  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.
