# Vienna Theory Lunch Seminar

by Daniel Lechner (UV), Olaf Krüger (UV), Philipp Stanzer (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

 Oct 9 2018UV Christopher Lepenik(Uni Vienna) Classical and Modern Techniques for Multiloop Computations In this talk I will present some methods which are currently used for evaluating multiloop Feynman integrals. Apart from looking for beyond the standard model physics by trying to produce elementary particles heavier than the top quark with accordingly high energies, one way to find new physics is to increase the accuracy of experimental measurements and theoretical calculations to a degree where deviations from pure standard model predictions can be detected. One obvious way to increase the accuracy of perturbative calculations is to increase the order of the expansion in the respective coupling constant, i.e. to compute Feynman diagrams with a higher number of loops. As these computations can be very cumbersome and this approach becomes evermore important, old techniques were improved and new ones were developed in the last decades. I will introduce, in a pedagogical way, some powerful tools to make multiloop calculations more feasible and maybe sometimes even entertaining. Oct 16 2018TU Aditya Pathak(Uni Vienna) Nonperturbative Corrections to the Groomed Jet Mass Spectrum With the advent of resummation and factorization techniques in QCD in the 80s and 90s we developed powerful tools to calculate event shape distributions with ever increasing accuracy, such as thrust or jet mass. This has paved the way for precise measurement of parameters such as top mass or the strong coupling, α s . While a successful program at LEP, the more complex collider environment of the LHC has called for exploring techniques that can help reduce the contamination of an event shape measurement from sources such as the underlying event or pile-up. One category of such a technique that has been amenable to theoretical calculations is the "Jet grooming". Jet grooming involves selecting certain particles in a given event, throwing away others, and calculating the event shape variable on the selected set of particles. This is done in a way such that what gets thrown away closely resembles the "contamination", and the key information, accessible to perturbative QCD calculations, is preserved by the kept particles. This results in reduced correction to the perturbative cross section from hadronization and underlying event. However, despite these advantages, due to complicated nature of the algorithm, it is a non trivial problem to include consistently a description of these nonperturbative effects. This has so far been done exclusively by making use of Monte Carlo Event Generators and an analytical understanding is lacking. In this talk I address these issues, deriving results solely from field theory wherever possible. Oct 23 2018UV Verena Schenzinger(Uni Vienna) From correlation to causality: Finding the golden thread Abstract: There's something correlated in the state of the atmosphere! However, what exactly is it? A new method promises to shed light on the distinction between a potentially spurious correlation between timeseries in contrast to an actual cause and effect relationship. Based on momentary information transfer, network graphs depicting independent interaction mechanisms can be established. The potential of the method is explored in the context of atmospheric physics, specifically the teleconnection of the quasi-biennial oscillation, a phenomenon in the equatorial stratosphere, and the Northern Hemisphere winter circulation. Oct 30 2018TU Clemens Jochum(TU Wien) Investigating DNA-based dendrimers: theory (and experiment) In this talk, I'm going to present some aspects of my research on DNA-based dendrimers [1]. These charged dendrimers, which are synthesized via enzymatic ligation of Y-shaped DNA building blocks [2], hold high promise in bringing about targeted self-assembly of soft-matter systems in the bulk and at interfaces [3]. A coarse-grained bead-spring model is employed and Molecular Dynamics simulations are performed in order to determine conformational characteristics of these all-DNA dendrimers. The obtained results are compared to dynamic and static light scattering experiments. Furthermore, the influence of salt concentration on the structural properties in equilibrium is analyzed. Finally, effective potentials between the dendrimers are calculated using a theoretical approach based on the Widom particle-insertion method [4]. [1] C. Jochum, N. Adžić, E. Stiakakis, T. L. Derrien, D. Luo, G. Kahl, and C. N. Likos, Nanoscale (advance article), (2018) [2] Y. Li, Y. Tseng, and D. Luo, Nat. Mater. 3, 38, (2004) [3] B. Mladek, M. Neumann, G. Kahl, and C. Likos, Phys. Rev. Lett. 96, 045701, (2006) [4] B. Mladek and D. Frenkel, Soft Matter, 7, 1450, (2011) Nov 6 2018UV Federico Ambrogi(HEPHY) Recent Developments in Tools for Beyond the Standard Model Phenomenology Studies The search for Beyond the Standard Model (BSM) physics is one of the main challenges for both experimentalists and theorists in particle physics. Astrophysical observations show that around 25% of the energy content of the universe includes a form of particle dark matter (DM), that cannot be explained by the Standard Model. In this talk I will present the recent developments in three tools commonly used for the study of the phenomenology of BSM physics: MadAnalysis 5, SModelS and MadDM. These packages provide a direct connection between the data collected by e.g. the detectors at the LHC or by DM indirect detetection experiments and the theoretical predictions of new physics models, allowing to efficiently constrain their large parameter space. Nov 13 2018TU Andreas Zoettl(TU Wien) Hydrodynamics of swimming bacteria Studying the locomotion of microorganisms has become a largely growing field (“active matter”) in physics in the last years. Due to the micron size their motion in Newtonian fluids (such as water) is dominated by viscous drag rather than inertia. In the absence of external forces non-reciprocal deformations of their cell body create fluid flow around them and enable them to move, they are therefore intrinsically out of equilibrium. Important examples of such motile “micro-swimmers” are sperm cells or bacteria with periodically beating appendages (“flagella”) at the back of their cell body. In the human body they swim through mucus which is a viscoelastic hydrogel mainly consisting of water and a crosslinked network of long (up to some micron) linear bio-polymers. I will first introduce the basic physical mechanisms how bacteria can move force- and torque-free in simple Newtonian viscous fluids, and will then show that - contrary to intuition - bacteria can move faster in polymeric fluids compared to water, despite the fact that the viscosity in polymeric fluids is larger. Nov 20 2018UV Markus Kitzler(TU Wien) Applications of ultrashort intense laser pulses in light-matter interaction The electronic response in nanoscopic objects, such as nanostructured surfaces, molecules and atoms lies in the Petahertz (1 PHz = 103 THz = 1015 Hz) frequency range and supports bandwidths reaching several hundred Terahertz. Such electronic dynamics is best accessed with ultrashort intense laser pulses which provide an extremely high temporal resolution reaching into the attosecond (1 as = 10-18 s) domain. Despite the moderate average powers of laser beams, the high peak electric field strength of an ultrashort laser pulse can ionize an atom or break a chemical bond. Thus, intense light pulses supply a driving force as well as a temporal reference in experiments exploring ultrafast processes. In my talk, I will provide an overview over technical aspects of the generation of ultrashort intense laser pulses and will discuss several examples where the unique opportunities offered by these pulses are beneficially exploited for the study of ultrafast dynamics in atoms and molecules. Nov 27 2018TU Josef Kaufmann(TU Wien) Green's Function Measurement in Hybridization-Expansion Continuous-Time Quantum Monte Carlo (CT-HYB) The Anderson impurity model (AIM) plays an important role in the the- ory of correlated electrons in solids. Despite the simplicity of this model, its solution is a challenging task, and it captures interesting physics related to magnetic impurities in metals. Continuous-time quantum Monte Carlo (CT-QMC) is a state-of-the-art ap- proach to the solution of the AIM, frequently employed in computational solid state physics. Specifically, the hybridization expansion is widely used in dynamical mean-field theory (DMFT) calculations. After an introduction of the algorithm, an overview over recent developments and applications will be given. Dec 4 2018UV Maciej Maliborski(Uni Vienna) Instability of Anti-de Sitter spacetime I will review the problem of instability of Anti-de Sitter spacetime and list several open problems in the context. Dec 11 2018TU Andreas Banlaki(TU Wien) On Mathieu Moonshine and Gromov-Witten invariants Mathieu moonshine describes the surprising connection between the elliptic genus of K3 and the largest Mathieu group M 24, found by Eguchi, Ooguri and Tachikawa in 2010. Through string duality this gets linked to the counting of holomorphic embeddings of Riemann surfaces into Calabi-Yau 3-folds, i.e. Gromov-Witten invariants. In this talk I will describe some of the details of this correspondence ending with two concrete examples. Dec 18 2018 -- Jan 1 2019 No Lunch Seminar Weihnachtsferien - Christmas Break Jan 8 2019TU Kevin Pichler(TU Wien) Coherent perfect absorption in disordered media – a random anti-laser Absorbing radiation efficiently is a goal with many applications ranging from spectral filtering to the conversion of sunlight into heat. A very promising phenomenon that has attracted considerable attention in the field of non-Hermitian wave engineering is that of coherent perfect absorption. A coherent perfect absorber (CPA) can be understood as the time-reverse of a laser operating at its lasing threshold, i.e., as a device that completely absorbs a specific injected signal without any reflection or transmission, thus featuring an ultimate level of absorption. While CPAs have already been realised in a number of setups, there was so far the notable exception of a CPA in a disordered medium that lacks any engineered structure. The talk will focus on the theoretical background of coherent perfect absorption as well as on the experimental realisation of a random anti-laser. Jan 15 2019UV Thorsten Schimannek(Uni Vienna) A brief introduction to string dualities There are five perturbative theories of superstrings. Another theory that is called M-theory does not contain strings at all. Yet all of these theories are considered to be limits of one and the same underlying theory that is usually called string theory. Maps that translate a physical setup in one theory into an equivalent yet vastly different configuration within another are called dualities. The goal of this talk is to provide a very elementary introduction to these various viewpoints and the dualities among them. Jan 22 2019TU Benedikt Hartl(TU Wien) Predictive molecular self-assembly at solid-liquid interfaces under electrochemical control - an evolutionary approach Gaining control over the adsorption and structural transition of organic molecules on solidsurfaces is both of fundamental as well as of technological relevance, especially in fieldssuch as corrosion, catalysis and nano–electronics (to name a few). In recent experiments[1, 2] the spontaneous and reversible transition between two- and three-dimensional self-assembly scenarios of a supramolecular system (PQP+ClO−4) at a solid–liquid interfaceunder electrochemical conditions [Au (111) in 0.1 M HClO4] has been demonstrated.In our theoretical investigations effort is dedicated to rationalize these experimentalobservations. The self-assembly scenarios of a molecular model at an interface is studiedand identified via optimization tools that are based on evolutionary algorithms. The com-plexity of the problem forces us to use advanced techniques of these optimization tools,including a basin hopping memetic approach combining the heuristic nature of evolutio-nary strategies with deterministic local gradient descent algorithms: The gradient descentmethod deterministically evaluates every basin’s local minimum with high accuracy whilethe evolutionary search gradually adapts its population to the energetically most favor-able solution, exploring the search space for the global optimum [3, 4]. Our theoreticalresults allow predicting, on the basis of the model above, the experimental conditionsunder which the various self-assembly scenarios in [1, 2] can be observed.[1] K. Cui, K. S.Mali, D. Wu, X. Feng, K. Müllen, M. Walter, S. De Feyter and S. F. L. Mertens, Angew. Chemie (Int.Ed.)53, 12951 (2014)[2] K. Cui, K. S. Mali, O. Ivasenko, D. Wu, X. Feng, M. Walter, K. Müllen, S. De Feyter and S. F. L. Mertens, Small13,1702379 (2017)[3] D.Wales, J.Doye, J. Phys. Chem. A,101, 5111 (1997)[4] M.Antlanger et al., Phys. Rev. Lett.117, 118002 (2016) Jan 29 2019UV Christoph Regner(Uni Vienna) Renormalons and R-Evolution Perturbation series in QCD are generally known to be divergent and can at best be considered asymptotic. The asymptotic behaviour is caused by contributions from small and large loop momenta in perturbative calculations and manifests itself in poles of the corresponding Borel transform. These poles are usually referred to as renormalons and lead to ambiguities in the definition of perturbative series. In this talk we discuss the implications of these ambiguities and investigate how well renormalon ambiguities can be quantified. Furthermore it will be shown how the introduction of a new scale R can be used to improve the poor convergence behaviour of perturbative series suffering from renormalon ambiguities. From the solution of renormalization group equations with respect to the scale R (R-evolution) one can deduce an analytic all-order expression for the Borel transform of perturbative series that can be used as a test for renormalon ambiguities. We will analyze this Borel transform in the context of the large-$\beta_0$ approximation.