We briefly discuss the recent discovery of reducible contributions to QED effective actions due to the presence of external electromagnetic fields at tree level and higher loop-order. We classify the physical effects of these contributions for various field configurations and discuss the strong field asymptotic limit.
The contributions of confining as well as nonconfining nonperturbative self-interactions of stochastic background fields to the shear and bulk viscosities of the gluon plasma in SU(3) Yang-Mills theory are calculated. The nonconfining self-interactio
ns change (specifically, diminish) the values of the shear and bulk viscosities by 15%, that is close to the 17% which the strength of the nonconfining self-interactions amounts of the full strength of nonperturbative self-interactions. The ratios to the entropy density of the obtained nonperturbative contributions to the shear and bulk viscosities are compared with the results of perturbation theory and the predictions of {cal N}=4 SYM.
We study the electron propagator in quantum electrodynamics in lower dimensions. In the case of free electrons, it is well known that the propagator in momentum space takes the simple form $S_F(p)=1/(gammacdot p-m)$. In the presence of external elect
romagnetic fields, electron asymptotic states are no longer plane-waves, and hence the propagator in the basis of momentum eigenstates has a more intricate form. Nevertheless, in the basis of the eigenfunctions of the operator $(gammacdot Pi)^2$, where $Pi_mu$ is the canonical momentum operator, it acquires the free form $S_F(p)=1/(gammacdot bar{p}-m)$ where $bar{p}_mu$ depends on the dynamical quantum numbers. We construct the electron propagator in the basis of the $(gammacdot Pi)^2$ eigenfunctions. In the (2+1)-dimensional case, we obtain it in an irreducible representation of the Clifford algebra incorporating to all orders the effects of a magnetic field of arbitrary spatial shape pointing perpendicularly to the plane of motion of the electrons. Such an exercise is of relevance in graphene in the massless limit. The specific examples considered include the uniform magnetic field and the exponentially damped static magnetic field. We further consider the electron propagator for the massive Schwinger model incorporating the effects of a constant electric field to all orders within this framework.
A method of constructing a canonical gauge invariant quantum formulation for a non-gauge classical theory depending on a set of parameters is advanced and then applied to the theory of closed bosonic string interacting with massive background fields.
Choosing an ordering prescription and developing a suitable regularization technique we calculate quantum guage algebra up to linear order in background fields. Requirement of closure for the algebra leads to equations of motion for massive background fields which appear to be consistent with the structure of string spectrum.
We investigate deviations from the plane wave model in the interaction of charged particles with strong electromagnetic fields. A general result is that integrability of the dynamics is lost when going from lightlike to timelike or spacelike field de
pendence. For a special scenario in the classical regime we show how the radiation spectrum in the spacelike (undulator) case becomes well-approximated by the plane wave model in the high energy limit, despite the two systems being Lorentz inequivalent. In the quantum problem, there is no analogue of the WKB-exact Volkov solution. Nevertheless, WKB and uniform-WKB approaches give good approximations in all cases considered. Other approaches that reduce the underlying differential equations from second to first order are found to miss the correct physics for situations corresponding to barrier transmission and wide-angle scattering.
A Mellin-type representation of the graviton bulk-to-bulk propagator from Ref. 1 in terms of the integral over the product of bulk-to-boundary propagators is derived.