Vienna Theory Lunch Seminar

by Christopher Lepenik (UV), Olaf Krüger (UV), Alexander Soloviev (TU)
and David Toneian (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.

Oct 10 2017


Aditya Pathak
(University of Vienna)

Precision Top Mass Measurement at the LHC

Abstract: The top quark mass is one of the most important parameters of the Standard Model and has been measured at the Large Hadron Collider (LHC) to a sub percent precision. While the experimental precision has been steadily improving over the years we are faced now with addressing an important contribution to the uncertainty in the top mass: the lack of precise knowledge of the mass renormalization scheme the current measurements are expressed in. This theory uncertainty is of the same order as the experimental uncertainty and has been difficult to address due to use of Monte Carlo based methods in the top mass measurements. Direct comparison of theory calculations with data has been lacking so far given the complex environment of the LHC in which the top quarks are produced in. In this talk I will describe our work on attacking the problem of the top mass measurement at the LHC by carrying out analytical calculations using observables with desired sophistication that have been made possible only recently thanks to the powerful Effective Field Theory (EFT) methods and new developments in the jet-substructure studies.

Oct 17 2017


Marcus Huber
(IQOQI, Austrian Academy of Sciences)

Autonomous Quantum Clocks

Abstract: In this talk I will discuss fundamental limitations of measuring time within the framework of quantum mechanics. These limitations naturally connect to the theory of quantum thermodynamics and give a quantitative insight into the connection between the arrow of time and the second law of thermodynamics.

Oct 24 2017


Kurt Hingerl
(University Linz)

A classical model for depolarization by temporal and spatial decoherence

Abstract: A finite spectral resolution and/or an imperfectly collimated beam /and or an (areal) extended light source / and or an (areal) extended detector and/ or a sample with a varying thickness can produce depolarization in any ellipsometric or polarization measurement. Despite these experimental findings, there are to our knowledge no physical models published which trace the origin of depolarization back to the atomic properties. Therefore, in the talk I will explain crosspolarization - and subsequently depolarization by considering the common- not separable- effect between the light beam and the sample, described by coherence length and coherence area. For inhomogeneous samples with dimensions smaller than the coherence area, the fields have to be added coherently. However, inner and non-planar boundaries give rise to evanescent fields in the vicinity of these boundaries. Parallel and perpendicular field components oscillate and decay differently in the vicinity of the boundaries, therefore cross- polarization (incident s- polarized light excites reflected p-polarized light and vice versa) occurs. In inhomogeneous samples the Fresnel reflectances are not correct any more, these strictly rely on homogeneity (i.e. arbitrary shifts of the sample along any surface direction change the measurement).

However, in optics we never measure the electric fields, because our available detectors are much too slow, but we measure their statistical second moments. In homogeneous samples with thick transparent overlayers it turns out that depolarization arises through the temporal decoherence of photons, and the measured Müller matrix (MM) elements are given by a convolution of the spectral width of the light source and a sample property: the thickness of the transparent overlayer.

For inhomogeneous samples, when the sample sizes or structures are larger than the coherence area, then the different local polarization states in reflection coming from different materials have to be added incoherently- partially depolarized light results. Also here the measured Müller matrix is given by a convolution of a light source property (i.e. the coherence area) and a sample property: the structure size). For both depolarization mechanisms mathematical models will be presented, allowing to predict the polarization response, i.e. all MM elements for periodic structures, metamaterials and thick films.

Oct 31 2017


Borivoje Dakic
(Institute for Quantum Optics and Quantum Information (IQOQI), Vienna)

CANCELLED, sorry for the inconvenience!

Single-Copy Entanglement Detection

Abstract: A main focus of current practical quantum information research is on the generation of large-scale quantum entanglement involving many particles with the goal of achieving real applications of quantum technologies. An important instance of this challenge is the verification problem: how to reliably certify the presence of quantum resources, in particular quantum entanglement. The plausibility of standard verification schemes (for example, based on entanglement witnesses) is questionable, since they require repeated measurement on large ensemble of identically prepared copies, which is highly demanding to achieve in practice when dealing with large-scale entangled quantum systems. In this talk, I will present our recent work [1] where we develop a novel method by formulating verification as a decision procedure, i.e. entanglement is seen as the ability of quantum system to answer certain “yes-no questions”. We show that for a variety of large quantum states even a single copy suffices to detect entanglement with a high probability by using local measurements. For example, a single copy of a 24-qubit linear cluster state suffices to verify entanglement with more than 95% confidence. Our method is applicable to many important classes of states, such as cluster states or ground states of local Hamiltonians in general.
References: A. Dimic, and B. Dakic, “Single-copy entanglement detection”, arXiv:1705.06719 (2017).

Nov 7 2017


Elke Aeikens
(University of Vienna)

Charged-Lepton Decays from Soft Flavour Violation in a Two-Higgs Doublet Seesaw Model

Abstract: Extensions of the Standard Model with right-handed neutrinos in the framework of the seesaw mechanism are popular to explain the smallness of the neutrino masses. In our model, we add a second Higgs double and in order to avoid lepton flavour-changing neutral-scalar interactions at tree level, we allow lepton flavour violation solely in the non-flavour-diagonal Majorana mass matrix of the right-handed neutrinos whereas all Yukawa-coupling matrices are lepton flavour-diagonal. We show explicitly in that framework that the branching ratios of the charged-lepton decays l1⁻--> l2⁻ + l1⁺ + l3⁻ can be close to their experimental upper bounds, while the branching ratios of other lepton flavour-changing decays, like l1--> l2 + photon, are invisible because they are suppressed by mR⁻⁴, where mR is the seesaw scale. Furthermore, considering the anomalous magnetic moment of the muon, in our model the contributions from the extra scalars can remove the discrepancy between its experimental and theoretical values.

Nov 14 2017


Daniel Fernandez
(University of Iceland)

Information geometry and Holography

Information Geometry allows for families of probability distributions to be imbued with a natural measure: the Fisher metric. In particular, it is possible to define a Fisher metric on the instanton moduli space of field theories, which in many cases leads to hyperbolic geometries. Furthermore, Euclidean AdS emerges naturally as the metric of the SU(2) Yang Mills instanton moduli space. These are possible first steps towards understanding the emergence of holographic dualities.

Nov 21 2017


Katharina Prochazka
(University of Vienna)

Modelling language shift, or: How I learned to stop worrying and love bad data

Abstract: Language shift is the process whereby people give up use of one language in favour of another. But why do people stop using a language? One way to try and answer this question is mathematical modelling which lets us make visible trends and mechanisms of language shift on a large scale. In this talk, I will use the example of language shift in Carinthia, Austria, to discuss the challenges in choosing adequate mathematical models for linguistic applications: Where do we get data, especially from the past? How do we measure language use? What kind of model can we build and what do the mathematical results tell us?

Nov 28 2017


Danijel Marković
(University of Vienna)

Perturbative Yang-Mills theory without Faddeev-Popov ghost fields

Abstract: A modified Faddeev-Popov path integral density for the quantization of Yang-Mills theory in the Feynman gauge is presented, the modification consisting in the presence of specific finite contributions of the pure gauge degrees of freedom. Translating back the path integral density into unconstrained gauge fields, a perturbation theory without Faddeev-Popov ghost fields is obtained. An explicit perturbative calculation to O(g^2) shows the equivalence of the usual Faddeev-Popov scheme and its modified version.

Dec 5 2017


Daniel Lechner
(University of Vienna)

Electroweak Sudakov Logarithms in e+e- --> ttbar at NLO

Abstract: Top quark pair production is one of the benchmark processes at future linear collider projects like ILC or CLIC, allowing for high-precision determinations of top quark parameters like the top mass or its coupling to the Higgs boson. Concerning radiative corrections, QCD calculations have advanced very far by application of theoretical tools like effective field theories. On the electroweak side far less progress has been made up to this point, but the envisioned precision to be reached by the linear collider studies strongly calls for a more systematic inclusion of electroweak effects in theoretical calculations. In this talk we present a first step in this direction by considering the next-to-leading order (NLO) corrections in alpha to e+e- --> ttbar in the continuum and the high-energy region, with emphasis on the structure of the calculation and the mechanism of UV- and gauge-cancellations.

Dec 12 2017


Khrystyna Gnatenko
(Ivan Franko National University of Lviv, Ukraine)

One- and many-particle problems in rotationally invariant noncommutative space

Abstract: In noncommutative space of canonical type there is a problem of rotational symmetry breaking. To preserve the symmetry the idea of generalization of parameters of noncommutativity to a tensor is considered. We propose to construct the tensor of noncommutativity involving additional coordinates which are governed by a rotationally invariant system. For simplicity this system is considered to be harmonic oscillator with large frequency. As a result, we construct noncommutative algebra which is rotationally invariant and equivalent to the noncommutative algebra of canonical type. In the rotationally invariant noncommutative space we examine influence of noncommutativity on the properties of physical systems. Namely, we find exactly the spectrum of a particle in uniform field and show that noncommutativity effects on the mass of the particle. Also corrections to the energy levels of hydrogen atom and exotic atoms are found in rotationally-invariant noncommutative space. The problem of describing the motion of a composite system, called the soccer-ball problem, is studied.

Dec 19 2017


Isabella Floss
(TU Vienna)

High harmonic generation in solids

Dec 26 2017 -- Jan 2 2018

No Lunch Seminar

Weihnachtsferien - Christmas Break

Jan 9 2018


Isabella Floss
(TU Vienna)

High harmonic generation in solids

Abstract: When irradiating crystalline insulators with strong and short laser pulses the non-linear response of the material gives rise to high harmonic generation. In recent experiments, spectra with pronounced peaks at harmonic frequencies have been observed from crystals. In my talk I will discuss the challenges when simulating the underlying processes and present a route to improvements. This encompasses starting from the microscopic level and including mesoscopic effects of the extended system resulting from the pulse propagation in the dense medium and the inhomogeneous illumination of the crystal which have been neglected in previous simulations.

Jan 16 2018


Angelika Widl
(University of Vienna)

Top Quark Mass Measurements at Future Linear Colliders

Abstract: Two future linear colliders, ILC and CLIC, are currently in planning and promise large improvements on the measurements of Higgs boson and top quark properties. In particular, they will measure the top quark mass with unprecedented accuracy using a threshold scan of the top quark pair production cross section. While the inclusive cross section is known directly at threshold from NRQCD and at higher energies from QCD, a description of the intermediate region above threshold has been missing so far.
In this talk I will first give an introduction to future linear colliders and the physics value they add to the measurements from LHC. Then I will focus on the NRQCD calculation at threshold and finally present our approach to match the NRQCD to the QCD cross section in the region above the threshold.

Jan 23 2018


Chiara Cardelli
(University of Vienna)

Are proteins such unique polymers? - The role of directional interactions in the designability of generalized heteropolymers

Abstract: Proteins are designable heteropolymers: they are able to fold in a unique and very precise 3D structure and encode it (design process) in a 1D sequence of different building blocks (the 20 amino acids) along a polymer chain. Until now, no other natural nor artificial polymer is known to be designable and to fold with the same precision and variability of proteins. Here we try to answer the following questions: "What synthetic heteropolymers lack compared to bio-heteropolymers that grant the latter such precision and versatility? In other words: is the specific spatial arrangement of the atoms in amino acids the only possible realization to obtain designable polymers?" We show through our computational approach that directional interactions are critical to the answer. Moreover, we introduce a simple qualitative criterion to discriminate a priori designable from not-designable polymer backbones. We propose that our criterion can be of guidance for the engineering of new types of self-assembling modular polymers that can open new applications for polymer-based materials science.

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