Vienna Theory Lunch Seminar

by Frederic Brünner, Sebastian Frank, Philipp Köhler, Karoline Mühlbacher and Florian Preis

Tuesdays 12:15-13:30

held alternatively at:

TU Wien ( Wiedner Hauptstr. 8-10, yellow area, 10th floor, seminar room E136 )

Uni Wien ( Boltzmanngasse 5, 5th floor, Schrödinger HS / large seminar room )


We thank our kind sponsors:





Wie auf vielen Universitäten praktiziert wollen wir ein Lunch-Seminar etablieren, das aktuelle Themen der Theoretischen Physik, die von DiplomandInnen, DoktorandInnen und PostDocs behandelt werden, aufgreift.

Das Niveau soll so gewählt werden, dass jeder Student und jede Studentin am Ende des Studiums dem Vortrag folgen kann. Die Vortragenden werden ermutigt keinen "perfekten" Vortrag zu halten, und sollen hauptsächlich Ihre Motivation, warum sie dieses Thema gewählt haben, wiedergeben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden.

Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza, Sushi) gratis zur Verfügung gestellt.

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.

The seminar is designed for graduate students but should also be comprehensible to advanced undergraduate students. Pre-diploma students are particularly encouraged to attend so that they receive an overview of research activities conducted at both universities. Speakers are also encouraged to focus on their motivation for choosing their particular topic and to present open questions.

In order to avoid any "loss of time" we provide a free lunch (pizza, sushi).

Wie kann ich teilnehmen?

How can I join?

Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailinglist eintragen oder Mail an fpreis at oder an philipp.martin.koehler at

Just attend! To receive informations via email go to the Mailinglist or drop an email to fpreis at or philipp.martin.koehler at


6 Mar 2012

Uni Wien

Denis Parganlija
(TU Wien)

Scalar and other mesons in vacuum

Abstract: Scalar mesons have been one of the most hotly debated issues of low-energy QCD for decades. Experimental data show the existence of six scalar isosinglet states -- next to the famous sigma meson, there are five states with the same quantum numbers but higher energies than the energy of the sigma. If we consider the u and d quarks as degenerate and work in a theoretical framework that inlcudes strange states as well, then we can construct two scalar \bar q - q states. Thus constructed scalars can, of course, describe at most two out of the mentioned six experimentally known states - but the question is: Which two? I will present a model with (pseudo)scalar and (axial-)vector mesons that allows us to pursue an answer to this question. Scalar (and other) mesons are important not only in vacuum but also at finite temperatures and densities as they build order parameters for the chiral transition, that will also be briefly discussed.

13 Mar 2012

TU Wien

Andreas Braun
(TU Wien)

String dualities, singularities and gauge symmetries

Abstract: After a brief introduction to string theory and string dualities I will present two classic examples in which non-abelian gauge symmetries correspond to a singular compactification manifold on one side of the duality. This leads to an interesting connection between representation theory and topology. Physically, the non-abelian gauge bosons come from branes wrapped on the vanishing cycles. This talk intends to motivate and explain these results in a pedagogic fashion.

20 Mar 2012

Uni Wien

Mario Bertin
(Universidade Federal do ABC)

Hamilton-Jacobi formalism for singular systems

Abstract: In this talk it will be shown how the construction of Carathéodory's complete figure of variational calculus, which culminates in the Hamilton-Jacobi theory, provides an adequate theoretical background to analyze systems with constraints. Initial focus will be the analysis of integrability, which can be seen as the constraint analysis itself. Other special features, such as the construction of symplectic structures, infinitesimal canonical transformations, and gauge transformations will be addressed as well.

27 Mar 2012

TU Wien

Martin Schwarz
(Uni Wien)

Preparing projected entangled pair states on a quantum computer

Abstract: We present a quantum algorithm to prepare injective PEPS on a quantum computer. To be efficient, our algorithm requires well-conditioned PEPS projectors and, essentially, an inverse-polynomial spectral gap of the PEPS' parent Hamiltonian. Injective PEPS are the unique groundstates of their parent Hamiltonians and capture groundstates of many physically relevant many-body Hamiltonians, such as e.g. the 2D AKLT state. Even more general is the class of G-injective PEPS where symmetry group G acting on virtual tensor indices leaves the PEPS invariant. G-injective PEPS are powerful enough to represent topologically ordered quantum states. As our second result we show how to prepare G-injective PEPS under similar assumptions as well.

17 Apr 2012

Uni Wien

Alejandra Castro
(McGill University Montreal)

Precision holography with higher spin theories

Abstract: In this talk I will give a short survey of higher spin theories in AdS spacetime. Vasiliev's higher spin theories provide a new venue to examine our expectations about quantum gravity. In relation to the holographic principle, this setup has allowed us to investigate in more depth the dictionary and consequences of this correspondence. The dual conformal theories are in principle solvable, and hence it opens the possibility of tracking the emergence of space-time from the boundary theory, among other effects. Our focus here will be in the three-dimensional version of AdS higher spin gravity, and hence its two-dimensional dual CFT. This is arguably the simplest setup of the correspondence from the bulk point of view, which has allowed a better understanding of physical phenomena in Vasiliev theory. The main topic to be discussed are the spectrum of both the bulk and boundary theory, and perhaps resolve the more puzzling question: what is a black hole in Vasiliev theory?!

24 Apr 2012

TU Wien

Arash Nikoubashman
(TU Wien)

Non-Equilibrium Computer Experiments of Soft Matter Systems

Abstract: In this talk, I will present the non-equilibrium characteristics of selected soft matter systems by the means of extensive computer simulations. During the last two decades, the field of soft matter physics developed into a highly active research field, attracting the interest of a vast number of different disciplines, such as chemistry, material science, biology and medicine. Soft matter systems are ubiquitous in our daily lives, ranging from biological matter as DNA, proteins, and blood to man-made substances like liquid crystals, soap and lubricants. In striking contrast to hard matter systems, whose properties are essentially fixed by the electronic structure of the atoms, the situation in soft matter systems is distinctly more complex, since a variety of different length scales are involved and relevant.

8 May 2012

Uni Wien

Rodrigo Picanco Negreiros
(Johann Wolfgang Goethe University, Frankfurt)

Impact of rotation-driven particle repopulation on the thermal evolution of pulsars

Abstract: Driven by the loss of energy, isolated rotating neutron stars (pulsars) are gradually slowing down to lower frequencies, which monotonically increases the tremendous compression of the matter inside of them. This increase in compression changes both the global properties of rotating neutron stars as well as their hadronic core compositions. Both effects may register themselves observationally in the thermal evolution of such stars, as demonstrated in this work. The rotation-driven particle process which we consider is the direct Urca (DU) process, which is known to become operative in neutron stars if the number of protons in the stellar core exceeds a critical limit of around 11% to 15%. We find that neutron stars spinning down from moderately high rotation rates of a few hundred Hertz may be creating just the right conditions where the DU process becomes operative, leading to an observable effect (enhanced cooling) in the temperature evolution of such neutron stars. We also find that the rotation-driven DU process can comfortably explain the temperatures observed for the neutron star in Cas A provided the mass of this neutron star is assumed to be around 1.5 -- 1.9 \msun. Finally, a preliminary estimate of the temperature of the recently discovered massive neutron star PSR J1614-2230, which rotates at 317 Hz, indicates that the rotation-driven DU process may have cooled this neutron star to remarkably low temperatures of ~ 10^6 K.

15 May 2012

TU Wien

Joyce Myers
(University of Groningen)

Chiral and deconfinement transitions in QCD-based theories with chemical potential in a small volume

Abstract: The phase diagram of QCD at non-zero chemical potential is difficult to calculate because the coupling strength is large, preventing ordinary perturbation theory, and the action is complex, leading to the "sign problem" and preventing conventional lattice simulations. To understand better how to deal with complex actions and to obtain a qualitative picture of the phase diagram of QCD it can be useful to study the theory in a very small spatial volume, which allows for perturbation theory to be employed at all temperatures. We consider QCD and QCD-based theories on S^1 x S^3 for small S^3 from one-loop perturbation theory. The quark number, free energy, and Polyakov lines as a function of the temperature and chemical potential provide a sketch of the phase diagram for QCD and supersymmetric QCD in the limit of large Nc and Nf. Finally we consider the effect of additional interactions which lead to spontaneous chiral symmetry breaking for sufficiently small chemical potentials.

22 May 2012

Uni Wien

Michael Vanner
(Uni Wien)

Quantum State Orthogonalization and a Toolset for Quantum Optomechanical Phonon Control

Abstract: In this talk a scheme to perform a controllably weighted superposition of phonon subtraction, addition and identity to the vibrational mode of a mechanical resonator will be presented. Using this quanta level manipulation tool we propose a method for (i) quantum state orthogonalisation, i.e. for an arbitrary pure state (with partial a priori knowledge of the state's mean) a single application of our tool produces an orthogonal state, and (ii) arbitrary quantum state transformation, i.e. for an arbitrary pure input state any target state can be realized by using repeated applications of our tool. While this presentation will concentrate on optomechanics these concepts can be readily applied to other areas of quantum optics. [arXiv:1203.4525]

5 Jun 2012

TU Wien


Hannes Grimm-Strele
(Uni Wien)

Challenges in Numerical Simulations of Astrophysical Flow


Abstract: Recent progress in multi-dimensional modelling of stars shows that the traditional one-dimensional approach is not sufficient to properly describe the structure of the star. But due to the wide range of length and time scales involved and the mixed hyperbolic-parabolic-elliptic character of the governing equations, these simulations are very difficult and expensive to carry out. Insufficient resolution gives qualitatively wrong results, whereas numerical schemes of low order are not efficient. Boundary conditions have a large influence on the solution and must be designed suitably. Finally, the numerical grid must stay simple (i.e., structured) such that high-order methods are applicable, but must also offer some flexibility to cover more complicated domains. The simulation code ANTARES which is developed at the faculty of Mathematics of the University of Vienna is designed to face these problems, and will be presented in this talk together with some results and applications.

12 Jun 2012

Uni Wien

Denise Neudecker
(TU Wien)

Towards Ascertaining Uncertainties - Open Questions in Nuclear Data Evaluation

Abstract: Nuclear Data Evaluations merge model information and experimental knowledge to provide evaluated quantities of best knowledge - e.g. cross sections - and associated uncertainties for dosimetry, nuclear waste incineration, medical applications, etc. The development of new nuclear technologies (e.g. Fusion or GenerationIV reactor, Accelerator Driven Systems) resulted in demands for an extension of the energy range of the evaluations from 20 MeV up to 200 MeV and for reliable evaluated uncertainties in the form of covariance matrices. The latter are essential to estimate safety and economic margins. The extension of the energy range is not straightforward as experimental data are often scarce above 20 MeV leading to a strong model dependence of the evaluated data in this energy regime. Hence, uncertainties of the model have to be estimated in a reasonable manner. While uncertainties in the model parameters are fairly well-understood and estimated in most evaluation methodologies, uncertainties due to model deficiencies are an open issue. Another important issue is the estimation of experimental covariance matrices as correlated systematic uncertainties are often underestimated and lead to unreasonably small evaluated uncertainties and can also result in pathologically small evaluated mean values - termed Peelle's Pertinent Puzzle. In this seminar talk, recent research and developments undertaken at the Vienna University of Technology concerning these issues will be presented.

19 Jun 2012

TU Wien

Leonard Fister
(Uni Heidelberg)

Yang-Mills Theory at Non-Vanishing Temperature

Abstract: We study the temperature dependence of correlators in Yang-Mills theory. For this purpose we utilise a purely thermal renormalisation group flow equation, and obtain the full thermal propagators. Interestingly, the electric screening mass is sensitive to the confinement-deconfinement phase transition. We also compute thermodynamic quantities such as the pressure.

Program Winter Semester 2011/2012

Program Summer Semester 2011

Program Winter Semester 2010/2011