General Physics Colloquium Tuesday 4th November 14:15-15 Fys. Aud. 'Quantum electrodynamical effects at critical background electromagnetic fields' Antonio Di Piazza Max-Planck-Institut für Kernphysik, Heidelberg, Germany Quantum electrodynamics (QED) is a well-established theory and its predictions have been confirmed experimentally in various regimes. However, there are still areas of QED that deserve theoretical and experimental investigation, especially when processes occur in the presence of background electromagnetic fields of the order of the so-called critical fields of QED [1]. In view of the increasingly stronger available laser fields it is becoming feasible to employ them to test QED under the extreme conditions supplied by intense fields [1]. A fundamental problem in electrodynamics is the so-called "radiation reaction" problem: classically, when a charged particle (an electron, for definiteness) is accelerated by an external field, it emits radiation and this emission alters the motion of the electron. What is the quantum analog of radiation reaction? We have answered this question in the realm of strong-field QED [2], by connecting radiation reaction to the emission by an electron of multiple photons (see also [3]), and we have shown the existence of the so-called quantum radiation dominated regime. In this regime, the laser field amplitude is effectively of the order of the critical field of QED, and quantum recoil and radiation-reaction effects both dominate the dynamics of the electron. The role of the stochasticity of the multiple photon emission in quantum radiation reaction has been also elucidated [4] together with its consequences for observing quantum radiation reaction itself. Finally, since in the presence of incoming particles with energies much larger than the electron rest energy, the laser field amplitude can be effectively boosted to the critical value, the interplay between the strong field provided by a highly-charged ion and by an ultraintense laser beam has been investigated in the process of electron-positron photo-production (Bethe-Heitler process) [5]. It has been shown that, unexpectedly, the presence of the laser field can strongly suppress the Bethe-Heitler cross section, an effect analogous to the wellknown Landau-Pomeranchuk-Migdal effect. [1] A. Di Piazza, C. Müller, K. Z. Hatsagortsyan, and C. H. Keitel, Rev. Mod. Phys. 84, 1177 (2012).[2] A. Di Piazza, K. Z. Hatsagortsyan, and C. H. Keitel, Phys. Rev. Lett. 105, 220403 (2010). [3] F. Mackenroth and A. Di Piazza, Phys. Rev. Lett 110, 070402 (2013). [4] N. Neitz and A. Di Piazza, Phys. Rev. Lett. 111, 054802 (2013); N. Neitz and A. Di Piazza, Phys. Rev. A 90, 022102 (2014). [5] A. Di Piazza and A. I. Milstein, Phys. Lett. B 717, 224 (2012). Wine and cheese will be served at 3 pm