Review of the semiclassical formalism for multiparticle production at high energies

These notes provide a comprehensive review of the semiclassical approach for calculating multiparticle production rates for initial states with few particles at very high energies. In this work we concentrate on a scalar field theory with a mass gap. Specifically, we look at a weakly-coupled theory in the high-energy limit, where the number of particles in the final state scales with energy, $n\sim E\to \infty$, and the coupling $\lambda\to 0$ with $n \lambda$ held fixed. In this regime, the semiclasical approach allows us to calculate multiparticle rates non-perturbatively.