Vienna Theory Lunch Seminar by Christopher Lepenik (UV), Maximillian Löschner (UV), Alexander Soloviev (VUT) Tuesdays 12:1513:30
held alternately at:
We thank our kind sponsors: 
Idee: 
Wie auf vielen Universitäten praktiziert wollen wir ein LunchSeminar etablieren, das aktuelle Themen der Theoretischen Physik, die von DiplomandInnen, DoktorandInnen und PostDocs behandelt werden, aufgreift.

We want to establish a lunch seminar as practiced at other universities. The focus is on recent theoretical research done by Master students, PhDs and PostDocs.

Wie kann ich teilnehmen? 
Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailingliste eintragen. 
Just attend! To receive informations via email register for the Mailinglist. 
Oct 11 2016 
Arjun Bagchi 
Tensionless strings and related fun things I will talk about the formulation of the tensionless limit of closed bosonic and superstring theory from the point of view of worldsheet symmetries. The talk would be based principally on the papers 1507.04361 and 1606.09628. 
Oct 18 2016 
Prof. Dr. Jörg Schmiedmayer 
Scrambling, Relaxation and the emergence of thermalization in an isolated many body quantum system The evolution of an isolated quantum system is unitary. This is simple to probe for small systems consisting of few noninteracting particles. But what happens if the system becomes large and its constituents interact? In general, one will not be able to follow the evolution of the complex many body eigenstates. Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are onedimensional Bosegases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their nonequilibrium evolution. In our experiments we study how the coherence created between the two isolated onedimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available. We find that a onedimensional quantum system relaxes to a prethermalisatized quasi steady state which emerges through a light cone like spreading of ’decoherence’. The prethermalized state is described by a generalized Gibbs ensemble. Finally, we investigate the further evolution away from the prethermalized state. On one hand we show that by engineering the Quasiparticles we can create many body quantum revivals. On the other hand, we point to two distinct ways for further relaxation towards a final state that appears indistinguishable from a thermally relaxed state. The system looks like two classically separated objects. This illustrates how classical physics can emerge from unitary evolution of a complex enough quantum system. We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by scrambling of the eigenmodes of long lived excitations (quasi particles). 
Oct 25 2016 
Benjamin Ramberger 
RPA forces for solids In electronic structure simulations, one is usually interested in the groudstate energy of a many electron system which can often only be calculated approximately. A particular useful method to approximate the energy of a quantum many body system is the random phase approximation (RPA). For many systems it is superior to the widely used density functional theory (DFT), especially when vanderWaals interactions are important. However, the computational demand for RPA simulations of solids was far out of reach for many years. Fortunately there has been a lot of progress in the development of computer simulations employing the RPA recently, so that RPAcorrelation energies became accessible for large systems. For solids however, there is not yet an efficient way to calculate forces in the RPA. Since the calculation of forces is crucial for the simulation of elastic and vibrational properties as well as for structure relaxations, this is a promising area for further investigation. The problem of force calculations within the RPA is adressed by a subproject of the Special Research Program (SFB) Vienna Computational Materials Laboratory (ViCoM) supported by the Austrian Science Fund (FWF). After a general introduction to the topic, the challanges of RPA forces calculations will be discussed and recent results of the project will be presented. 
Nov 1 2016 
No Lunch Seminar 
Allerheiligen  All Saints' Day 
Nov 8 2016 
Esteban Castro Ruiz 
Entanglement of quantum clocks through gravity In general relativity, the picture of spacetime for a given observer assigns an ideal clock to each spacetime point. Being ideal, gravitational effects due to these clocks are ignored and the flow of time according to one clock is not affected by the presence of surrounding clocks. However, if time is defined operationally, as a pointer position of a physical clock that obeys the laws of quantum mechanics and general relativity, such a picture is at most a convenient fiction. We show that the massenergy equivalence implies gravitational interaction between the clocks, while the superposition of energy eigenstates leads to a nonfixed metric background. Based only on the assumption that both quantum mechanics and general relativity are valid in this situation, we show that the clocks necessarily get entangled through time dilation effect, which eventually leads to a loss of coherence of a single clock. Hence, the time as measured by a single clock is not welldefined. However, the general relativistic notion of time is recovered in the classical limit of clocks. 
Nov 15 2016 
No Lunch Seminar 
Tag des Landespatrons  Day of the Patron Saint of Vienna 
Nov 22 2016 
Peter Eigenschink 
Global existence for the EinsteinVlasov System with massive and massless particles The Cauchy problem in general relativity is of fundamental interest. What is known depends crucially on the choice of a reasonable matter model coupled to the Einstein equation. One of the simplest matter models one can choose is Vlasov matter, which represents collisionless gas and is governed by a continuity equation, the Vlasov equation. For a spherically symmetric spacetime and appropriate initial data it is known that for either massive or massless Vlasov matter the EinsteinVlasov system has global solutions without singularities. But the way those results are obtained are inherently different. For that reason it is of interest if global solutions without singularities also exist in the case of a combination of massive and massless Vlasov matter. We show that for certain initial data, global stability of solutions of the spherically symmetric EinsteinVlasov system with a combination of both, massive and massless matter, still holds. As an intermediate result, which is crucial for the final conclusion, we also show that for such initial data, massive and massless particles separate after finite time in an appropriate way. 
Nov 29 2016 
Philipp Kleinert 
Thermalisation of Wightman TwoPoint Functions in AdS/CFT In this talk, we will discuss applications of the AdS/CFT correspondence to farfromequilibrium physics. In the first part, we address the issue of generalizing the holographic dictionary to outofequilibrium situations. In the second part, we consider a concrete example of a holographic model of nonequilibrium physics: thermalization following a global quench in 2d CFTs dual to AdSVaidya spacetime. At first sight, nonlocal and local probes in these holographic models of thermalization reach their equilibrium values on very different time scales. As an example of nonlocal probes, we study the evolution of Wightman twopoint functions and show that these twopoint functions, after being Fourier transformed to momentum space, decay towards their thermal values with a rate set by the quasinormal modes of the final state black hole spacetime. The quasinormal modes also set the thermalisation time scale of local probes which suggests a unified picture of thermalisation times of one and twopoint functions. Based on: 1412.2806 and 1511.08187 
Dec 6 2016 
Carla Schuler & Gabriel Sommer 
Electroweak models with classical scaleinvariance We study versions and extensions of the standard model (SM) with classical scaleinvariance, which are of great interest nowadays. In these models the Lagrange density does not contain any mass terms, which are responsible for the breakdown of scaleinvariance. The generation of the masses of all particles is then described by the ColemanWeinberg mechanism, where spontaneous symmetry breaking (SSB) occurs as the consequence of quantum corrections rather than the negative Higgs mass term in the Lagrangian. With the particle content of the SM, it is not possible to formulate the SM as a classically scaleinvariant theory with the experimentally measured mass values of the Higgs boson and the tquark. An enlarged SM HiggsSector with additional scalar fields and an extension of the SM gaugegroup by (non)Abelian factors provide possible solutions. 
Dec 13 2017 

Hilbert Series and SUSY gauge theories The Hilbert series counts gauge invariant chiral operators and encodes information on the moduli space of supersymmetric vacua of a theory. In this talk I will introduce the use of Hilbert series in example of 4d N=1 theories. Thereafter, I proceed to 3d N=4 theories and focus on the Coulomb branch Hilbert series. 
Jan 10 2017 
Bernadette Kolbinger 
Measuring the Ground State Hyperfine Splitting of Antihydrogen
The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration at the Antiproton Decelerator at CERN aims to test the CPT symmetry by measuring the ground state hyperfine structure of antihydrogen which, according to the theorem of CPT, is predicted to have the same electromagnetic spectrum as hydrogen. 
Jan 17 2017 
Friedrich Kupka 
Numerical time integration and studies of stellar convection Convection is one of the main physical mechanisms operating in stars to transport heat and angular momentum, to mix the plasma they consist of, to generate magnetic fields, and to drive and damp global oscillations that are used to study their interior structure. A deeper understanding of the physics of these processes requires numerical simulations and their main challenge is the huge spread of scales in space and time they operate on. After an overview on this basic challenge we report on new numerical methods for efficient and reliable time integration, originally developed for the ANTARES general purpose hydrodynamical simulation code, which are of general use for similar mathematical classes of equations. We demonstrate the versatility of ANTARES with examples from general hydrodynamics, solar physics, and the physics of white dwarf stars. Specifically, these are a demonstration of layer formation in double diffusive flows, how to catch driving and damping of stellar osciallations, and how fluid is mixed by convection into stably stratified regions of a star. Each of these examples is also motivated by explaining the general physical and specific astrophysical interests in underlying questions driving that kind of research. 
Jan 24 2017 
Prof. Stefan Fredenhagen 
Aspects of higher spins Interacting gauge theories of fields of higher spins are difficult to construct, and the only known examples are the Vasiliev theories. I will try to explain the approach taken in these constructions, in particular the socalled unfolded formulation of the equations of motion. Although the Vasiliev theories in principle provide consistent nonlinear systems of higherspin gauge fields, attempts to extract concrete equations of motion have run into difficulties, and I will sketch some of the resulting conceptual challenges. 
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