Sommerfeld Theory Colloquium (ASC)

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The Search for New Interactions at the LHC: The top quark Window Feb 24, 2016

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New Physics searches at the LHC are mostly being performed with the aim of detecting new states. A complementary strategy is to look for new interactions, something that typically involves precise measurements. In this talk I argue how many of the current SM (and BSM) measurements in the top sector could be used to efficiently and consistently determine of the couplings of an effective field theory and in particular that of the SM at dimension 6.

Ergodicity, Entanglement and Many-Body Quantum Dynamics in Localization Dec 02, 2015

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Do quantum many-body systems necessarily come to thermal equilibrium after a long enough time evolution? The conventional wisdom has long been that they do and that, in the process, any quantum information encoded in the initial state is lost irretrievably. Thus the dynamics of many-interacting particles becomes effectively classical. But these ingrained notions of thermalization and ergodicity have recently been called into question. In this talk I will discuss how ergodicity can break down in disordered quantum systems through the phenomenon of many-body localization. In contrast to thermalizing fluids, quantum correlations can persist through time evolution of the localized state even at high energy densities. Thus, investigating the many-body localization transition offers a concrete route to address fundamental unsolved questions concerning the boundary between classical and quantum physics in the macroscopic world. I will emphasize the important role that quantum entanglement plays in current attempts to understand this fascinating dynamical phase transition. Finally I will present recent progress in confronting the emerging theoretical understanding of many-body localization with experimental tests using systems of ultra-cold atoms.

Quantum Mechanics and Geometry of Spacetime Nov 19, 2015

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Quantum mechanics is important for determining the geometry of spacetime. We will review the role of quantum fluctuations that determine the large scale structure of the universe. In some model universes we can give an alternative description of the physics in terms of a theory of particles that lives on its boundary. This implies that the geometry is an emergent property. Furthermore, entanglement plays a crucial role in the emergence of geometry. Large amounts of entanglement are conjectured to give rise to geometric connections, or wormholes, between distant and non-interacting systems.

Supermassive Black Holes: From Jets to the Event Horizon Nov 16, 2015

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Highest resolution Event Horizon Telescope (EHT) observations will probably soon tell us more about the supermassive black hole at the Galactic Centre (Sgr A*) and the cores of active galactic nuclei (AGN). It might also help to clarify the long-standing question whether the central massive objects in AGN are instead close pairs of black holes. Mergers of supermassive black hole pairs would provide the strongest gravitational wave signals. I will present examples of how we identify potential close binary black hole candidates based on the combined analysis of high resolution radio interferometric (VLBI) observations and multi- wavelength data. I will also provide an outlook on the scientific prospects with regard to future EHT-observations.

Extreme Light and Quantum Fields Jun 18, 2015

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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.

Turbulence without Linear Instability Jun 11, 2015

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All flows show a transition from a laminar phase to a turbulent one for sufficiently high flow speeds. In many cases turbulence develops in a succession of instabilities that create flows of increasing temporal and spatial complexity (Lord Rayleigh, Sommerfeld, Heisenberg, Taylor, Landau etc), For the classroom example of pipe flow and several other flows, the linear stability analysis of the laminar profile does not reveal any instability, so that the very first point in that cascade of instabilities is absent. Over the last decade much of the mystery of the transition in pipe flow has been resolved, primarily thanks due to suitable adaptations and extensions of ideas from nonlinear dynamics. Numerical and experimental data corroborate a scenario where the appearance of new classes of fully 3d solutions and their increasing entanglement provides the key ingredients for the transition. Further studies on the spatio-temporal dynamics in the transition region, where turbulence is not space-filling, reveal intriguing similarities to the directed percolation transition in statistical mechanics.

Two-Dimensional Melting Transition: New Algorithms, New Insights Apr 30, 2015

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The hard-disk model has exerted outstanding influence on computational physics and statistical mechanics. Decades ago, hard disks were the first system to be studied by Markov-chain Monte Carlo methods and by molecular dynamics. It was in hard disks, through numerical simulations, that a two-dimensional melting transition was first seen to occur even though such systems cannot develop long-range crystalline order. Analysis of the system was made difficult by the absence of powerful simulation methods. In recent years, we have developed powerful Monte Carlo algorithms for hard disks and related systems. I will in particular show how the event-chain Monte Carlo algorithm has allowed us to prove that hard disks melt with a first-order transition from the liquid to the hexatic and a continuous transition from the hexatic to the solid. I will finally describe how a new factorized Metropolis filter transforms the event-chain algorithm into a paradigm for general Monte Carlo calculations. First results with the generalized algorithm have allowed us to establish the phase diagram for two-dimensional soft disks and Yukawa particles.

Phenomenological and Cosmological Implications of String Theory Models Dec 04, 2014

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Cosmology with Type Ia Supernovae: where do we stand today? Nov 07, 2014

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The role of Type Ia supernovae in observational cosmology has evolved from being ”avant-garde” in the early 1990’s until today’s mature status of precision cosmology. Several large transient surveys have been detecting supernovae routinely, near and far, with the aim of probing what is causing the accelerated expansion of the Universe. With time, the focus has changed towards addressing the intricacies of astrophysical effects that could bias the fits of cosmological parameters, most notably the nature of dark energy. In this talk, aimed at high-energy physics theorists(!), I will try to convey the status of the field, high-lighting some progress and set-backs, based on studies of the closest Type Ia SN in modern time that exploded in the beginning of 2014 in the near-by galaxy M82.

Lattice gauge theory insights Oct 21, 2014

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Various aspects of lattice gauge theory will be briefly discussed including, general principles, sources of systematic errors, dynamical fermions, QCD phenomenology, the FLAG project and, if time allows, some applications of lattice theory to other non-perturbative BSM phenomena.

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