Particle creation, classicality and related issues in quantum field theory: I. Formalism and toy models

Understand

The quantum theory of a harmonic oscillator with a time dependent frequency arises in several important physical problems, especially in the study of quantum field theory in an external background.

• While the mathematics of this system is straightforward, several conceptual issues arise in such a study.
• We present a general formalism to address some of the conceptual issues like the emergence of classicality, definition of particle content, back reaction etc.
• In particular, we parametrize the wave function in terms of a complex number (which we call excitation parameter) and express all physically relevant quantities in terms it.