We discuss pair creation in a strong laser background. Using lightfront field theory, we show that all the physics is contained in the lightfront momentum transfer from the laser, and probe, to the produced pair. The dependence of this momentum trans
fer on the geometry of the laser leads to resonance and diffraction effects in pair production spectra. The lightfront approach naturally explains the interpretation of laser-stimulated pair production as a multi-photon process creating pairs of an effective mass.
We consider stimulated pair production employing strong-field QED in a high-intensity laser background. In an infinite plane wave, we show that light-cone quasi-momentum can only be transferred to the created pair as a multiple of the laser frequency
, i.e. by a higher harmonic. This translates into discrete resonance conditions providing the support of the pair creation probability which becomes a delta-comb. These findings corroborate the usual interpretation of multi-photon production of pairs with an effective mass. In a pulse, the momentum transfer is continuous, leading to broadening of the resonances and sub-threshold behaviour. The peaks remain visible as long as the number of cycles per pulse exceeds unity. The resonance patterns in pulses are analogous to those of a diffraction process based on interference of the produced pairs.
We discuss a two-fold extension of QED assuming the presence of strong external fields provided by an ultra-intense laser and noncommutativity of spacetime. While noncommutative effects leave the electrons intensity induced mass shift unchanged, the
photons change significantly in character: they acquire a quasi-momentum that is no longer light-like. We study the consequences of this combined noncommutative strong-field effect for basic lepton-photon interactions.
We discuss various limits which transform configuration space into phase space, with emphasis on those related to lightfront field theory, and show that they are unified by spectral flow. Examples include quantising in `almost lightfront coordinates
and the appearance of lightlike noncommutativity from a strong background laser field. We compare this with the limit of a strong magnetic field, and investigate the role played by lightfront zero modes.
