For fermions and photons we have:
🗒️ Note: to simplify the diagrams we use an electron though it could be any fermion
We can also have interactions between $W$-bosons and photons
Electromagnetic force interacts with all charges particles (fermions, antifermions and $W$ bosons)
For fermions and photons we have:
🗒️ Note: to simplify the diagrams we use an electron though it could be any fermion
We can also have interactions between $W$-bosons and photons
💎 Conclusion: Electric charge and fermion flavour is conserved at each interaction vertex for EM
Lets now show how we can re-order these diagrams to represent different interactions
💼 Case: we consider our first interaction, fermion-photon where again we use an electron for clarity
We can draw first rotate the diagram to change the absorptions and emissions i.e.
We can swap the fermions for antifermions in the interaction
We can consider annihilation and production of $f\overline f$
We can consider product annihilation of $f\overline f \gamma$
🗒️ Note: here on the left we have the vacuum generating 3 particle which is allowed by the uncertainty principle though only for a very short amount of time
Again here you can repeat this for all other fermions
<Fill later>
💼 Case: consider our $f\overline f \gamma$ interaction
Every vertex of a Feynman diagram has an associated coupling strength, in the context of the EM force we simply have
$$ \text{coupling strength}\equiv q_f $$
where $q_f$ is the charge of fermion
🗒️ Note: there is no general symbol for coupling strength it is different for each force
💃 Example: for electrons the coupling strength $=$ the electric charge $e$
🗒️ Note: this is in natural units otherwise it is $e/\sqrt{\epsilon_0}$ with units $\sqrt{\text{energy}\cdot\text{length}}$
We define the fine structure constant (coupling constant of EM interactions) as
$$ \alpha_\text{em} = \frac{e^2}{4\pi\epsilon_0 \hbar c} \approx \frac{1}{137} $$
🗒️ Note: in general we have $\text{coupling constant} = \frac{(\text{coupling strength})^2}{4\pi \hbar c}$