PX156 - G1 - introduction

the probability of a process to occur in particle physics is frequently difficult to calculate
feynmann diagrams were designed to make the calculation easier
each element is associated with a particular mathematical term
a feynmann diagram can be viewed on two levels:
- mathematical calculation
- a cartoon of what is going on in an interaction
a diagram represents a complex number which tells the probability of transitioning from a given initial state to a final state
this number is called the amplitude or the matrix element:

A (M) (i \to f)

P (i \to f) = | A (i \to f) |^{2}

if there are multiple routes for $i \to f$ , then all routes must be accounted for in the probability:

P (i \to f) = {| \sum_{t} A (i \to t \to f) |}^{2}

anatomy of a feynmann diagram

A \to B + C + D

the middle part will contain every way to go from $A$ to $B + C + D$ consisted with conservation laws
terminology:
- the lines representing A, B, C, D are 'external lines', and represent real particles
- lines in the middle are 'internal lines', and represent the transfer of quantum numbers and energy