Gluons

Gluons are the exchange
particles for the color
force between quarks,
analogous to the exchange of photons in the electromagnetic
force between two charged particles. The gluon can be considered to be
the fundamental exchange particle underlying the strong
interaction between protons and neutrons in a nucleus. That
short-range nucleon-nucleon interaction can be considered to be a residual
color force extending outside the boundary of the proton or neutron. That
strong interaction was modeled by Yukawa
as involving an exchange of pions,
and indeed the pion
range calculation was helpful in developing our understanding of the
strong force.

Gluon interactions are often represented by a Feynman
diagram. Note that the gluon generates a color change for the
quarks. The gluons are in fact considered to be bi-colored, carrying
a unit of color and a unit of anti-color as suggested in the diagram
at right. The gluon exchange picture there converts a blue quark to
a green one and vice versa. The range of the strong force is limited
by the fact that the gluons interact with each other as well as with
quarks in the context of quark
confinement. These properties contrast them with photons, which
are massless and of infinite range. The photon does not carry
electric charge with it, while the gluons do carry the "color
charge".

Within their range of about a fermi, the gluons can interact with each
other, and can produce virtual quark-antiquark pairs. The property of
interaction with each other is very different from the other exchange
particles, and raises the possibility of gluon collections referred to as
"glueballs". The internal state of a hadron
is viewed as composed of a fixed net number of quarks, but with a dynamic
cloud of gluons and quark-antiquark pairs in equilibrium.

Index

Particle
concepts

Reference
Griffiths
Section
2.3

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Feynman Diagrams

Feynman diagrams are graphical ways to represent exchange
forces. Each point at which lines come together is called a vertex,
and at each vertex one may examine the conservation
laws which govern particle interactions. Each vertex must conserve
charge, baryon number and lepton number.

Developed by Feynman to decribe the interactions in quantum
electrodynamics (QED), the diagrams have found use in describing a
variety of particle interactions. They are spacetime diagrams, ct vs
x. The time axis points upward and the space axis to the right.
(Particle physicists often reverse that orientation.) Particles are
represented by lines with arrows to denote the direction of their
travel, with antiparticles having their arrows reversed. Virtual
particles are represented by wavy or broken lines and have no
arrows. All electromagnetic interactions can be described with
combinations of primitive diagrams like this one.

Only lines entering or leaving the diagram represent observable
particles. Here two electrons enter, exchange a photon, and then
exit. The time and space axes are usually not indicated. The
vertical direction indicates the progress of time upward, but the
horizontal spacing does not give the distance between the particles.

Other electromagnetic process can be represented, as in the examples
below. A backward arrow represents the antiparticle, in these cases a
positron. Keep in mind that time progresses upward, and that a downward
arrow is not a particle progressing downward, but an antiparticle
progressing upward ( forward in time).

After being introduced for electromagnetic processes, Feynman diagrams
were developed for the weak and strong interactions as well. Forms of
primitive vertices for these three interactions are

Particle interactions can be represented by diagrams with at least two
vertices. They can be drawn for protons, neutrons, etc. even though they
are composite objects and the interaction can be visualized as being
between their constituent quarks.

Feynman diagrams for:

Lamb
shift

Neutron
decay

Weak
interaction

Quark
decay

Strong
interaction

Twisted
Feynman diagrams and crossing
symmetry

Index

Particle
concepts

References
Griffiths
Ch
2

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Intermediate Vector Bosons

The W
and Z particles are the massive exchange
particles which are involved in the nuclear weak
interaction, the weak force between electrons and neutrinos. They were
predicted by Weinberg, Salam, and Glashow in 1979 and measured at CERN
in 1982. The prediction included a prediction of the masses of these
particles as a part of the unified theory of the electromagnetic and weak
forces, the electroweak
unification. "If the weak and electromagnetic forces are essentially
the same, then they must also have the same strength. The fact that the
experimentally observed strengths seem quite different is attributed to
the masses of the W and Z particles-under certain conditions a force of
large strength can have the appearance of a force of small strength if the
particle that carries the force is very massive. Theoretical calculations
show that at a fundamental level the weak and electromagnetic forces have
the same strength if the W and Z particles have masses of 80 and 90 GeV
respectively." The masses measured at CERN were 82 and 93 GeV, a brilliant
confirmation of the electroweak unification.

The experiments at CERN detected a total of 10 W bosons and 4 Z bosons.
In the extended experiment at Fermilab's Tevatron
known as "Run 1" (1992-96), the D0
detector facility measured over 100,000 W particles. The D0 value for
the mass of the W is 80.482 +/- 0.091 GeV. Current values combining the
experiments at the Tevatron and at CERN's LEP electron-positron collider
are MW = 80.41 +/- 0.18 GeV and MZ = 91.1884 +/-
0.0022 GeV.

Index

Particle
concepts

Reference
Ohanian
Sec
46.5

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Properties of W and Z

The W and Z particles are called intermediate
vector bosons and are the exchange
particles for the weak
interaction. The weak interaction visualized in the Feynman
diagram below is responsible for the decay
of the neutron and for beta
decay.

The charged W bosons participate in the transformation
of quarks in which the flavor of the quark
is changed. The neutron Z boson does not participate in changing the
flavor of quarks, so its interactions are harder to detect. It interacts
by influencing the scattering cross sections for neutrinos in what are
called "neutral
currents".

The W bosons can decay by a number of processes, and this
provides a variety of decay paths for those particles which decay by
the weak interaction. An interesting example is the decay of the D
meson.

Feynman
diagrams of quark
transformations

Index

Particle
concepts

Reference
Rohlf
Ch.
18

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Photon

Photon is the name given to a quantum of light or other electromagnetic
radiation. The photon energy is given in the Planck
relationship. The photon is the exchange
particle responsible for the electromagnetic
force. The force between two electrons can be visualized in terms of a
Feynman
diagram as shown below. The infinite range
of the electromagnetic force is owed to the zero rest mass of the photon.
While the photon has zero rest mass, it has finite momentum,
exhibits deflection by a gravity field, and can exert a force.

The photon has an intrinsic angular momentum or "spin" of 1, so
that the electron
transitions which emit a photon must result in a net change of 1
in the angular momentum of the system. This is one of the "selection
rules" for electron transitions.

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Graviton

The graviton is the exchange
particle for the gravity
force. Although it has not been directly observed, a number of its
properties can be implied from the nature of the force. Since gravity is
an inverse
square force of apparently infinite range,
it can be implied that the rest mass of the graviton is zero.

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Particle
concepts

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