Fundamental Interactions
PAN 2015 Workshop
Dr. Robert McTaggart
South Dakota State University
July 28, 2015
Forces and Interactions
• “Action at a distance”
• Four known forces
•
•
•
•
Gravity
Electromagnetism
Strong Force
Weak Force
• Standard Model
• How do you detect something you cannot see?
What is a force?
• Classically, a force is a push or a pull
that affects another body.
• Forces permit the transfer of energy
from one object to another, or the
• Forces allow us to solve problems via
transformation of one kind of energy
• Newton’s Laws
into another.
• Work-Energy Theorem
• Conservation Laws for energy,
momentum, and angular momentum
Action at a Distance
• Classically, each force has a “field”
that emanates from one object and
influences the other object.
M
• The force on an object is related to
the fields at that location produced
by other bodies.
• This is referred to as “action at a
distance”
E
Forces in particle physics
• We may describe interactions by an
exchange of momentum.
• This momentum exchange occurs via
the emission and absorption of
particles.
Particle exchange
• The total force is related to the rate of emitting/absorbing these
momentum bundles.
F = ∆p / ∆t
• Every force has a unique set of particles that is responsible for
exchanging these bundles of momentum.
• Gravity – graviton
• Electromagnetism – photon
Gravity vs. Electromagnetism
What is similar
What is different
• Inverse-square law
• Works over long distances
• Acts as a vector
• Both are conservative (i.e. they
have a scalar potential)
• Both obey quantum mechanics,
but it is difficult to see this in
gravity.
• There are 2 charges (+/-), and
two poles (north/south).
Gravity has one positive charge
(a.k.a. mass).
• In the nucleus, E&M is much
stronger, and gravity can be
neglected.
Why do we need a strong force?
• The nucleus is made up of protons
and neutrons.
• Protons repel other protons.
• Without another force, nuclei could
not stick together!
Strong force
• It is attractive.
• Protons attract protons.
• Neutrons attract neutrons.
• Neutrons attract protons and vice-versa.
• Adding neutrons provides more strong force to counter the repulsion of
all the protons.
• The periodic table ends: The long-distance repulsion of the protons
overcomes the short-range attraction of the strong interaction.
Chart of the Nuclides
What relationship do you see
between proton and neutron
number?
What are the neutrons doing?
Why does the periodic table
end?
http://www.nndc.bnl.gov/nudat2/
“magic numbers”
• Nuclei that are more stable tend to have one or more of the following
characteristics
• They have more neutrons.
• They have an even number of protons and/or an even number of neutrons.
• They have a “magic number” or protons and/or neutrons.
• 2, 8, 20, 28, 50, 82, 126
• Lead-208 has 82 protons and 126 neutrons, so it is a doubly-magic
isotope.
Island of Stability
• Does a larger magic number exist?
• Would produce additional
elements in the periodic table with
new properties.
Binding Energy:
How difficult is it to remove a nucleon?
Gluons
• The particles that deliver the strong force are called gluons.
• The color wheel (red, blue, green) is used to describe the nature of
the gluons.
• Because gluons carry color, they are also subject to the strong nuclear
force!!!
Why do we need a weak force?
• Not all nuclei are stable.
• Radioactive beta decay cannot be explained using the strong force or
electric repulsion.
• We need another force!
• If the resulting nucleus is still unstable, subsequent decays occur until
the final product is stable.
Characteristics of radioactive decay
• The total entropy of the universe
increases.
• One particle in small volume 
multiple particles in a larger volume
• No kinetic energy beforehand,
multiple particles moving
afterwards.
• Stored nuclear energy is being
converted into new particles with
kinetic energy.
• Momentum is conserved.
Why do nuclear reactions release so much
energy?
• We can estimate this using the uncertainty principle.
xp  
15
x  d  2r  2  10 m

p 
E
2
6.626  10
2

2
2m
2mx 
34
J * s
2
1eV
1MeV
E
*
*
2

19
6
 27
15
21.67  10 kg 2  10 m  1.602  10 J 10 eV
E  205MeV
Particle detectors
• Radiation detectors collect charge, light, or heat when particles
interact with matter.
• After the detector has been calibrated, the amount of charge, light, or
heat is proportional to the energy/momentum of the original
particles.
Example: Scintillation detector with a
photomultiplier tube (PMT)
Gas-Filled Particle Detectors –
Geiger Counters
• These detectors depend on
an amount of ionization
being produced and then
collected.
• All detectors have a unique
capacitance and resistance
(on top of what we may add
ourselves).
• The charge collected
produces a voltage pulse of
V=Q/C.
Time
Pulse
Height
Dead Time
Recovery Time
Resolving Time