Hi. I'm self-studying particle physics.Just been looking at some questions where a reaction is listed and the questions asks to draw a Feynman diagram for the reaction and state which force is involved. I have the answers but they all seem so random and I would like to know how to decide which force is involved. If possible I would like any explanation for the following examples - 1 - an electron and positron annihilate forming a photon which then decays to a muon and an antimuon This is an EM interaction but I have read that only the Weak Interaction changes particle flavour. But hasn't the flavour changed here from electron to muon ? 2- an electron and positron move to the right (time axis) they are joined by a vertical line and 2 photons are emitted at the vertices. Is this electron-positron annihilation ? Why the vertical line ? Why do the electron and positron just not meet and annihilate to form a photon ? 3 π+ → νμ + μ+ via a W+ boson. I have read that "the weak force cannot turn quarks into leptons and vice vera". But in this reaction hasn't quarks and antiquarks turned into leptons and antileptons ? Thanks PS I have the books by Martin & Shaw , Thompson , Barr & Devenish but none of them really explain how to construct Feynman diagrams just from a given reaction.