QFT treats particles as excited states (also called quantum levels) of their underlying quantum fields, which are more fundamental than the particles. The equation of motion of the particle is determined by minimization of the action computed for the Lagrangian, a function of fields associated with the particle. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding quantum fields. Each interaction can be visually represented by a Feynman diagram according to perturbation theory in quantum mechanics.