Higher-order Qed Effects and Nonlinear Qed 1 Higher-order Qed Eeects

After a brief review of higher-order QED eeects, a survey is made of novel aspects of QED with emphasis on recent experimental results in strong-eld QED: nonlinear Compton scattering and multiphoton pair creation by light. Quantum Electrodynamics (QED) describes the interactions between charged particles that are mediated by quanta (photons) of the electromagnetic eld. The simplest QED processes involve only a single photon, as represented by exchange and annihilation diagrams (Fig. 1). These processes involve two vertices of the form ee (where e represents an electric charge, not necessarily an electron). The process that consists only of emission of a real photon by a charged particle is forbidden by energy-momentum conservation. We may then say that higher-order QED is any process that involves more than two ee vertices. Higher-order QED processes are often divided into two groups: tree and loop. Loop processes involve one or more virtual pairs of charged particles; examples are shown in Fig. 2. Tree processes, such as that of Fig. 3, have no loops. Higher-order QED tree processes are often called radiative corrections. Any basic interaction can be modiied by the emission of one or more photons, and the probability of emission of a soft photon approaches unity. Hence, the empirical measurement of the \basic" processes is dependent on an extrapolation to the improbable case of no radiative