Fundamental production creating a particle-antiparticle pair. Often described

Fundamental FOrce Mediating
particles

Abstract

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The currently known fundamental forces of
nature are the electromagnetic, weak, strong, and gravitational forces. This
paper will give a basic overview of the current standard model that is used to
describe these forces. It will also explore some of the history behind the
developments.

Introduction

As quantum mechanics was developing, it
became clear that the interactions between particles could be described as the exchange
of particles. The particles that mediate the fundamental interactions are known
as gauge bosons. Gauge bosons often behave as virtual particles during
interactions; this is where they mediate a force but cannot be directly observed
1.

Feynman Diagrams will be used throughout
as they are a useful visual conveyor of information. They will have time on the
x-axis and space on the y-axis. For those unfamiliar with these diagrams, a few
things are important to note: The arrows on lines for electrically charged
particles represents the direction of flow of negative electric charge; and some
particles may be illustrated to seemingly travel in space but not in time, these
are virtual particles.

The standard model may be presented as
having two parts. These are the Electroweak Model, which includes both the weak
and electromagnetic interactions, and Quantum Chromodynamics, which include the
strong interaction. The Electroweak Model is the one that will be discussed first.

Electroweak
Interactions

Quantum Electrodynamics

Photons are the mediators of the electromagnetic
force. The theories around these interactions between light and matter are
sometimes referred to by the mouthful of a name, quantum electrodynamics. Photons
have an infinite range, but if they have a high enough energy they can undergo
pair production creating a particle-antiparticle pair. Often described as
packets of energy, they carry only energy with no mass and no quantum numbers.
What is specifically notable is that they don’t carry electrical charge, yet
they mediate the electromagnetic force – this is not the case in other gauge
bosons which will be discussed later.

Here is an illustration
showing an example electromagnetic interaction:

?

 

e-

 

e-

 

e-

 

e-

 

Figure X. A Feynman diagram showing
electron scattering.

The photon behaves as a virtual particle,
and mediates electromagnetic repulsion between the two electrons. It is because
the photon is behaving as a virtual particle that it can be drawn as a vertical
line where no time passes. A similar diagram may be drawn to represent an
attraction between opposite charge.

A similar interaction could be illustrated
for any charged particle

 

Electroweak Unification

Between the years 1961 and 1967 the first
part of the standard model, the Electroweak theory, was developed. This was
done mainly by Abdus Salam, Sheldon Glashow, and Steven Weinberg. This theory
unified the electromagnetic and weak interactions into a single theory. It
showed that although at low energies there is a big distinction between the
weak and the electromagnetic interactions; at very high energies the
distinction wasn’t present. It predicted the weak force mediators, the W+,
W-, and Z0 bosons. These particles were later discovered
in 1983 in CERN with the masses predicted. 6 1

These bosons differ greatly form the
photon in that they are massive. This is explained by the Higgs boson which I
will not cover here, but further reading can be found here: 6.

Some, namely W+ and W-, carry electric
charge therefore they themselves can interact by the electromagnetic interaction.
They also have what is known as a weak charge, as do all particles that interact
by the weak force. Interestingly enough, this means that they can also interact
with themselves by the weak force. 3

Here is an illustration showing a weak
interaction:

u

 

d

 

e-

 

?? e

 

W-

 

Figure X. A Feynman diagram showing a weak
interaction, specifically ?- decay of a down quark into an up quark.
Here you can see it’s also conventional to use wiggly lines to represent the
weak force bosons. This fits nicely due to the unification of the two forces.

In this example the boson produces a pair
of particles.  The anti-electron neutrino
must be produced to conserve the electron lepton quantum number. A similar
interaction might happen but in reverse, where a up quark decays into a down
quark. This would instead produce a W+ boson to conserve charge, and
that would proceed to decay into a positron electron-neutrino pair.

Strong
Interactions

The Yukawa Interaction

The first suggestion of a force mediator
for the strong interaction came from Hideki Yukawa. In his paper “On the Theory
of Elementary Particles. I” 4, he suggested that a theoretical quantised particle
could act as a mediator for the strong interaction. At this time the quark
model was yet to be developed, so his theory was developed around hadrons being
fundamental particles. Nevertheless, he managed to predict the later discovered
pi-meson (or pion), which does indeed mediate the interaction between hadrons
as expected. This type of interaction is now known as a Yukawa interaction. 1
5

As time progressed it could be seen that the
lepton family formed compact, nicely organised group, but the hadrons not so
much. There were many reasons to suspect an underlying structure for hadrons,
but one main obvious one was that newly discovered hadrons continued to pile
up. In 1964, George Zweig and Murray Gell-Mann independently developed a theory
which in effect defined the substructure of hadrons. Known as the quark model,
this model in a more developed form is incorporated in what is now the second
half of the standard model. 5

Quantum Chromodynamics

Quantum chromodynamics describes the
strong interaction as the exchange of gluons. 
Gluons, like photons, are massless. One of the main parts of Quantum
Chromodynamics (QCD) revolves around the quantum number ‘colour charge’ which will
be referred to as colour. Like electric charge, colour is always conserved in interactions.

Both quarks and gluons have colour. There
are three types of colour, each with a corresponding ‘anticolour’. These are:
red, antired, green, antigreen, blue, and antiblue.  While quarks carry unit colour (or anticolour
for antiquarks), gluons consist of a colour-anticolour combination. The allowed
combinations give a total of 8 possible gluons – these are two colour neutral
gluons, and three with corresponding antiparticles. A baryon will have a quark
of each colour, while a meson will have a colour and its corresponding
anticolour – they are therefore both colour neutral overall. 1

g

 

qR

 

qB

 

qB

 

qR

 

 

Figure X – A Feynman Diagram Showing the
interaction between a red quark and a blue quark   mediated by a gluon.

The colour of the gluon could be either RB?
or BR?. It may be helpful to think of it as one particle travelling in one
direction being identical to its antiparticle travelling in the opposite
direction.

 

 

 

 

 

 

g

 

qG?

 

qB?

 

qG

 

qB

 

Figure X –
A Feynman Diagram showing the interaction between a green quark and an
antigreen quark.

The colour
of the gluon could be either GB? or BG

 

Conclusion

This piece has only been a brief dip of
the toes into the basics of particle physics with a focus on the gauge bosons. The
standard model is good, but far from complete. Currently there is no theory not
based on conjecture that unifies QCD with the electroweak theory, but this is
something that many people hope to achieve. Such theories are known as ‘grand
unified theories’ – If I succeeded in developing one I would definitely embrace
the pretentious name. Some of these include estimations for proton decay
without conserving baryon number, but this is of course near impossible to
verify experimentally because the predicted half life of the proton is much
longer than the estimated life of the universe.  The number of protons that would need to be observed
at once in conditions free of interfering particles is likely not achievable. 3

One step to have a more complete particle
model of the fundamental interactions would be to verify the existence of the theoretical
particle the graviton. It is theorised to be the force mediating particle of
the gravitational force. Successfully doing this would involve developing a theory
to take over from the large-scale approximation in Einstein’s theory of general
relativity. But things relating to gravity on small scales are very difficult to
do due to the tiny impact that gravity has at these scales. 2

What the reader should take away from this
is: Whoever you are, and whatever your knowledge level, there is sure to be
more for you to discover.