Hadrons

Hadrons are particles
made from quarks
and/or gluons, bound
together by their strong
interactions.

Hadrons have no net strong charge (or color
charge)  but they do have
residual strong interactions due to their color-charged
substructure. Many types of hadrons were discovered in the 50's and
60's.  The idea of the quarks was first proposed to explain the
many observed hadrons. There are two classes of hadrons: baryons
and mesons.

Baryons

Baryons are particles
made from a basic structure of three quarks (and anti-baryons from three
antiquarks.)

Baryons carry an odd half quantum unit of angular momentum
(spin)
and, hence, are fermions, which means that they obey the Pauli
Exclusion Principle rules.

Sample Fermionic
Hadrons
Baryons () and Anti-baryon ()

Symbol

Name

Quark
Content

Electric
Charge

Mass
(GeV/c2)

Spin

proton

1

0.938

1/2

anti-proton

-1

0.938

1/2

neutron

0

0.940

1/2

lambda

0

1.116

1/2

omega

-1

1.672

3/2

The proton is the only baryon that is stable in isolation.
Its basic structure is two up quarks and one down
quark. Neutrons are
also baryons. Although neutrons are not stable in isolation, they can be
stable inside certain nuclei. A neutron's basic structure is two down
quarks and one up quarks.

More massive baryons may be made from any set of three
quarks. Baryons containing more massive quarks are all unstable because
these quarks decay via weak
interactions. There are also more massive baryons that have the same
quark content as a proton or a neutron but have additional angular
momentum. These are typically very short-lived because they can decay to
a proton or a neutron and a meson via residual strong interactions.

Mesons

Mesons are color-neutral particles with a basic structure
of one quark and one antiquark. There are no stable mesons. Mesons have
integer (or zero) units of spin, and hence are bosons, which means that they do not obey Pauli
exclusion principle rules.

Sample Bosonic
Hadrons
Mesons ()

Symbol

Name

Quark
Content

Electric
Charge

Mass
(GeV/c2)

Spin

pion

+1

0.140

0

kaon

-1

0.494

0

rho

+1

0.770

1

D+

+1

1.869

0

eta-c

0

2.979

0

The most common mesons are:

Pions (or pi mesons), made from up and down type quarks
and antiquarks only (for example, a pi-plus meson is a u and an anti-d
quark).

K mesons, which contain one u or d type quark or
antiquark and one s type (for example K-plus is a u and an anti-s
quark).

These are the only types of mesons which are long-lived
enough to be seen directly by their tracks in a detector.

More massive mesons with the same quark content but higher
angular momentum, as well as others containing one or more of the more
massive quark types, are all very short lived. They  have
been  found by studies of their decay products, but they decay too
quickly to leave a track that can be  detected.

Particles that contain the more massive quark types, for
example B, D, or Nc particles decay via weak interactions
slowly enough that their production can be inferred from the fact that
the decay product tracks emerge from a vertex point outside the beam collision
region. Vertex
detectors are precision tracking devices designed chiefly to detect
such decay vertex points.

Complex
Structure

The quarks inside a meson or baryon are continually
interacting with one another via the strong force field. At any instant
in time, they may contain many virtual
particles: gluons and additional quark-antiquark pairs.

The picture of a proton as made of three quarks is thus a
gross simplification. For example, we know from measurements that in a
high-momentum proton only about half the momentum is carried by quarks,
the rest is carried by gluons.

Glueballs

Theoretically, there are additional types of hadrons that
have a basic structure that is made purely from gluons. These are called
glueballs. Such particles do contain virtual quark-antiquark pairs, at
least some of the time, so it is very difficult to find an experimental
signature that can distinguish such a particle from an ordinary meson.
There are, however, some observed particles that are thought to be
chiefly glueball-like.

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