Vienna Theory Lunch Seminar

by Daniel Lechner (UV), Angelika Widl (UV), Josef Leutgeb (TU)
and Susanne Wagner (TU)

Tuesdays 12:15-13:30

held alternately at:

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

University of Vienna (UV): Boltzmanngasse 5, 5th floor, Schrödinger Lecture Hall

We thank our kind sponsors:

Dean of physics, TU

Faculty of Physics, UV

Daniel Grumiller, TU



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 Beginn des Masterstudiums dem Vortrag folgen kann. BachelorstudentInnen können besonders von dem Seminar profitieren, da es ihnen ermöglicht einen Eindruck in die Forschungsarbeit beider Universitäten zu erhalten. Die Vortragenden werden dabei auch ermutigt darüber zu sprechen, warum sie ein gewisses Forschungsgebiet gewählt haben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden und es ist nicht notwendig einen Vortrag über eine "perfekte", abgeschlossene Arbeit zu halten.

Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza) 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. Undergraduate 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).

Wie kann ich teilnehmen?

How can I join?

Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailingliste eintragen.

Just attend! To receive informations via email register for the Mailinglist.

Mar 5 2019


Wolfgang Waltenberger

Can we machine-learn the Next Standard Model?

The most recent breakthrough in particle physics, the discovery of the Higgs boson, was driven by a highly predictive model with only the mass of the Higgs as a free parameter of the theory. This time, however, we are facing a situation unlike in the past. Our simpler, most straightforward proposals for a Next Standard Model, like "natural" minimal supergravity models, are by now in contradiction with data. We are confronted with a large number of equally plausible theories and no clear experimental hints of new physics, yet we are confident that the Standard Model cannot be the end of the story. In this talk, I shall propose to employ Bayesian learning on the space of all viable Lagrangians to construct the prospective Next Standard Model in a data-driven manner. I wish to discuss the technical and theoretical developments that are missing to implement this proposal.

Mar 12 2019


No Lunch Seminar

Rektorstag (Uni Wien) - Rector's Day (Uni Vienna)

Mar 19 2019


Tanja Rindler-Daller
(Astrophysics Institute, UV)

Cosmology of ultralight scalar field dark matter

The particle nature of dark matter remains a fundamental problem in modern physics and cosmology. There is recently great interest in ultralight scalar-field dark matter (SFDM) models, in which structure formation is supposed to be similar to standard CDM on large scales, while suppressed on small scales by quantum effects. In this talk, I will summarize some of the key features of these models, contrasting them with the more familiar axion dark matter, in terms of the cosmological evolution. It turns out that SFDM can change the expansion history of the early Universe, with surprising implications for detectability of gravitational waves from inflation, as well as facilitating a first-order phase transition at the electroweak scale. Along the way, I will demonstrate how these models can be tested by using observables probed by measurements of the cosmic microwave background, Big Bang nucleosynthesis and gravitational waves observatories, such as LIGO.

Mar 26 2019


Stefan Donsa
(TU Wien)

Observing the time domain build-up and spectral phase of Fano resonances

Asymmetric absorption lines in atomic spectra were first observed by Beutler and are nowadays frequently encountered in a wide variety of research fields. In atomic and molecular systems they emerge as an interference effect between direct ionization and a delayed ionization from an auto-ionizing state as first theoretically described by Fano. They are a direct consequence of electronic correlations which lie at the heart of auto-ionization processes. Together with the asymmetric line shape the Fano profile features also a rapid phase jump by π at the minimum of the transition amplitude. I will show experimental results on how the temporal build-up has been observed and a theoretical proposal how to measure the phase-jump.

Apr 2 2019


Daniel Whalen
(Institute of Cosmology and Gravitation, Portsmouth)

The Origin and Evolution of the First Quasars in the Universe

Supermassive black holes are found at the centers of most massive galaxies today. But over 160 quasars powered by billion solar-mass black holes have now been discovered at redshifts z > 6, less than a billion years after the Big Bang. How black holes this massive formed by such early epochs poses serious challenges to current theories of cosmological structure formation. I will review current thinking on the origin and of these objects and present new radiation hydrodynamical simulations of their subsequent evolution over the first billion years of cosmic time. I will also present synthetic near infrared (NIR) observables for each stage of their evolution (supermassive primordial star, direct collapse black hole and growth to a billion solar masses) and discuss their prospects for detection by the James Webb Space Telescope (JWST), Euclid and the European Extremely Large Telescope (E-ELT) in the coming decade.

Apr 9 2019


Stefan Riemelmoser
(Uni Vienna)

Range-Separated Density Functional Theory

In solid state physics, we encounter both UV and infrared divergences in the evaluation of Feynman diagrams. These are related to the very nature of the Coulomb potential, namely its infinite range and the singularity at zero distance respectively. Even though the divergences are integrable, they drive up the cost of numerical methods significantly. Either sort of divergence can be cured by splitting the Coulomb potential in a long-range and complementary short-range part. Then, the part of diagrams that contains the divergence is approximated by density functional theory. I will present popular applications of range-separation in solid state physics and talk about technical and theoretical aspects of range-separation techniques in general.

Apr 16 -- Apr 23 2019

No Lunch Seminar

Osterferien - Easter Break

Apr 30 2019


Gaurav Shrivastav
(TU Wien)

Yielding of glass under shear: a directed percolation transition, formation of shear bands and effect of aging

Glassy materials, ranging from soft matter systems to metallic alloys, yield under external mechanical loading. Yielding transition in glasses has been an area of intense research in recent years, especially the nature of this transition is highly debated. Using extensive molecular dynamics simulations, we identify the dynamical heterogeneities (regions with contrasting mobilities) in glasses under imposed shear. At a critical strain, these highly mobile regions percolate and the material yields. We give evidence that this yielding transition belongs to the universality class of directed percolation. During yielding, these materials often respond via formation of inhomogeneous flow patterns (shear bands) which are precursors to the catastrophic failure. At low shear rates, the percolating cluster, formed at the onset of flow, evolves into a transient shear band. We also demonstrate that transient shear bands are visible only under specific combinations of age of glass sample, ambient temperature and the imposed shear, signifying the dependence of material properties of glasses on the history of preparation.

May 7 2019


Valerie Smejkal
(TU Wien)

Second harmonic generation in two-dimensional semiconductors

Starting from the first fabrication of graphene, a single layer of graphite, the research of two-dimensional crystals has become a playing field for both theoretical and experimental physics. In my talk I will focus on the class of semiconducting transition-metal dichalcogenides which exhibit interesting optical properties that can be influenced by mechanical strain. When they are illuminated by a strong laser pulse, second harmonic radiation can be generated due to their crystal symmetry. I will explain the challenges in capturing their linear and non-linear optical properties, since the reduced screening in two dimensions leads to enhanced Coulomb attraction between the excited electron and hole pair. Finally, I will present a joint theoretical and experimental study investigating the influence of strain on the second harmonic response in the visible spectrum.

May 14 2019


Christiane Maria Schultze
(Uni Vienna)

Leggett-Garg Inequalities and Neutrino Oscillation

The oscillation of neutrinos was predicted in the mid of the last century. Since then they were intensively studied both theoretically and experimentally since a couple of phenomena like e.g. CP violation (charge-conjugation-parity) are conjectured. Also, it is not known which neutrino is the heaviest, formulated as the mass hierachy problem. I will focus on how tools from foundations of quantum mechanics can give answers to these riddles in neutrino physics. In particular, a type of the Leggett-Garg inequalities, kind of time-like versions of Bell inequalities, will be investigated for neutrinos propagating through matter.

May 21 2019


Panagiotis Betzios
(University of Crete)

Emergent Fields from Hidden Sectors

We will entertain the possibility of describing gravitons, axions and other gauge fields as composites of hidden sector fields. Our general setup is motivated by Holography and the AdS/CFT correspondence, but is a more general phenomenon that does not necessarily need a holographic theory to operate. We will describe in detail our general setup and then focus on the concrete examples of emergent gravitons and axions. In these cases the hidden sector stress energy and instanton density have an effective IR description in terms of emergent gravitons and axions respectively. We will use both a linearised as well as a fully non-linear approach to study them.

May 28 2019


Joachim Bosina
(Atominstitut, TU Wien)

qBounce: Gravitational Resonance Spectroscopy with UCN - A Brief Introduction

This talk focuses on the control and understanding of a gravitationally interacting elementary quantum system using the techniques of gravitational resonance spectroscopy (GRS) and ultracold neutrons (UCN). It offers a new way of looking at gravitation at short distances based on quantum interference. In the past years, the qBounce collaboration designed and built a new Ramsey-type GRS experiment at the Institute Laue-Langevin (Grenoble). In 2018, we were able the measure gravitational state transitions with the complete assembled experiment for the first time. Furthermore we searched for a non-zero electric charge of the neutron with a modified setup. In June 2019 another 200 days of measurements will start which will increase the statistical sensitivity on various dark matter and dark energy models.

Jun 4 2019


Niccolò Cribiori
(TU Wien)

Introduction to Supersymmetry and its Breaking

I will review basic ingredients of supersymmetry in four dimensions and some consequences of its spontaneous breaking. The talk is meant to be introductory and I will mainly proceed by discussing simple examples.

Jun 11 2019

No Lunch Seminar

Pfingsten - Pentecost

Jun 18 2019


Elisa Meninno

Heavy-Flavour Studies with the ALICE Experiment at the LHC

Heavy quarks (charm and beauty) are powerful tools to study the properties of the Quark-Gluon Plasma (QGP), formed in heavy ion collisions at the LHC. Because of their large masses, heavy quarks are produced in initial hard-scattering processes, and they subsequently experience the whole system evolution, interacting with the medium constituents. The measurement of the nuclear modification factor of open heavy-flavour hadrons can provide important information about the properties of the parton in-medium energy loss. The charmed baryon-to-meson ration \Lambda^+_c -to- D^0 is sensitive to hadronisation mechanisms. In particular, it is expected to be enhanced with respect to the proton-proton baseline if charm quarks hadronise via recombination with the surrounding light quarks in the QGP. Measurements of charm-hadron production in small systems (pp and p-Pb collisions) are a fundamental reference for measurements in Pb-Pb collisions and allow to disentangle cold nuclear matter effects from those deriving from the presence of the QGP. The ALICE detector has excellent performance in terms of particle identification capabilities and of vertexing performance and it operates at low magnetic field (B = 0.5 T). All these characteristics allow one to study the heavy flavour production at very low transverse momentum, complementing results reported by other LHC experiments at higher momenta. It is therefore possible the reconstruction of charmed mesons (D^0, D^+, D^{*+}, D^+_s) and baryons (\Lambda_c and \Xi^0_c) and of electrons from heavy-flavour hadron decays at central rapidity down to very low p_T. Moreover, heavy-flavour decay muons at forward rapidity are measured via the ALICE muon spectrometer.

Jun 25 2019


Marie-Christine Roehsner
(Uni Vienna)

Quantum Advantage for Probabilistic One-Time Programs

One-time programs, computer programs which self-destruct after being run only once, are a powerful building block in cryptography and would allow for new forms of secure software distribution. However, ideal one-time programs have been proven to be unachieveable using either classical or quantum resources. We relax the definition of one-time programs to allow some probability of error in the output and show that quantum mechanics offers security advantages over purely classical resources. We introduce a scheme for encoding probabilistic one-time programs as quantum states with prescribed measurement settings, explore their security, and experimentally demonstrate various one-time programs using measurements on single-photon states. By combining quantum and classical technology, we demonstrate that quantum techniques can enhance computing capabilities even before full-scale quantum computers are available.

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