In quantum field theory, wave function renormalization is a rescaling (or renormalization) of quantum fields to take into account the effects of interactions. For a noninteracting or free field, the field operator creates or annihilates a single particle with probability 1. Once interactions are included, however, this probability is modified in general to Z ≠ ≠ 1. This appears when one calculates the propagator beyond leading order ; e.g. for a scalar field,