In particle physics, a hadron (listen) (Greek:
ἁδρός, hadrós; "stout, thick") is a
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
P+c(4380) and P+c(4450), 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 environment, mesons such as pions are
produced by the collisions of cosmic rays
with the atmosphere.
== Etymology ==
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:
Notwithstanding the fact that this report
deals with weak interactions, we shall frequently
have to speak of strongly interacting particles.
These particles pose not only numerous scientific
problems, but also a terminological problem.
The point is that "strongly interacting particles"
is a very clumsy term which does not yield
itself to the formation of an adjective.
For this reason, to take but one instance,
decays into strongly interacting particles
are called non-leptonic.
This definition is not exact because "non-leptonic"
may also signify "photonic".
In this report I shall call strongly interacting
particles "hadrons", and the corresponding
decays "hadronic" (the Greek ἁδρός signifies
"large", "massive", in contrast to λεπτός
which means "small", "light").
I hope that this terminology will prove to
be convenient.
== Properties ==
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 +​2⁄3,
for a total of +​4⁄3 together) and one
down quark (with electric charge −​1⁄3).
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
+​1⁄3 for quarks and −​1⁄3 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 ==
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 = ​1⁄3,
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,
P+c(4380) and P+c(4450), both discovered in
2015 by the LHCb collaboration.
== Mesons ==
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 = ​1⁄3 − ​1⁄3
= 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 ==
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
