Sommerfeld Theory Colloquium (ASC)

2015 is the International Year of Light, and of its purposes is “to raise

awareness of optical technologies”. One such technology, high-power

lasers of the petawatt class and beyond, provides the most intense light sources created by humankind so far. The intensities and field

strengths in question are in excess of 1022 W/cm2 and 1014 V/m, respectively

magnitudes that correspond to concentrating the total solar

radiation on a pinhead. This talk will present an overview of the uses

and consequences of such extreme environments within the realm of

particle physics. The relevant theory is strong-field QED, with the

laser beams providing a rather peculiar electromagnetic background

field. The magnitudes above are such that a nonperturbative treatment

of the background becomes a necessity. Using appropriate theoretical

tools, a number of phenomena will be addressed, including

radiation reaction, nonlinear Thomson/Compton scattering, laserstimulated

pair production and photon-photon scattering. A particular

incarnation of the latter, polarisation flip forward scattering,

or vacuum birefringence, will be discussed in some detail, with an

outlook on a planned experiment at the European XFEL at DESY.

With the discovery of the Higgs boson and the determination of its mass, the Standard Model is complete and its parameters are now

known and over-constrained. I will review the status and future directions in precision electroweak physics both at high and low en- ergies. There is strong evidence that the Standard Model is correct at the level of quantum corrections, and that in the absence of major conspiracies, any new physics beyond it is either significantly heavier than the electroweak scale or very weakly coupled.

Several new techniques are currently being employed to probe the strong gravitational fi�eld in the vicinity of black holes. Long baselineinterferometry at sub-millimeter wavelengths sets constraints on the

silhouette of the black holes in the Galactic center (SgrA*) and M87.

Stars which get tidally disrupted as they orbit too close to a single

black hole are being discovered at cosmological distances. Electromagnetic counterparts of black hole binaries in galaxy mergers are being identifi�ed, and can be used to calibrate the rate of gravitational wave sources. Most interestingly, the recoil induced by the

anisotropic emission of gravitational waves in the �nal plunge of binaries leaves unusual imprints on their host galaxies. Finally, the

lecture will describe new constraints on the contribution of primordial black holes to the dark matter.

Three-dimensional Einstein gravity has no local dynamical degree of

freedom. Yet, it is far from being trivial when the cosmological constant is negative. (i) It admits black hole solutions. (ii) It easily allows for consistently interacting and tractable higher-spin extensions.

(iii) It possesses remarkable asymptotic properties at infi�nity where

an infi�nite-dimensional symmetry algebra emerges. These features

make three-dimensional gravity a perfect "theoretical laboratory" in which to explore the conceptual issues related to (i), (ii) and (iii) in

a simpler context. The talk will not only discuss three-dimensional

gravity assuming no previous knowledge on the subject, but will also

provide access to recent work where new results on points (i), (ii)

and (iii) are developed.

In this seminar I will describe some of the hidden structures recently

discovered in the scattering amplitudes of elementary particles, such

as those measured at the Large Hadron Collider. These structures are

responsible for the mysterious simplicity of these quantities, which is

completely obscured by a calculation based on textbook techniques

such as Feynman diagrams. I will then move on to discuss form factors. These are slightly off-shell quantities and, similarly to amplitudes,

are also much simpler than what expected based on conventional

approaches. In particular I will focus on form factors of particular (half-BPS) operators in a special theory, known as N=4 super Yang-Mills, and briefly discuss some unexpected connections

to scattering amplitudes in phenomenologically relevant theories.

Remarkably, a strong candidate for Dark Matter exists within the Standard Model. Theoretical arguments suggest that QCD forces in the flavor-singlet sector may be strong enough that the H-dibaryon is a deeply-bound, compact state which is absolutely stable with a mass < 2mp. This possibility has gotten recent support from lattice QCD studies, which – although not yet at high enough resolution and sensitivity to confront the hypothesis – show a deeper binding than other states. As I will show, reasonable assumptions about the H’s mass and wave function lead to the observed dark matter density and DM-to- (3-quark) baryon ratio. I will discuss why H-DM is not excluded by the various experimental and observational limits, and discuss what its observational signatures would be.

Tremendous progresses have been achieved in the last decade in real- ising and manipulating stable and controllable quantum systems, and these made possible to experimentally study fundamental questions posed in the early days of quantum mechanics. We shall theoretical discuss recent cavity QED experiments on non-demolition quantum measurements. While they nicely illustrate postulates of quantum mechanics and the possibility to implement efficient quantum state manipulations, these experiments pose a few questions such as: What does it mean to observe a progressive wave function collapse in real time? How to describe it? What do we learn from them? Their analysis will allow us one hand to link these experiments to basics notions of probability or information theory, and on the other hand to touch upon notions of quantum noise. As an illustration, we shall look at quantum systems in contact with a heat bath subject to quan- tum transitions between energy levels upon absorption or emission of energy quanta. Isolating the two indispensable mechanisms in com- petition, we shall describe the main physical features of thermally activated quantum jumps.

The restricted three body problem has an intriguing dynamics. Glo- bal surfaces of section are a tool to reduce the study of the dynamics on a three dimensional energy hypersurface to the study of an area preserving map of a two dimensional surface. The existence of such a global surface of section is far from obvious. So far mostly per- turbative methods have been used to prove its existence. However, recently developed global tools originating from Gromov-Witten the- ory combined with the fact that energy hypersurfaces are of contact type can be applied to produce new global surfaces of section. In this talk I explain the restricted three body problem, the notion of a global surface of section and the new results we can obtain by bringing holomorphic curves and the contact form into play.

Ultra-fast optical spectroscopy is a powerful tool for the observation of dynamical processes in several kind of materials. The basic time- resolved optical experiment is the so-called “pump-probe”: a first light pulse, the “pump”, resonantly triggers a photo-induced process. The subsequent system evolution can be monitored, for example, by the time–dependent transmission changes of a delayed “probe” pulse. The pump pulse photon energy, spectral width and peak intensity cre- ates a certain density of electron-hole pairs in a more or less localized region of space. After the creation of the initial carrier density the time evolution of the single-particle and many-particle excitations is now governed by a non-trivial interplay between electron-electron and electron-phonon scatterings. In this talk I will present a novel approach based on the merging of Non-Equilibrium Green’s func- tion theory and Density Functional Theory to investigate the carrier dynamics following a pump excitation. The case of bulk Silicon, a paradigmatic indirect gap semiconductor, is studied by using the Baym-Kadanoff equations. Both the electron-electron (e-e) and elec- tronphonon (e-p) self-energies are calculated fully Ab-Initio by using a semi-static GW approximation in the e-e case and a Fan self-energy in the e-p case. By using the generalized Baym-Kadanoff ansatz the two-time evolution is replaced by the only dynamics on the macro- scopic time axis. The enormous numerical difficulties connected with a real-time simulation of realistic systems is overcomed by using a completed collision approximation that further simplifies the mem- ory effects connected to the time evolution. The carrier dynamics is shown to reduce in such a way to have stringent connections to the well-known equilibrium electron-electron and electron-phonon self- energies. This link allows to use general arguments to motivate the relative balance between the e-e and e-p scattering channels on the basis of the carrier energies.

In Regensburg there exists a large Lattice-QCD group (SFB/TR-55) working in many fields, ranging from the development of energy efficient super-computers to specialized Lattice studies of SU(N) gauge theories with N>3 for matching to AdS/CFT predictions. The bulk of the work is focused on hadron phenomenology [generalized parton distributions (GPDs), distribution amplitudes (DAs), transverse momentum dependent parton distributions (TMDs)] and QCD thermodynamics, especially in a magnetic background field. (Like a chemical potential a B-field affects quark and gluon degrees of freedom differently thus allowing to investigate their connection. In contrast to the chemical potential, however, it is easy to implement on the lattice. The aim of the talk is to give an overview and to possibly identify topics of mutual interest.