1.1 Feynman Diagrams Flashcards
What is a Feynman diagram?
A graphical technique used to calculate the amplitude for transitioning from an initial state i to a final state f
What are the three regions of a Feynman diagram?
The initial state
Physics state
Final state
What is the mathematical form of a decay and a scatter?
Decay: 1 -> n
Scatter: 2 -> n
Where n >= 2
What is a vertex in a Feynman diagram, and why is it important?
Where the particle lines meet, and all conservation laws must be obeyed at every vertex
What are Feynman diagram external lines?
Lines representing real particles in the initial and final states
- Observable in detectors
What are Feynman diagram internal lines?
Virtual lines that connect verticies inside a diagram
- Represent the transfer of quantum numbers
- Not observable in detectors, do not carry a direction arrow
What do we mean by real and virtual lines in relation to FD?
Real lines have a unique rest mass defined by the energy - momentum - mass relationship
Virtual lines do not obey this relationship
What is a Feynman diagram vertex?
A point at which quantum numbers are transferred
- Each vertex is associated with a coupling factor that represents the Force strength
Why are coupling factors important?
They are used in the amplitude calculation
How are particle and antiparticle lines represented on a FD?
Particle lines travel forwards
Antiparticle lines travel “backwards”
What do we mean by preserving the “sense” of the arrows in a FD?
You should be able to follow a fermion line from one to another without being stopped at a vertex
Describe the hierarchy of the Feyman diagram types
Leading order diagram
Next-to-LO diagram (NLO)
Next-to-next-to-LO diagram
Why are the different types of Feyman diagrams improtant?
Because they all possible ways of a particle transitioning from state i to state f must be considered
What are the coupling factors for the four fundamental forces?
EM (fine structure constant) = 1/137
Weak = 1/5
Strong and gravity are complicated
For the weak and EM interaction, why can we get away with only calculating the LO diagram?
Because the series converges as the coupling factor is «_space;1
What are the main steps for drawing a FD?
- Work out what force(s) can be involved by looking at the initial and final states
- Analyse the states - what has been destroyed/created?
FD Strategy: If a photon is involved, what must the force be?
EM
FD Strategy: If neutrinos are involved, what must the force be?
Charged weak W+-
FD Strategy: If there is a flavour change, what must the force be?
Charged weak W+-
State the heirarchy for choosing the force involved in a FD
Strong -> EM -> Weak
How many vertices must be present in order to consider LO, NLO and NNLO diagrams?
2 for LO
4 for NLO
6 for NNLO
How can we calculate the amplitude of an interation?
Amplitude is proportional to the coupling factor squared
What is a FCNC and why is it important?
There is no flavour changing neutral current that exists in the SM
