Confinement of Color: Recent Progress

Recent progress done in Pisa on the subject is presented. It is shown that dual superconductivity of the vacuum or absence of it is an intrinsic property of QCD vacuum, independent of the choice of the abelian projection. The order of the deconfining phase transition in $N_f=2$ QCD is studied as a key to understand the mechanism of confinement.Comment: Talk at Confinement 2003, Tokyo, july 2003, to appear in the proceedings. 10 pages, 2 figure

Nonlinear Breit-Wheeler pair production in a tightly focused laser beam

The only available analytical framework for investigating QED processes in a strong laser field systematically relies on approximating the latter as a plane wave. However, realistic high-intensity laser beams feature much more complex space-time structures than plane waves. Here, we show the feasibility of an analytical framework for investigating strong-field QED processes in laser beams of arbitrary space-time structure by determining the energy spectrum of positrons produced via nonlinear Breit-Wheeler pair production as a function of the background field in the realistic assumption that the energy of the incoming photon is the largest dynamical energy in the problem. A numerical evaluation of the angular resolved positron spectrum shows significant quantitative differences with respect to the analogous result in a plane wave, such that the present results will be also important for the design of upcoming strong laser facilities aiming at measuring this process.Comment: 6 pages, 1 figur

First-order strong-field QED processes in a tightly focused laser beam

In [Phys. Rev. Lett. \textbf{117}, 213201 (2016)] we have determined the angular resolved and the total energy spectrum of a positron produced via nonlinear Breit-Wheeler pair production by a high-energy photon counterpropagating with respect to a tightly focused laser beam. Here, we first generalize the results in [Phys. Rev. Lett. \textbf{117}, 213201 (2016)] by including the possibility that the incoming photon is not exactly counterpropagating with respect to the laser field. As main focus of the present paper, we determine the photon angular resolved and total energy spectrum for the related process of nonlinear Compton scattering by an electron impinging into a tightly-focused laser beam. Analytical integral expressions are obtained under the realistic assumption that the energy of the incoming electron is the largest dynamical energy of the problem and that the electron is initially almost counterpropagating with respect to the laser field. The crossing symmetry relation between the two processes in a tightly focused laser beam is also elucidated.Comment: 24 pages, no figure

Quantum Limitation to the Coherent Emission of Accelerated Charges

Accelerated charges emit electromagnetic radiation. According to classical electrodynamics if the charges move along sufficiently close trajectories they emit coherently, i.e., their emitted energy scales quadratically with their number rather than linearly. By investigating the emission by a two-electron wave packet in the presence of an electromagnetic plane wave within strong-field QED, we show that quantum effects deteriorate the coherence predicted by classical electrodynamics even if the typical quantum nonlinearity parameter of the system is much smaller than unity. We explain this result by observing that coherence effects are also controlled by a new quantum parameter which relates the recoil undergone by the electron with the width of its wave packet in momentum space.Comment: 6 + 3 SM pages, 3 figure