1) Draw diagram in time-ordered quantum mechanics
   and Feynman diagram for a+b -> c+d process in t-channel
   explain differences

2) What is the simplest form for Lorentz Invariant Matrix Element.
   Could you explain it?

3) Explain the concept of chirality and helicity. When they are equal?
   Could you write down the definitions?
   What does it mean that "helicity is conserved in the vertex
   of QED interactions"?

4) In the QED, cross-section for e+e- -> mu+mu- is calculated
   averaging over initial spin states and summing over the final
   spin states.
   What is the form for the differential cross-section dsigma/dOmega
   could you draw it and explain contributions.
   What is the form for total (integrated) cross-section, how it
   behaves with increasing energy in the centre-of-mass frame of
   annihilating electrons