= Perturbative quantum field theory
Perturbative quantum field theory approximates an interacting <quantum field theory> by expanding around a solvable reference theory, usually a free field theory. Powers of interaction couplings organize the contributions to <correlation functions> and <scattering amplitudes>. <Feynman diagrams> encode the terms, their internal <propagators> and their combinatorial factors.
At leading order, tree diagrams often reveal the main process; loop diagrams give quantum corrections. Divergent intermediate expressions require a specified <regularization> and <renormalization> prescription before comparison with physical quantities. A small coupling is helpful but is not the only validity condition: large logarithms, thresholds or long evolution can invalidate a fixed-order approximation. Some phenomena, including certain bound states and tunnelling effects, require resummation or methods beyond an ordinary power series.
Back to article page
