In
particle physics, a
hadron ( , from the ,
hadrós, "stout,
thick") is a particle made of
quarks held together by the
strong force (similar to how
molecules are held together by the
electromagnetic force). Hadrons are
either
mesons (made of one quark and one
antiquark) or
baryons (made of three quarks). Other combinations,
such as
tetraquarks (an
"exotic" meson) and
pentaquarks (an
"exotic"
baryon), may be possible but no evidence conclusively suggests
their existence . The best known mesons are
pions and
kaons, while the best
known baryons are
protons and
neutrons.
Introduction
All types of hadrons have zero total color charge.
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 + ) 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". There are two ways to accomplish this:
three quarks of different colors, or a quark of one color and an
antiquark carrying the corresponding
anticolor. Hadrons based on the former are called
baryons, and those based on the latter are called
mesons.
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), and
C, the charge conjugation (or
C-parity), and the particle's
mass,
m. 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 (or
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 (groups of
three quarks) have
B = 1 while 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 particle physics experiments. Resonances decay extremely quickly
(within about 10
−24 seconds) via
the strong nuclear force.
In other
phases of
QCD 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 at the energy scales between a
GeV and a
TeV.
All free hadrons except the proton are unstable.
Baryons
All known baryons are made of three valence quarks, and are
therefore
fermions. They have
baryon number B = 1, while
antibaryons (composed of three antiquarks) have
B = −1. In principle, some baryons (or
antibaryons) could be composed of further quark–antiquark pairs in
addition to their three quarks (or antiquarks). Baryons containing
a single additional quark–antiquark pair are called
pentaquarks. Several pentaquarks candidates were
found in the early 2000s, but upon further review these states have
now been established as non-existent. (This does not rule against
pentaquarks in general, only the candidates put forward). No
evidence of baryon states with even more quark–antiquark pairs has
been found.
Mesons
Mesons are
bosons composed of a
quark–antiquark pair. They have baryon number
B = 0. Examples of mesons commonly produced in
particle physics experiments include
pions and
kaons. The former also play a role holding
atomic nuclei together via the
residual strong force. Hypothetical
mesons have more than one quark–antiquark pair; a meson composed of
two of these pairs is called a
tetraquark. Several tetraquark candidates were
found in the 2000s, but their status in under debate.
Several hypothetical
"exotic" mesons
lie outside the quark model classification. These include
glueballs and
hybrid
mesons (mesons bound by excited
gluons).
See also
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