% miko:type: atomic
% miko:title: Feynman Diagrams
% miko:summary: Graphical bookkeeping for perturbative quantum field theory amplitudes.
% miko:tags: particle
% miko:difficulty: undergrad
% miko:related: complete-particle-physics, standard-model, qcd-asymptotic-freedom
% miko:keywords: scattering, amplitude, perturbation theory

\documentclass{article}
\usepackage{amsmath}
\title{Feynman Diagrams}
\begin{document}
\maketitle

\section{Purpose}

Feynman diagrams are not literal pictures of particle trajectories. They are
bookkeeping devices for terms in a perturbative expansion of a scattering
amplitude.

\section{Basic Structure}

External lines represent incoming and outgoing particles. Internal lines
represent propagators. Vertices represent interaction terms in the Lagrangian.

\section{Amplitude}

The observable cross section is built from the squared amplitude, schematically

$$
\sigma \propto |\mathcal M|^2.
$$

\end{document}
