, which is known as asymptotic freedom, has
been experimentally confirmed in the energy range between 1 GeV
(gigaelectronvolt) and 1 TeV (teraelectronvolt).


All free hadrons except (possibly) the proton and antiproton are
unstable.


                              Baryons
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Baryons are hadrons containing an odd number of valence quarks (at
least 3). Most well known baryons such as the proton and neutron have
three valence quarks, but pentaquarks with five quarks - three quarks
of different colors, and also one extra quark-antiquark pair - have
also been proven to exist. Because baryons have an odd number of
quarks, they are also all fermions, 'i.e.', they have half-integer
spin. As quarks possess baryon number 'B' = , baryons have baryon
number 'B' = 1.

Each type of baryon has a corresponding antiparticle (antibaryon) in
which quarks are replaced by their corresponding antiquarks. For
example, just as a proton is made of two up-quarks and one down-quark,
its corresponding antiparticle, the antiproton, is made of two
up-antiquarks and one down-antiquark.

As of August 2015, there are two known pentaquarks,  and , both
discovered in 2015 by the LHCb collaboration.


                               Mesons
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Mesons are hadrons containing an even number of valence quarks (at
least 2). Most well known mesons are composed of a quark-antiquark
pair, but possible tetraquarks (4 quarks) and hexaquarks (6 quarks,
comprising either a dibaryon or three quark-antiquark pairs) may have
been discovered and are being investigated to confirm their nature.
Several other hypothetical types of exotic meson may exist which do
not fall within the quark model of classification. These include
glueballs and hybrid mesons (mesons bound by excited gluons).

Because mesons have an even number of quarks, they are also all
bosons, with integer spin, 'i.e.', 0, 1, or �1. They have baryon
number 'B' =  �  = 0. Examples of mesons commonly produced in particle
physics experiments include pions and kaons. Pions also play a role in
holding atomic nuclei together via the residual strong force.


                              See also
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* Exotic hadron
* Hadron therapy, a.k.a. particle therapy
* Hadronization, the formation of hadrons out of quarks and gluons
* Large Hadron Collider (LHC)
* List of particles
* Standard model
* Subatomic particles


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