High Energy Particle Physics
At this most basic level, we are looking for the stuff from which all matter is made; the
basic forces and constituent particles making up all of matter. The players identified so
far are summarized in the following table:
Families of particles:




Leptons: “low mass” particles like the electron. No apparent internal structure.
Mesons: “intermediate mass” particles exchanged by baryons(below). Quarkantiquark pairs (π+ = uđ, π- = ūd)
Baryons: “high mass” particles like protons, composed of three quarks. (p=uud,
n=udd)
Vector Bosons: particles exchanged when a force is exerted. Carry momentum,
energy and other quantities between interacting particles.
Fundamental particles:
particles
charge
Quarks
Leptons
Force particles
2/3
-1/3
-1
0
0
-1 +1 0
eνe
γ photon
up(u)
down(d)
W- W + Zo
mass(GeV) .003
.006
5.1E-4 1E-8
μνμ
charm(c) strange(s)
g gluon
1.3
0.1
0.106
2.5E-4
τντ
top(t)
bottom(b)
G graviton
174
4.3
1.78
.02
The families of fundamental particles on the left are all spin 1/2, making them fermions.
This means that, like electrons, they can only have 2 particles in the same energy level.
The force particles on the right are all integer spins; spin 2 for the graviton and spin 1 for
the rest. Such particles are called bosons and may have any number of particles in an
energy level.
Antimatter
This is our reduced periodic table. In addition, each of the particles listed has an
antiparticle with opposite charge and spin but the same mass. The neutral bosons on
the right side are their own antiparticles.
When a particle and its antiparticle meet, they annihilate one another and the energy is
given off as photons. Similarly, under conditions where momentum and energy are
conserved, gamma rays (high energy photons) can produce particle-antiparticle pairs; a
dramatic confirmation of Einstein’s E = mc2.
Forces are represented as the exchange of particles which carry momentum, energy
and other quantities. It was found that two new forces were needed to represent
nuclear interactions: the weak force responsible for beta decay and the strong nuclear
force responsible for holding nuclei together (and ultimately to hold the quarks in the
neutrons and protons themselves together).
All the forces we have discussed and all those that physicists have investigated boil
down to a total of four fundamental forces:
 gravity
 electromagnetic
 weak nuclear
 strong nuclear (Quantum Chromo Dynamics)
For each force, there is a particle which is exchanged and certain combinations of
particles which the particle can connect. These are easiest to represent in Feynman
diagrams.
Feynman diagrams display interactions of objects in a space-time plot. For example, the
collision of two billiard balls can be diagrammed this way:
Balls a and b exchange momentum during the collision.
But at the more fundamental level, what is a collision?
The force of the collision is due to the repulsion of the
electrons in the two balls. This gives the more detailed
picture below, ignoring the rest of the bulk of the balls. As
depicted, we normally omit the space and time axes.
These fundamental forces are the subject of study in High Energy Physics.
The allowed vertices or connections for the fundamental forces are summarized below.
Force
Particle
electromagnetic
photon

weak
Allowed Vertices
W+
WZo
strong (QCD)
gluon
g
gravity
graviton
G
All diagrams of interactions must satisfy conservation laws. These include the
conservation of
 electric charge
 lepton number
 baryon number
 momentum (not always obvious in diagram)
 energy (for long times)
 angular momentum (spin)
There are other, approximate, conservation laws which are broken under certain
circumstances. These instances led to new, deeper understandings of the forces
involved, but we will not have time to investigate these further.
With all this, we can begin to look at various diagrams:
Repulsion between electrons:
For clarity, arrows were added to show
the direction of travel.
Now suppose we rotate the diagram, keeping in mind that time is upward on the vertical
axis. The labeled electrons and photon are fine, but what are those other particles?
Remember, time is vertical, so in the middle of the diagram, only the photon
exists. Those other particles with the backward arrows must then be
antielectrons (positrons). This correspondence of an antiparticle to its
conjugate particle moving backward in time is also present in the
mathematics! Remember that with only a couple of particles around, there is
no entropy to define the direction of time!
This diagram shows the annihilation of an electron and positron followed by
pair creation (creating an electron and positron from a photon).
An antiparticle is equivalent to a particle moving backwards in time!
Now let’s rotate again:
This is just the repulsion of two positrons, another possible
interaction.
The next rotation would give another example of annihilation and pair creation.
A pattern emerges from such diagrams which gives another set of conservation laws.
Lepton number is conserved. Assign +1 to each e-, μ-, τ-, νe, νμ and ντ and -1 to each
of their antiparticles. For every valid interaction, the sum of these numbers is the same
before and after interaction.
Baryon number is conserved. In a similar way, assign a +1 to each 3 quark
combination (like p=uud, n=udd, etc.) and -1 to each combination of 3 antiquarks
( p  uud , etc.); the sum of these baryon numbers is the same before and after each
interaction.
This same scheme of looking at diagrams and their rotations can be applied to other
interactions. Try those below.
electron capture
muon decay