We present the first Dyson\char21{}Schwinger calculation of the three-gluon vertex in Landau-gauge QCD in which its full covariant structure is back-coupled self-consistently. We truncate a Bose-symmetrized version of the Dyson\char21{}Schwinger equation at the level of one-loop diagrams, model the four-gluon vertex, and neglect terms that contain nonprimitively divergent $n$-point functions; the ghost-gluon vertex is taken bare to good approximation. Fit functions for the ghost and gluon propagators that interpolate between scaling and decoupling are presented. In all aspects of our study, Bose symmetry is manifest, from the truncation to the basis decomposition and to the momentum invariants. We explore the uniform and soft-collinear infrared limits and obtain the expected infrared exponents. The presence of a zero crossing in the tree-level component of the vertex is confirmed for both scaling- and decoupling-type scenarios. The zero crossing appears at a scale $$\sim${}1\text{ }\text{ }\mathrm{GeV}$; however, its location might be sensitive to the four-gluon vertex and missing components in the Dyson\char21{}Schwinger equation.

Electron-positron pair production in oscillating electric fields is investigated in the nonperturbative threshold regime. Accurate numerical solutions of quantum kinetic theory for corresponding observables are presented and analyzed in terms of a proposed model for an effective mass of electrons and positrons acquired within the given strong electric field. Although this effective mass cannot provide an exact description of the collective interaction of a charged particle with the strong field, physical observables are identified which carry direct and sensitive signatures of the effective mass.

Recent studies of the dynamically assisted Schwinger effect have shown that particle production is significantly enhanced by a proper choice of the electric field. We demonstrate that optimal control theory provides a systematic means of modifying the pulse shape in order to maximize the particle yield. We employ the quantum kinetic framework and derive the relevant optimal control equations. By means of simple examples we discuss several important issues of the optimization procedure such as constraints, initial conditions or scaling. By relating our findings to established results we demonstrate that the particle yield is systematically maximized by this procedure.

Electron-positron pair production in strong electric fields, i.e., the Sauter-Schwinger effect, is studied using the real-time Dirac-Heisenberg-Wigner formalism. Hereby, the electric field is modeled to be a homogeneous, single-pulse field with subcritical peak field strength. Momentum spectra are calculated for four different polarizations—linear, elliptic, near-circular elliptic or circular—as well as a number of linear frequency chirps. With details depending on the chosen polarization, the frequency chirps lead to strong interference effects and thus quite substantial changes in the momentum spectra. The resulting produced pairs’ number densities depend nonlinearly on the parameter characterizing the polarization and are very sensitive to variations of the chirp parameter. For some of the investigated frequency chirps, this can provide an enhancement of the number density by 3 to 4 orders of magnitude.

The Dirac-Heisenberg-Wigner formalism is employed to investigate electron-positron pair production in cylindrically symmetric but otherwise spatially inhomogeneous, oscillating electric fields. The oscillation frequencies are hereby tuned to obtain multiphoton pair production in the nonperturbative threshold regime. An effective mass, as well as a trajectory-based semiclassical analysis, is introduced in order to interpret the numerical results for the distribution functions as well as for the particle yields and spectra. The results, including the asymptotic particle spectra, display clear signatures of ponderomotive forces.