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Transcript
How Particles Interact
Particle Physics Lesson 5
Today’s Objectives



Describe how forces are caused by particle exchange.
State that for the electromagnetic force, virtual photons
are the exchange particles
Draw Feynman Diagrams for the following
interactions:





Neutron-neutrino interaction
Proton-antineutrino interaction
β− decay
β+ decay
Electron capture
Homework



Know what exchange particles do  PPQ
Learn the four fundamental forces of nature and
their exchange particles.
Learn the Feynman diagrams for the following
interactions:
 Neutron-neutrino interaction
 Proton-antineutrino interaction
 β− decay
 β+ decay
 Electron capture
How Hard Am I Trying?
Effort Descriptor
Estimated Result Example Result
(Target Grade = B)
Complete extra questions out of the book. (about Target Grade + 1
1 hour independent study per hour in class.)
A
Complete all homework questions, seeking help
when you are stuck or don’t understand
something at workshop or on Wikispaces.
B
Target Grade
Complete all homework leaving questions blank
Target Grade – 1
when you can’t do them, asking questions in class.
C
Complete homework half the time, got stuck but
couldn’t get help. Passively listening in class.
Target Grade – 2
D
Generally don’t complete homework but try your
best to learn it all just before exams.
Target Grade – 3
E
What homework? Oh I lost the sheet!
Target Grade – 4
U
What does the Evidence show?

I have about half the homework – which I have
marked – it was done pretty well  target grade
performance.

Does this mean half the class is on track to get a
Target Grade -3 or -4?
Solve the Following
charge flowing into a junction = charge flowing out
Video

In Search of Giants (12 of 15) QED - The Jewel
of Physics.wmv
Exchange Particles
What causes forces?



Newton’s 2nd Law states that force is equal to
the rate of change of momentum.
Momentum is calculated by multiplying the mass
and the velocity of a particle.
Richard Feynman (1918 – 1988) came up with
the idea that the forces are transmitted by
exchange particles.
Feynman Diagram



Is a pictorial representation of what is going on.
First devised by American particle physicist
Richard Feynman (1918 – 1988).
The forces between electrically charged particles
are thought to be transmitted by photons,
which are emitted and absorbed by the
particles.
Weak Nuclear Force

Which force is involved when a neutron changes
to a proton or proton  neutron?

Can’t be electromagnetic because neutrons have
no charge.
Must be weaker than the strong force otherwise
it would affect stable nuclei.
Therefore it is called the weak nuclear force.


Weak Nuclear Force II

Due to the exchange of particles called W
bosons.

They are unlike photons in that they have:Non zero rest mass
A very short range (less than 0.001 fm!)
Are positively or negatively charged.



Four Fundamental Forces
Type of
Interaction
Strong
Exchange Particle/ Particles Affected
Gauge Boson
Gluon
Hadrons only
Electromagnetic
Photon
Weak
W+,W−,Z0
Charged particles
only
All types
Gravity
Graviton?
All types
Feynman diagrams




They were developed by Richard Feynman to decribe
the interactions in quantum electrodynamics (QED).
The diagrams are used to describe a variety of particle
interactions.
Virtual particles are represented by wavy or broken
lines and have no arrows.
Graphical ways to represent exchange forces, time is
upward but the lines don’t represent the paths the
particles take.
Neutron-Neutrino Interaction
Proton-Antineutrino Interaction
−
β
decay
+
β
decay
Electron Capture
Exchange Particles




Exchange particles do the following:Transfer momentum
Transfer Energy
Transfer Charge (in the case of W+ and Wbosons.)
Summary

The are four fundamental forces of nature:
gravity, electromagnetic, strong & weak.

Each force is thought to have a corresponding
exchange particle that transfer the momentum.

Feynman diagrams are a way of summarising the
interactions.