Past Talks (2015-2019)

17.12.2019 {$\hspace{0.5cm}$} Nico Gneist (University of Cologne) {$\hspace{0.2cm}$} Slides

Title: Cluster functional renormalization group approach to Quantum Spin Liquids

Abstract: The pseudofermion functional renormalization group (pf-FRG) has been put forward as a semi-analytical scheme that, for a given microscopic spin model, allows to discriminate whether the low-temperature states exhibit magnetic ordering or a tendency towards the formation of quantum spin liquids. However, the precise nature of the putative spin liquid ground state has remained hard to infer from the original (single-site) pf-FRG scheme. In this talk, I will present a cluster pf-FRG approach, which allows for a more stringent connection between a microscopic spin model and its low-temperature spin liquid ground states. In particular, it allows to calculate spatially structured fermion bilinear expectation values on spatial clusters, which are formed by splitting the original lattice into several sublattices, thereby allowing for the positive identification of a family of bilinear spin liquid states. In this talk I will present the distinct features of the method as well as basic notions of quantum spin liquids in general. In addition I will show how we were able to successfully capture the spin liquid physics of the SU(N) model and also how we apply this method to the J1-J2 model, where we find evidence for the absence of a bilinear quantum spin liquid order.

10.12.2019 {$\hspace{0.5cm}$} Christian-Marcel Schmied (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: A short story of non-thermal fixed points

Abstract: Far from equilibrium, comparatively little is known about the possibilities nature reserves for the structure and states of quantum many-body systems. Starting from a far-from equilibrium initial configuration a quantum many-body system can approach a non-thermal fixed point exhibiting universal scaling in time and space. Such fixed points have been discussed analytically as well as numerically and have recently been observed in experiments. Different underlying physical configurations and processes can lead to the universal scaling characterizing the evolution at the fixed point. In particular, the dynamics can be driven by the reconfiguration and annihilation of (topological) defects populating the system. By quenching a spinor Bose gas across a quantum phase transition we introduce instabilities in the spin degree of freedom which lead to the formation of such defects. The subsequent temporal evolution of the associated characteristic length scales exhibits universal scaling and is thus characterized by universal scaling exponents. In my talk, I will give an introduction to the concept of non-thermal fixed points and present results of numerical simulations of one-dimensional spin-1 Bose gases with ferromagnetic spin interactions using the semi-classical truncated Wigner method.

26.11.2019 {$\hspace{0.5cm}$} Julian Urban (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Prospects of Lattice Field Theory Simulations powered by Deep Neural Networks

Abstract: Traditional Monte Carlo simulations of lattice quantum field theories based on importance sampling are often plagued by prohibitively long autocorrelation times. Generative machine learning methods offer a promising approach to tackle this problem. However, they bring along their own set of issues regarding statistical exactness and performance. Nevertheless, recent work has demonstrated the feasibility of simulating with such frameworks. In this talk, we will explore the statistics of independence Metropolis samplers. Two conceptually similar approaches based on Generative Adversarial Networks (GAN) and Invertible Neural Networks (INN) will be presented. Potential advantages of both methods versus traditional sampling algorithms will be evaluated and compared. We will discuss persisting issues and perspectives on future research into this topic.

19.11.2019 {$\hspace{0.5cm}$} Ricardo Costa de Almeida (University of Trento) {$\hspace{0.2cm}$} Slides

Title: Multipartite entanglement certification in quantum many body systems using quench dynamics

Abstract: Entanglement detection is a central problem for current experiments exploring quantum many-body physics. Though entanglement witnesses provide a framework to handle this task, their direct use is often problematic due to practical considerations. We overcome such limitations for the quantum Fisher information(QFI), a witness for multipartite entanglement, by introducing a protocol to measure it using quench experiments. In particular, the QFI of thermal states becomes accessible via measurements of the response to quenches in the linear regime. To showcase this technique, we apply it to the one-dimensional Fermi-Hubbard model and calculate the QFI across the phase diagram. We introduce QFI bounds adapted to fermionic systems as previous connections between QFI and multipartite entanglement focused on spin systems. As such, this allow us to certify the presence of multipartite entanglement in different regions of the phase diagram. We assess the sensitivity to thermal effects and compare the performance of different observables. Our protocol paves the way to experimentally accessing entanglement that can provide quantum enhancement for metrological devices.

23.10.2019- 15:15 (Note the special date) {$\hspace{0.5cm}$} Manuel Reichert (CP3-Origins, University of Southern Denmark) {$\hspace{0.2cm}$} Slides

Title: Quantum Gravity meets Dark Matter

Abstract: Over the last two decades, the asymptotic safety scenario has evolved into a viable candidate for the fundamental theory of quantum gravity. Evidence for the existence of a non-trivial UV fixed point has been collected in more and more evolved truncations. I will review the status of asymptotically safe quantum gravity and it’s coupling to Standard Model matter. A particular attractive feature of asymptotic safety is that it can make predictions for certain matter couplings in the infrared. I will apply this predictive power to two simple dark matter models and show the constrains on the mass of the dark matter candidate.

22.10.2019 {$\hspace{0.5cm}$} Yu Hamada (Kyoto University) {$\hspace{0.2cm}$} Slides

Title: Stable magnetic monopole in two Higgs doublet models

Abstract: Two Higgs doublet model (2HDM), in which one more Higgs doublet is added to the Standard Model(SM), is one of the most simple extensions of the SM. We show that there is a stable magnetic monopole solution in a certain parameter range of 2HDM. In particular, the stability of the monopole is topologically protected when the Higgs potential has a U(1) symmetry and a $\mathbb{Z}_2$ symmetry. Since this monopole has a mass of about a few TeV, it might be discovered in future monopole searches. This talk is based on arXiv:1904.09269 [hep-ph] .

16.07.2019 {$\hspace{0.5cm}$} Kevin Geier (Heidelberg University)

Title: Analog reheating of the early universe in the laboratory

Abstract: The early universe has undergone a transition from a super-cooled state after cosmic inflation to a hot, thermal state. We propose an analog experimental implementation of this cosmic reheating using an ultra-cold Bose gas. In our mapping, a Bose-Einstein condensate plays the role of the inflaton field, which describes the state of the universe after inflation. The expansion of the universe as well as the dynamics of the inflaton field are encoded in the time-dependence of the atomic interaction, which can be tuned via Feshbach resonances. We illustrate by means of classical-statistical simulations that the dynamics of the system involves the known stages of reheating. At early times, parametric instabilities lead to the production of Bogoliubov quasi-particles as excitations on top of the condensate, mimicking cosmological particle production by the decaying inflaton field. At later times, the system develops a turbulent cascade transporting energy to higher momenta in a self-similar way. The final stage of the dynamics, where the system relaxes to thermal equilibrium, is dominated by quantum fluctuations and therefore not captured by the classical-statistical approximation, which motivates an experimental study of this process using a quantum simulator.

09.07.2019 {$\hspace{0.5cm}$} Paul Wittmer (Heidelberg University)

Title: Non-Equilibrium Dynamics in a Holographic Superfluid

Abstract: We use holographic methods to study the real-time dynamics of two- and three-dimensional strongly correlated many-body quantum systems in a genuinely non-perturbative framework. A superfluid is described holographically in terms of a higher-dimensional gravitational system with an Abelian scalar Higgs model. A fast numerical implementation of the bulk equations of motion in the probe limit is used to evolve the systems on large grids. The two-dimensional system is put in a far-from-equilibrium state by preparing topological vortex defects as quench-like initial conditions for the superfluid’s dynamics. We focus in particular on the dynamics of vortex—anti-vortex annihilation processes. The results are discussed in terms of known equations of motion for vortices in Gross-Pitaevskii theory and compared to numerical results of such. The second part of this talk is focused on the three-dimensional system. The differences between the two- and three-dimensional systems are highlighted before we present and discuss how for the first time in a holographic superfluid the creation and evolution of vortex rings is observed. As for the two-dimensional system, we compare our findings to known results from Gross-Pitaevskii theory.

03.07.2019- 15:15 (Note the special date) {$\hspace{0.5cm}$} Benjamin Knorr (Radboud University, Nijmegen)

Title: Form Factors in Quantum Gravity and Matter

Abstract: Form factors are a central ingredient of the effective action, since they store the full momentum dependence of interactions. In particular in asymmetric configurations, where simple scale identifications might give a too naive picture, they are crucial to obtain correct predictions. In this talk I will give a classification of potentially relevant form factors in quantum gravity coupled to matter, and present some results obtained with non-perturbative renormalisation group techniques.

02.07.2019 {$\hspace{0.5cm}$} Konstantin Otto (University of Gießen)

Title: Compact Objects from the Functional Renormalization Group

Abstract: In recent years, the study of neutron stars has experienced a surge of interest. One major goal of modern research is to probe the equation of state (EoS) of neutron star matter with macroscopic observations such as masses and radii and thereby learn about its microscopic structure. This leads to yet unanswered questions like the possible existence of stars with a quark matter core and the unknown role of strangeness. In this talk we address these problems by employing EoS from the two- and three-flavor quark-meson models obtained from the functional renormalization group in local potential approximation. We study the influence of quantum fluctuations by comparing the results to mean-field calculations. Furthermore, we satisfy the beta equilibrium and charge neutrality conditions which naturally follow from the electroweak sector.

18.06.2019 {$\hspace{0.5cm}$} Emilio Torres (University of Cologne) {$\hspace{0.2cm}$} Slides

Title: Compatible orders in Dirac materials: symmetries and phase diagrams

Abstract: Chiral symmetry breaking patterns in Dirac systems can be realized by different interactions that can moreover appear simultaneously. In the simplest setting, where the gapless, semimetallic Dirac phase is adjacent to two gapped phases, the system exhibits a multicritical point (MCP) where all three phases meet. It is well known that the presence of Dirac fermions alters the critical behaviour of phase transitions in a way that is not accounted for by mere Landau Ginzburg theory, so it is natural to ask to what extent this MCP and the related phase diagram is affected by the gapless fermions. In this talk we address this question by functional renormalization group methods. We obtain the phase diagrams associated to this MCPs for interactions that induce different long range orders, and show that even in the absence of topological terms, the Dirac fermions have a symmetry-enhancing effect that extends to large portions of the phase diagram.

21.05.2019 {$\hspace{0.5cm}$} Torsten Zache (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Dynamical topological transitions in the massive Schwinger model

Abstract: Analog quantum simulators and digital quantum computers have the exciting prospect to access physical phenomena that lie beyond the reach of classical simulations. In the context of lattice gauge theories, the implementation of simple toy models, such as the massive Schwinger model (QED in one spatial dimension), is possible with current technology. Motivated by anomalous and topological properties of QCD, we consider the real-time dynamics of the massive Schwinger model with a topological theta term. Following a quench of the theta term, we identify so-called dynamical quantum phase transitions (DQPT), which have previously been found in various condensed matter models. We establish a general dynamical topological order parameter based on two-time correlation functions that allows to detect these transitions in the presence of interactions. Our numerical simulations employing exact diagonalization indicate that the dynamical topological transitions persist beyond the weak-coupling regime and show robust signatures on small, coarse lattices. These findings make this phenomenon an ideal target for near-future quantum simulator experiments.

14.05.2019 {$\hspace{0.5cm}$} Lukas Kades (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Spectral Reconstruction with Deep Neural Networks

Abstract: We explore artificial neural networks as a tool for the reconstruction of physical spectral functions from imaginary time Green's functions. We systematically investigate a reconstruction based on a straightforward supervised learning approach with feedforward neural networks. A detailed analysis of its performance on physically motivated mock data is provided along with a comparison to established methods of Bayesian inference. We find that the use of labelled training data in combination with an appropriate optimisation procedure can lead to a superior reconstruction accuracy, in particular at larger noise levels. The advantages and disadvantages of the supervised approach as well as potential improvements are discussed in detail.

07.05.2019 {$\hspace{0.5cm}$} Dietrich Roscher (University of Cologne) {$\hspace{0.2cm}$} Slides

Title: Fractionalization in spin systems: an FRG perspective

Abstract: In the last decades, a plethora of new phenomena and structures has been found in low-energy condensed matter systems that seem to defy the Landau paradigm of phase transitions. Oftentimes, effective theories with certain topological features and fractionalized degrees of freedom are best suited to describe the experimental findings. Unfortunately, writing down such an effective theory and understanding its relation to the accepted microscopic model for the actual material is usually not the same thing. In this talk, I show how functional renormalization group can be employed to systematically construct the effective action of spin systems in a so-called spin liquid phase. Unexpectedly, the common toolbox of spontaneous symmetry breaking is well suited for this task and can be adapted to develop an understanding of phenomena that are often considered to be inherently "topological". Besides making predictions for hitherto poorly understood physical systems, functional RG thus provides a neat perspective on fractionalization and emergent gauge fields in strongly correlated spin systems.

12.02.2019 {$\hspace{0.5cm}$} Lukas Barth (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: A geometric framework to compare classical field theories

Abstract: How to compare two physical theories? Is there a way to determine when they have something in common? Is it possible to "intersect" two theories to see if they share any structure? And could one transfer methods that are used to solve problems in one theory to another? In this talk, a mathematical framework is introduced that provides a possibility to answer the above questions for classical field theories (like electrodynamics, hydrodynamics, relativity theory, etc). In this framework the differential equations that usually describe the dynamics in a classical field theory are transformed into geometric objects (by understanding them as submanifolds of Jet Bundles). In this way, one can define a map between those submanifolds that can be interpreted as a correspondence under which two theories become comparable. In particular, this correspondence facilitates to define the intersection of theories as the intersection of two manifolds which is a well-defined notion under certain conditions. This intersection can then be investigated with geometric and cohomological methods to find out if it can be understood as a subtheory of the intersected theories. One can then show that solutions of this intersection can be transferred to solutions of both intersected theories which facilitates a transfer of methods.

29.01.2019 {$\hspace{0.5cm}$} Natalia Sánchez-Kuntz (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Prelude to the reference frame interpretation

Abstract: In this talk a revision of the main no-go theorems in Quantum Mechanics — namely those that deal with locality, contextuality and realism — is delivered. This revision aims at the possibility of constructing an interpretation of Quantum Mechanics that is consistent with the causal structure of Special Relativity. We show firstly that Bell’s theorem (and the EPR paradox) cannot be posed in a factual scenario , which we define. We give several motivations to regard physics and the phenomena it describes as factual, and impose factuality in Quantum Mechanics (non-factuality would lead to non-contextuality, which is ruled out by the no-go theorems of contextuality). Our second result is in the realms of the nature of the quantum state: is the quantum state an ontological description of the system or is it just an epistemological tool for prediction? We show that in order to conclude that a certain quantum state is ontological —through the PBR theorem [1] — the precise quantum system whose state is concluded to be ontological must have undergone a measurement. So it is shown that what PBR demonstrate is that systems which have been measured are described by states which are ontological. What this implies is that, by means of the PBR theorem, one cannot conclude the ontology of a quantum state which describes a system that has not been measured. Our revision has then a far-reaching consequence, which is what we call the reference frame interpretation . This interpretation is inspired by Bohr’s complementarity principle, and is still under construction. If time allows, the general structure of this interpretation will be given in the concluding part of the talk. [1] Pusey, M. F., Barrett, J., Rudolph, T.: On the reality of the quantum state. Nature Physics. 8, 475 (2012)

15.01.2019 {$\hspace{0.5cm}$} Kevin Keiler (University of Hamburg)

Title: State engineering of impurities in a lattice by coupling to a Bose gas

Abstract: After a brief introduction to the physics of ultracold atoms and our numerical method ML-MCTDHX, I will present the localization pattern of interacting impurities, which are trapped in a lattice potential and couple to a Bose gas. For small interspecies interaction strengths, the impurities populate the energetically lowest Bloch state or localize separately in different wells with one extra particle being delocalized over all the wells, depending on the lattice depth. In contrast, for large interspecies interaction strengths we find that due to the fractional filling of the lattice and the competition of the repulsive contact interaction between the impurities and the attractive interaction mediated by the Bose gas, the impurities localize either pairwise or completely in a single well. Tuning the lattice depth, the interspecies and intraspecies interaction strength correspondingly allows for a systematic control and engineering of the two localization patterns. The sharpness of the crossover between the two states as well as the broad region of their existence supports the robustness of the engineering. Moreover, we are able to manipulate the ground state's degeneracy in form of triplets, doublets and singlets by implementing different boundary conditions, such as periodic and hard wall boundary conditions.

18.12.2018 {$\hspace{0.5cm}$} Alexander Schuckert (Technische Universität München)

Title: Many body chaos near a second order thermal phase transition

Abstract: Chaos is one way of seeing why classical systems thermalize: details about the initial state are effectively forgotten as trajectories diverge exponentially quickly. Recently, the study of out-of-time-ordered correlation functions (OTOCs) in quantum mechanical systems has generalized the notion of chaos to many-body systems. Here, we study the classical analogue of OTOCs near the second order thermal phase transition of a real scalar field theory in two spatial dimensions. We show that even in this regime without well-defined quasi-particles, the OTOC exhibits a lightcone-like behaviour as has been previously found in quantum systems. We reveal a transition to chaos in the symmetric phase and a local maximum of the Lyapunov exponent (the "strength" of chaos) at the the phase transition. Both features are put into context to the fluctuations of the order parameter. Furthermore, the butterfly velocity (the speed at which chaos spreads through the system) has a global maximum at the phase transition. Lastly, a self-similar behaviour of the temporal fluctuations of the Lyapunov exponent is shown, with an exponent in agreement with the 2D KPZ universality class. To conclude the talk, a short introduction will be given into how OTOCs have been studied in quantum field theory and how current short comings might be overcome in future studies.

27.11.2018 {$\hspace{0.5cm}$} Ingolf Bischer (Max-Planck-Institut für Kernphysik) {$\hspace{0.2cm}$} Slides

Title: New neutrino interactions: Theoretical motivation and experimental probes

Abstract: I review the approach of general neutrino interactions as an effective parametrisation of new physics including aspects of their theoretical motivation. Furthermore, I discuss some prospects of upcoming experiments to test them.

20.11.2018 {$\hspace{0.5cm}$} Stefanie Czischek (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Violating Bell’s inequality with Langevin dynamics in a deep belief network

Abstract: A representation of quantum spin-1/2 states using artificial neural networks, specifically restricted Boltzmann machines, has been introduced in [1]. This approach can be used to numerically simulate ground states and dynamics after sudden quenches in quantum many-body systems. We implemented this ansatz to benchmark the method on a one-dimensional transverse-field Ising model and found that the simulation of dynamics struggles in the vicinity of the quantum phase transition, where also other simulation methods based on matrix-product-states fail [2]. To improve the neural network approach, we use a Langevin machine, which can be extended to deeper networks and enables the possibility to measure different bases using a deep belief network. We apply Langevin dynamics to sample the real parts of spin states from the network and add the complex phase via an additional reweighting approach. We show that we can violate Bell’s inequality using this network structure to represent a Bell-pair state. This suggests that the deep belief networks can be used to represent full quantum spin states, where the precision of the representation can be increased by going to deeper networks with more hidden layers, which is possible using the Langevin sampling. [1] G. Carleo, M. Troyer, Science 355, 602-606 (2017) [2] S. Czischek, M. Gärttner, T. Gasenzer, Phys. Rev. B 98, 024311 (2018)

16.10.2018 {$\hspace{0.5cm}$} Martin Roelfs (KU Leuven) {$\hspace{0.2cm}$} Slides

Title: Faddeev-Popov Matrix in Linear Covariant Gauge: First Numerical Results

Abstract: In order to make meaningful continuum predictions for observables in gauge theories, gauge fixing is required. This is done using the Faddeev-Popov procedure, which introduces unphysical particles called ghosts whose sole purpose is related to fixing the gauge. A popular gauge is the Landau gauge, and there are a lot of analytical and numerical predictions available for various quantities in this gauge. Remember, although observables in gauge theories should be gauge invariant, there are many quantities which are not. For example, the gluon and ghost two-point functions are gauge dependent quantities. It is therefore interesting to look at the broader class of Linear Covariant Gauges (LCG), of which Landau is a special case, to study this gauge dependence. Although there have been some numerical studies of the gluon two-point function in LCG, reliable data on the ghost propagator has remained elusive. This is because it turns out to be hard to define a lattice equivalent of the Faddeev-Popov operator in LCG. In a recent preprint (https://arxiv.org/abs/1809.08224) we proposed a possible definition of the Faddeev-Popov operator on the lattice, which has all the properties we know and love about the continuum version, and some preliminary data for SU(2) and SU(3) Yang-Mills theories has been obtained.

31.07.2018 {$\hspace{0.5cm}$} Yanick Volpez (University of Basel) {$\hspace{0.2cm}$}

Title: Designing Topological Phases in Condensed Matter Physics

Abstract: In recent years, the field of topological phases of matter has become one of the major research topics in condensed matter physics. It attracts a lot interest since it addresses fundamental questions, deepened our understanding of gapped electronic phases, and also bears the potential to be of great importance in quantum computation. Although the basic theoretical toy models describing topological insulators or superconductors are rather simple, they rely on materials with properties that are very hard to find, which is why many researchers focus on a different strategy, namely, the engineering or designing of topological materials. In our work [1], we propose a model for three-dimensional topological insulators that only uses a stack of conventional semiconducting materials, such as weakly coupled two-dimensional electron-gas layers with spin-orbit interaction. We show that in the presence of strong electron-electron interactions the system realizes a fractional strong topological insulator, where rotational symmetry and condensation energy arguments allow us to treat the problem as quasi-one-dimensional with bosonization techniques. [1] Y. Volpez, D. Loss, and J. Klinovaja, Phys. Rev. B 96,085422 (2017)

25.07.2018 {$\hspace{0.5cm}$} Carlos Mauricio Nieto Guerrero (SISSA, Trieste) {$\hspace{0.2cm}$} Slides

Title: In search of a UV completion of the Standard Model

Abstract: Asymptotically safe extensions of the Standard Model have been searched for by adding vector-like fermions charged under the Standard Model gauge group and having Yukawa-like interactions with new scalar fields. We study the corresponding renormalization group beta-functions to next and next-to-next to leading order in the perturbative expansion, varying the number of extra fermions and the representations they carry. We test the fixed points of the beta-functions against various criteria of perturbativity to single out those that are potentially viable. We show that all the candidate ultraviolet fixed points are unphysical for these models: either they are unstable under radiative corrections, or they cannot be matched to the Standard Model at low energies.

17.07.2018 {$\hspace{0.5cm}$} Anders Eller Thomsen (CP3 Origins, Odense) {$\hspace{0.2cm}$} Slides

Title: Beta functions at large N_f

Abstract: Including a large number (N_f) of vector-like fermions in a quantum field theory provides a limit where the beta functions of the theory becomes calculable as an expansion in 1/N_f. At next-to-leading order in the expansion, the gauge beta function develops a non-trivial zero, which has recently been used as the foundation for constructing ultraviolet safe theories. The talk will focus on the machinery behind the large N_f computations extended to generic gauge-Yukawa theories. For semi-simple gauge theories the phase diagram shows the persistence of the UV fixed point of simple gauge theories.

03.07.2018 {$\hspace{0.5cm}$} Jan Maelger (Centre de Physique Théorique) {$\hspace{0.2cm}$} Slides

Title: Generic features of the heavy quark QCD phase diagram in one-loop models and 2-loop Curci-Ferrari

Abstract: Quantum Chromodynamics exhibits two transitions of physical interest, related to chiral and center symmetry respectively. For large quark masses the effects of the chiral case can be neglected and the center symmetry alone already leads to a rich phase structure with ubiquitous features that are present in all one-loop models. These will be outlined in a generic scenario and the resulting predictions tested against results from higher order models with particular focus on the Curci-Ferrari Model.

26.06.2018 {$\hspace{0.5cm}$} Marc Schiffer (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Ultraviolet Dynamics of Fermions and Gravity

Abstract: The asymptotic safety scenario for gravity and matter might not only provide a consistent theory of quantum gravity, but also serve as an ultraviolet completion of the standard model. In this talk we will study the interplay of gravity and fermions in the ultraviolet. The considered system features an interacting fixed point, where fermions and gravity can be coupled to each other. Symmetry arguments suggest that this fixed point features non-zero interactions of fermions and curvature tensors. We investigate the viability of the asymptotic safety scenario under the inclusion of such a non-minimal interaction. Furthermore, we analyse structural similarities of two avatars of the Newton coupling, a gravitational self-coupling and a fermion-gravity coupling. We discover near effective universality for the Newton coupling at the interacting fixed point for one single fermion. This provides evidence for the physical nature of the discovered fixed point.

19.06.2018 {$\hspace{0.5cm}$} Sebastian Schmalzbauer (Goethe University, Frankfurt am Main)

Title: QCD with isospin asymmetry: phase diagram, extensions and applications

Abstract: I will give insight in QCD with isospin asymmetry and present results of the corresponding phase diagram studied with lattice QCD methods. Of special interest is the phase boundary between the vacuum and pion condensation phases and the chiral/deconfinement transition, as well as the crossover to color-superconductivity for high isospin asymmetries. Furthermore, I will discuss possible extensions to QCD with baryon chemical potentials and mention an application in the sector of compact stars.

29.05.2018 {$\hspace{0.5cm}$} Manuel Scherzer (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Approaching the sign problem by complexification

Abstract: The numerical sign problem plagues Monte Carlo simulations, due to presence of an imaginary part in the action. A priori this prohibits the usual probability interpretation of the path-integral measure. Some of the most promising approaches to the sign problem deal with this by complexifying the integration manifold. I will give an introduction to two of these approaches, the complex Langevin method and the Lefschetz thimble method. I will mainly focus on simple models to keep the talk simple and pedagoical. If time permits I will show some results from the Complex Langevin method in QCD.

15.05.2018 {$\hspace{0.5cm}$} Riccardo Martini (Friedrich Schiller University, Jena) {$\hspace{0.2cm}$} Slides

Title: A curvature bound from gravitational catalysis

Abstract: Gravitational catalysis expresses an interplay between the curvature of the spacetime and fluctuation-induced mass generation of quantum matter. I will show how a scale-dependent analysis of this phenomenon on local AdS backgrounds allow us to identify bounds on the curvature of local patches of spacetime, based on the requirement of long-range chiral symmetry. The bound will be expressed in terms of the ratio between the local scalar curvature and the gauge-invariant coarse-graining scale, pointing out a dependence of the result on the relevant modes of the observed Physics. I will show how this bound can be applied to constraint quantum gravity theories relying on the specific framework provided by Asymptotic Safety.

08.05.2018 {$\hspace{0.5cm}$} Bruno Faigle-Cedzich (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Dimensional crossover in ultracold Fermi gases from Functional Renormalisation

Abstract: After an introduction to the physics of ultracold atoms, the dimensional crossover from two to three dimensions in an ultracold Fermi gas is investigated. Our results are obtained from first principles within the framework of the Functional Renormalisation Group (FRG) and the confinement of the transverse direction is imposed by means of periodic boundary conditions. We calculate the equation of state, the gap parameter at zero temperature and the superfluid transition temperature across a wide range of transversal confinement length scales and the whole BCS-BEC crossover. Particular emphasis is put on the determination of the finite temperature phase diagram for different confinement length scales. In the end we compare our results with recent experimental observations.

24.04.2018 {$\hspace{0.5cm}$} Marc Steinhauser (Friedrich Schiller University, Jena)

Title: N=1 supersymmetric Yang-Mills theory on the Lattice

Abstract: Supersymmetric gauge theories are an important building block for extensions of the standard model. As a first step towards Super-QCD we investigate the pure gauge sector, in particular the bound states: meson-like gluinoballs, gluino-glueballs and pure glueballs. The talk contains an introduction to superymmetry and presents the necessary basics for lattice field theory. Furthermore, I will focus on different strategies to improve discretization artifacts, in which the chiral symmetry and the supersymmetry provide important guidelines. The supersymmetric continuum limit and particle masses are discussed and compared to predictions from effective field theory.

17.04.2018 {$\hspace{0.5cm}$} Lukas Kades (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Langevin type dynamics for continuous and discrete systems

Abstract: After a short introduction of the BrainScaleS (Brain-inspired multiscale computation in neuromorphic hybrid systems) project in Heidelberg, a possible computation of the Langevin equation on the neuromorphic hardware system is discussed. A Monte Carlo algorithm based on Gaussian noise is derived from the findings. The Langevin machine and a modified Ornstein-Uhlenbeck process represent two further useful achievements.

30.01.2018 {$\hspace{0.5cm}$} Martin Pospiech (TU Darmstadt) {$\hspace{0.2cm}$}

Title: Exploring the QCD phase diagram at finite temperature and density

Abstract: We discuss the structure of the QCD phase diagram at finite temperature and quark chemical potential using the Functional Renormalization Group. In the first part, I will present a Fierz-complete NJL-model study and show how the phase boundary is altered when Fierz-incomplete ansätze are considered. In the second part of my talk, I will then show first preliminary results for the phase diagram as obtained from a study including gauge-field dynamics. Finally, I discuss future improvements.

23.01.2018 {$\hspace{0.5cm}$} Alexander Stegemann (Goethe University, Frankfurt) {$\hspace{0.2cm}$}

Title: FRG beyond the local potential approximation at finite temperature

Abstract: The functional renormalisation group (FRG) is a non-perturbative method suited to describe strongly interacting theories like quantum chromodynamics (QCD) at finite temperature and chemical potential. We apply this method to the two-flavour quark-meson model, an effective low-energy model for QCD. In the local potential approximation (LPA), which is the lowest order truncation of the derivative expansion, inconsistencies are observed. In this talk, I will present my recent progress on improving this truncation by including mesonic wave function renormalisations.

16.01.2018 {$\hspace{0.5cm}$} Giovanni Rabuffo (DESY, Hamburg) {$\hspace{0.2cm}$}

Title: Spin foam models and their renormalization: an introduction

Abstract: Spin foam models provide a path integral formulation for quantum gravity. They are defined on a discretization of spacetime, which can be regarded as an irregular lattice. In the first part of the talk we will get a general introduction to the spin foam models, their construction and achievements. While very successful, some important aspects of the models are still not understood. In the second part we will then face the open question of the continuum limit. How can we `zoom out' from Planck scale to large scales? How does the theory change when we build it on different discretizations? To answer these questions, the Renormalization group techniques are an ideal tool, relating theories at different scales.

09.01.2018 {$\hspace{0.5cm}$} Igor Boettcher (Simon Fraser University, Vancouver) {$\hspace{0.2cm}$}

Title: Complex tensor order and quantum criticality in half-Heusler superconductors

Abstract: A revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spin-orbit coupling of electrons, such as the half-Heusler compounds YPtBi or LuPdBi. Due to band inversion, the low-energy degrees of freedom are electrons at a three-dimensional quadratic band touching point with an effective spin-3/2, which allows for Cooper pairs with spins ranging from 0 to 3. I will illuminate some of the unconventional superconducting properties that arise from this band structure and attractive short-range interaction: (i) At strong coupling, the system features an s-wave superconducting quantum critical point with non-Fermi liquid scaling of fermions and several other unusual scaling properties. (ii) The system may further undergo a transition into a phase with complex tensor order, which is a superconducting state captured by a complex-valued matrix order parameter describing Cooper pairs having spin-2. Here, the interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will discuss the mean-field phase structure as a function of doping and temperature, and relate our finding to experiments in YPtBi. Further, the critical properties of this new paradigm for superconductivity will be addressed.

19.12.2017 {$\hspace{0.5cm}$} Daniel Goeschl (Karl-Franzens-Universitaet Graz)

Title: Dual formulation of the SU(2) principal chiral model at finite density

Abstract: Monte Carlo simulations are a powerful quantitative tool for obtaining non-perturbative insight into quantum field theories. However, in the conventional lattice formulation of a QFT the sign problem restricts the applicability of Monte Carlo techniques to zero density. In recent years, dual representations have proven to circumvent the sign problem in various lattice field theories by an exact mapping of the theory to new degrees of freedom. In this dual formulation, matter fields are represented by loops and gauge fields by sheets of plaquettes. In this talk we derive the dualization of the SU(2) principal chiral model by using the recently developed Abelian color flux approach. We show that the inclusion of chemical potentials does not give rise to a sign problem in the dual formulation, hence, allowing for a simulation at finite density.

12.12.2017 {$\hspace{0.5cm}$} Esther Weil (JLU Gießen) {$\hspace{0.2cm}$}

Title: Dyson-Schwinger 101 - Research in the DSE approach

Abstract:

05.12.2017 {$\hspace{0.5cm}$} Alexander Lehmann (Heidelberg University) {$\hspace{0.2cm}$}

Title: NRQCD + Classical Statistical Fields: Real-Time Evolution of Heavy Quarkonium Bound States

Abstract: In order to investigate the time-evolution of the fireball in heavy ion collision from the initial glasma state to the quark gluon plasma over the hadronization until the final detection, usually several subsequent schemes are used, where one method provides the initial conditions for the subsequent one. But yet for the earliest time evolution after the collision assumptions are made for the "second" to be used scheme - a working very first scheme still has to be provided. We aim at providing such a scheme for the time evolution of heavy quarkonia, namely bottomonium. We argue that the high-occupancy of the gluon fields enables the use of classical statistical evolution schemes of the gauge fields. On top of that, we use combine it with NRQCD, an effective field theory for non-relativistic particles. I present the physical setup, the solution idea, some technical details for solving the appearing differential equation systems and some test results of our integration schemes.

21.11.2017 {$\hspace{0.5cm}$} Eduardo Grossi (Heidelberg University) {$\hspace{0.2cm}$}

Title: Causality of fluid dynamics for high-energy nuclear collisions

Abstract: Dissipative relativistic fluid dynamics is not always causal. We discuss the causality structure of high energy nuclear collision. When the fluid evolution equations are hyperbolic, one can bring them to a characteristic form describing the radial expansion of a fireball. This dynamics is causal if the characteristic velocities are smaller than the speed of light. We obtain a concrete inequality from this constraint and discuss how it can be violated for certain initial conditions. We argue that causality poses a bound to the applicability of relativistic fluid dynamics.

14.11.2017 {$\hspace{0.5cm}$} Johannes Lumma (Heidelberg University) {$\hspace{0.2cm}$}

Title: Higgs portal to scalar dark matter in asymptotically safe quantum gravity

Abstract: We investigate asymptotic safety of a singlet-scalar extension of a toy model of the Higgs sector including two real scalar fields under the impact of quantum gravity fluctuations. Employing functional renormalization group techniques, we search for fixed points of the system which feature quantum scale invariance and provide a tentative ultraviolet completion of the system. We find that in a particular regime of the gravitational parameter space all couplings in the scalar sector including the mass parameters become irrelevant at the ultraviolet fixed point. The infrared physics that can be reached from that fixed point is fully predicted and features no free parameters. In the remainder of the gravitational parameter space, the values of all quartic couplings in our model are predicted in terms of the two mass parameters. In light of these results, we discuss possible scenarios where the singlet-scalar can be a dark matter candidate.

24.10.2017 {$\hspace{0.5cm}$} Andreas Elben (IQOQI Innsbruck) {$\hspace{0.2cm}$} S. J. van Enk and C. W. J. Beenakker, Phys. Rev. Lett. 108, 110503 (2012)

Title: Renyi Entropies from Random Quenches in Atomic Hubbard and Spin Models

Abstract: In this talk, I discuss a technique for measuring nonlinear functionals of a many-body density matrix, such as Renyi entropies with direct connection to entanglement, without measuring and reconstructing the whole density matrix (i.e. without performing full quantum state tomography). The approach, which has direct connection to Random Matrix Theory and quantum chaos, consists in implementing an ensemble of random unitary evolution operators, applying them on the measured many-body state and extracting the desired functions from ensemble averaged observables [1]. Investigating the generation of such random unitary evolution operators and the scaling of errors in possible experiments, I show that our approach is readily implementable with current technology and widely applicable, in particular in systems where full state tomography is not available. Concretely, I present applications in one and two-dimensional Fermi (Bose-) Hubbard models and Spin models as realized by Rydberg atoms or trapped Ions. [1] S. J. van Enk and C. W. J. Beenakker, Phys. Rev. Lett. 108, 110503 (2012)

17.10.2017 {$\hspace{0.5cm}$} Nicolas Wink (Heidelberg University) {$\hspace{0.2cm}$}

Title: Finite temperature n-point functions from analytic continuation

Abstract: A formalism for the self consistent calculation of general non-perturbative n-point functions at real times from analytic continuation within the FRG framework is presented. Specifics concerning the analytic continuation of vertices and their spectral representation are discussed at the example of a scalar theory. It is shown how they can be solved numerically in a convenient manner by reformulating the problem as an integral equation for the spectral densities.

25.07.2017 {$\hspace{0.5cm}$} Simon Resch (Justus-Liebig-University Giessen)

Title: Mass Sensitivity of the QCD Phase Diagram

Abstract: The nature of the chiral phase transition of Quantum Chromodynamics (QCD) is still a hot topic of investigation with many open questions. It is known that the quark masses play a crucial role in e.g. the order of the phase transition. This dependence is typically summarized in the so called Columbia plot. Here I apply the 2+1 flavor quark-meson model as a low energy model of QCD to investigate the chiral phase diagram. By varying the strength of explicit symmetry breaking the mass dependence of the chiral transition can be resolved. Furthermore, I will discuss the state of the axial anomaly at finite temperature and its important role in the discussion of the Columbia plot. Non-perturbative fluctuations are captured within the functional renormalization group framework and the results compared to mean field calculations.

18.07.2017 {$\hspace{0.5cm}$} Christian Steinwachs (Albert-Ludwigs-University Freiburg) {$\hspace{0.2cm}$}

Title: Quantum UV properties of Lifshitz theories

Abstract: Lifshitz theories are characterized by an anisotropic scaling of space and time. Such theories break fundamental Lorentz invariance but show promising quantum UV properties. New methods and techniques have to be formulated to analyze general renormalization properties and to perform explicit calculations. I discuss some of these techniques and present new results with a special emphasis on Horava gravity.

11.07.2017 {$\hspace{0.5cm}$} Sebastian Schenk (Heidelberg University)

Title: Perturbation Theory and Geometry

Abstract: There is a vast variety of quantum mechanical systems that are typically studied using perturbation theory. Intriguingly in some quantum potentials the perturbative approach seems to naturally encode all non-perturbative information about higher non-perturbative sectors other than the perturbative vacuum. For a certain subclass of quantum mechanical systems this observation can be made moreprecise by means of geometry. In this talk we will explore these quantum systems by giving a pedagogical introduction to quantum spectral problems associated with genus one elliptic curves. We review arguments that for this particular class of quantum curves elementary classical geometry combined with all-orders WKB is enough to illustrate that the quantum action determines the quantum dual action order by order -- and vice versa -- to finally discuss some specific examples.

04.07.2017 {$\hspace{0.5cm}$} Bernhard Ihrig (Heidelberg University) {$\hspace{0.2cm}$}

Title: Chiral critical behavior of Dirac materials: Gross-Neveu-Yukawa model at three loops

Abstract: Dirac and Weyl Fermions appear as quasi-particle excitations in many condensed-matter systems as for example in graphene. They display various quantum transitions which give rise to new "chiral" universality classes. We study the bosonized version of the Gross-Neveu model - the Gross-Neveu-Yukawa model - at three-loop order and calculate critical exponents in D = 4 - \epsilon. Since this includes more than 1,500 diagrams we employ a computer algebra developed in high energy physics. The comparison to other approaches, namely FRG and Monte Carlo methods, paves the road to a more comprehensive understanding of this paradigmatic example of interacting QFTs. We discuss the applications of the results for the metal insulator transition in graphene to a charge density wave phase (CDW) and a spin density wave phase (SDW).

27.06.2017 {$\hspace{0.5cm}$} Gabor Almasi (GSI Darmstadt) {$\hspace{0.2cm}$}

Title: Modeling chiral criticality and its consequences for heavy-ion collisions

Abstract: A central question in heavy-ion physics is whether there is a chiral critical endpoint (CEP) in the QCD phase diagram and if there is, where it is located. To learn about the location of the CEP experimentally, fluctuation observables of conserved charges have been proposed. Around critical points, such as the CEP in QCD, higher order cumulants of the relevant quantities show universal nonanalytic behavior. The universal behavior of baryon number cumulants around the CEP of QCD can be studied in effective models of QCD that lie in the same universality class, and can be related to the net-proton fluctuations measured in heavy-ion collisions. Such an effective model is for example the Quark Meson model. In my talk, I discuss what one can learn from effective field theory studies of fluctuations and present my results obtained using the Functional Renormalization Group method in the Quark Meson model.

20.06.2017 {$\hspace{0.5cm}$} Walid Mian (University of Graz & Heidelberg University) {$\hspace{0.2cm}$}

Title: Formulating electroweak pion decays in functional methods and the influence of C-P-violation

Abstract: During binary neutron star merger, the dynamical backcoupling of the electroweak interaction influences the merger process and thus the form of the gravitational waves. After the recent successful detection of these, binary neutron star mergers come more into focus of the investigation. To take the dynamical back coupling of the electroweak sector onto the strong sector, a non-perturbative treatment of both sector is necessary. The functional methods in from of the Dyson-Schwinger-Equations, Bethe-Salpeter-Equations and the Functional-Renormalization-Group provide such a non-perturbative tool. The dominant process in the neutron star is the beta-decay. To proceed along this path, we consider the electroweak pion decay into an electron and neutrino as a first step, which has the same ingredients as the beta-decay. We hereby take the special features of the electroweak interaction into account, namely C- and P-violation. Our studies at the level of the quark propagator indicates, that the dynamical backcoupling of the C- and P-violation may lead to the change of handedness for different particles and thus modify the reservoir of particles, which are weakly interacting. Thus we have to be cautious with perturbative extrapolations.

13.06.2017 {$\hspace{0.5cm}$} Aline Ramires (ETH Zuerich) {$\hspace{0.2cm}$}

Title: Large-N: from a theoretical tool to the laboratory

Abstract: Heavy fermion systems are prototype materials for the study of strongly correlated systems. These systems have in their composition very localized electronic orbitals, and as a consequence very strong Coulomb repulsion. In my talk I will introduce the main concepts needed for the understanding of these materials, and the need for non-perturbative approaches, at which point large-N techniques become useful. If one requires the large-N treatment to preserve the fundamental properties of the spin under time reversal, one needs to introduce what is called the symplectic-N approach. I will discuss how this approach allows us to describe a broader range of phenomena in condensed matter, compared to the standard SU(N) generalization. I would then like to touch upon the question: Are these large-N generalizations just a theoretical tool or can they be present in real systems? To answer this question, I will talk about the realisation on symplectic symmetry in cold atomic systems.

06.06.2017 {$\hspace{0.5cm}$} Nicolo Defenu (Heidelberg University) {$\hspace{0.2cm}$}

Title: Nonperturbative RG treatment of amplitude fluctuations for phi^4 topological phase transitions

Abstract: The study of the Berezinskii-Kosterlitz-Thouless (BKT) transition in two-dimensional phi^4 models can be performed in several representations, and the amplitude-phase (AP) Madelung parametrization is the natural way to study the contribution of density fluctuations to non-universal quantities. We show how one can obtain a consistent phase diagram in the AP representation using the functional renormalization group scheme. Constructing the mapping between phi^4 and the XY models allows us to treat these models on equal footing. We estimate universal and non-universal quantities of the two models and find good agreement with available Monte Carlo results. The presented approach is flexible enough to treat parameter ranges of experimental relevance.

30.05.2017 {$\hspace{0.5cm}$} René Sondenheimer (Friedrich-Schiller University Jena) {$\hspace{0.2cm}$}

Title: Rethinking flavor physics

Abstract: Gauge-invariant perturbation theory is an extension of ordinary perturbation theory, which describes strictly gauge-invariant states in theories with a Brout-Englert-Higgs effect. Such gauge-invariant states are composite operators which have necessarily only global quantum numbers. As a consequence, flavor is exchanged for custodial quantum numbers in the standard model, recreating the fermion spectrum in the process. Here, we study the implications of such a description, possibly also for the generation structure of the standard model.

23.05.2017 {$\hspace{0.5cm}$} Aaron Held (Heidelberg University) {$\hspace{0.2cm}$} Viability of quantum-gravity induced ultraviolet completions for matter

Title: Viability of quantum-gravity induced ultraviolet completions for matter

Abstract: We highlight how the existence of an ultraviolet completion for interacting Standard-Model type matter puts constraints on the viable microscopic dynamics of asymptotically safe quantum gravity within truncated Renormalization Group flows. A first constraint – the weak-gravity bound – is rooted in the destruction of quantum scale-invariance in the matter system by strong quantum- gravity fluctuations. A second constraint arises by linking Planck-scale dynamics to the dynamics at the electroweak scale. Specifically, we delineate how to extract a prediction of the top quark mass from asymptotically safe gravity and stress that a finite top mass could be difficult to accommodate in a significant part of the gravitational coupling space.

16.05.2017 {$\hspace{0.5cm}$} Vladyslav Shtabovenko (Technical University of Munich) {$\hspace{0.2cm}$}

Title: Relativistic O(alpha_s^0 v^2) corrections to e+ e- to chi_{cJ} gamma in NRQCD

Abstract: Physics of heavy quarkonia belongs to the most interesting sectors of strong interactions. The progress in this field can be regarded as a measure for our understanding of QCD. Effective Field Theory (EFT) framework provides us with necessary theoretical tools to describe production and decay of these heavy quark bound states in a systematic way. Nonrelativistic QCD (NRQCD) is an EFT of QCD that takes full advantage of the nonrelativistic nature of charmonia and bottomonia and exploits wide separation of the relevant dynamical scales. These scales are $m_Q \gg m_Q v \gg m_Q v^2$, where $m_Q$ is the heavy quark mass and $v$ is the relative velocity of the heavy quarks in the quarkonium. In this sense $m_Q v$ is the typical size of the relative momentum in the heavy quarkonium rest frame, while $m_Q v^2$ corresponds to the binding energy of the state. In this talk we will present our new results on relativistic $\mathcal{O}(\alpha_s^0 v^2)$ corrections to the exclusive electromagnetic production of $\chi_{cJ}$ (spin triplet P-wave $c\bar{c}$ bound state) and a hard photon. Furthermore, we will show how matching calculations between QCD and NRQCD can be automatized using Mathematica package FeynCalc and several additional software tools that were developed specifically for this purpose. These techniques can be also useful for other nonrelativistic EFTs.

09.05.2017 {$\hspace{0.5cm}$} Niklas Müller (Heidelberg University) {$\hspace{0.2cm}$} The chiral anomaly, Berry's phase and chiral kinetic theory, from world-lines in quantum field theory

Title: The chiral anomaly, Berry's phase and chiral kinetic theory, from world-lines in quantum field theory

Abstract: We outline a novel chiral kinetic theory framework for systematic computations of the Chiral Magnetic Effect (CME) in ultrarelativistic heavy-ion collisions. The real part of the fermion determinant in the QCD effective action is expressed as a supersymmetric world-line action of spinning, colored, Grassmanian point particles in background gauge fields, with equations of motion that are covariant generalizations of the Bargmann-Michel-Telegdi and Wong equations. Berry's phase is obtained in a consistent non-relativistic adiabatic limit. The chiral anomaly, in contrast, arises from the phase of the fermion determinant; its topological properties are therefore distinct from those of the Berry phase. We show that the imaginary contribution to the fermion determinant too can be expressed as a point particle world-line path integral and derive the corresponding anomalous axial vector current. Our results can be used to derive a covariant relativistic chiral kinetic theory including the effects of topological fluctuations that has overlap with classical-statistical simulations of the CME at early times and anomalous hydrodynamics at late times.

02.05.2017 {$\hspace{0.5cm}$} Sebastian Wetzel (Heidelberg University) {$\hspace{0.2cm}$}

Title: Detecting Phase Transitions with Artificial Neural Networks

Abstract: In this talk we explore how it is possible to identify phase transitions in physical systems with artificial neural networks. The methods range from feed-forward neural networks in the context of supervised learning to variational autoencoders in the context of unsupervised learning. We present the results of applying the algorithms to the 2d Ising and the 3d XY Model.

25.04.2017 {$\hspace{0.5cm}$} Fleur Versteegen (Heidelberg University) {$\hspace{0.2cm}$}

Title: Quantum gravity signatures in the Unruh effect

Abstract: We study quantum gravity signatures emerging from phenomenologically motivated multiscale models, spectral actions, and Causal Set Theory within the detector approach to the Unruh effect. We show that while the Unruh temperature is unaffected, Lorentz-invariant corrections to the two-point function leave a characteristic fingerprint in the induced emission rate of the accelerated detector. Generically, quantum gravity models exhibiting dynamical dimensional reduction exhibit a suppression of the Unruh rate at high energy while the rate is enhanced in Kaluza-Klein theories with compact extra dimensions. We quantify this behavior by introducing the "Unruh dimension'' as the effective spacetime dimension seen by the Unruh effect and show that it is related, though not identical, to the spectral dimension used to characterize spacetime in quantum gravity. We comment on the physical origins of these effects and their relevance for black hole evaporation.

18.04.2017 {$\hspace{0.5cm}$} Tobias Denz (Heidelberg University) {$\hspace{0.2cm}$}

Title: Towards apparent convergence in asymptotically safe quantum gravity

Abstract: The asymptotic safety scenario in gravity is accessed within the systematic vertex expansion scheme for functional renormalisation group flows put forward in [1,2], and implemented in [3] for propagators and three-point functions. In the present work this expansion scheme is extended to the dynamical graviton four-point function. For the first time, this provides us with a closed flow equation for the graviton propagator: all vertices and propagators involved are computed from their own flows. In terms of a covariant operator expansion the current approximation gives access to $\Lambda$, $R$, $R^2$ as well as $R_{\mu\nu}^2$ and higher derivative operators. We find a UV fixed point with three attractive and two repulsive directions, thus confirming previous studies on the relevance of the first three operators. In the infrared we find trajectories that correspond to classical general relativity and further show non-classical behaviour in some fluctuation couplings. We also find signatures for the apparent convergence of the systematic vertex expansion. This opens a promising path towards establishing asymptotically safe gravity in terms of apparent convergence. [1] arXiv:1209.4038 [hep-th] [2] arXiv:1403.1232[hep-th] [3] arXiv:1506.07016 [hep-th]

06.02.2017 {$\hspace{0.5cm}$} Pascal Törek (University of Graz & FSU Jena)

Title: FMS mechanism in the standard model and beyond

Abstract: Gauge-invariant perturbation theory for theories with a Brout-Englert-Higgs effect, as developed by Fröhlich, Morchio and Strocchi, starts out from physical, exactly gauge-invariant quantities as initial and final states. These are composite operators, and can thus be considered as bound states. In case of the standard model, this reduces almost entirely to conventional perturbation theory. This explains the success of conventional perturbation theory for the standard model. However, this is due to the special structure of the standard model, and it is not guaranteed to be the case for other theories. This talk is about how gauge-invariant perturbation theory can be applied and to show that it is little more complicated than conventional perturbation theory, and that it is often possible to utilize existing results of conventional perturbation theory. Tests of the predictions of gauge-invariant perturbation theory using lattice gauge theory are presented and compared to conventional perturbation theory.

31.01.2017 {$\hspace{0.5cm}$} Giulio Schober (Heidelberg University)

Title: Microscopic theory of the refractive index

Abstract: We examine the refractive index from the viewpoint of modern first-principles materials physics. We first argue that the standard formula, $n^2=\epsilon\mu$, is generally in conflict with fundamental principles on the microscopic level. Instead, it turns out that an allegedly approximate relation, $n^2=\epsilon$, which is already being used for most practical purposes, can be justified theoretically in the long-wavelength limit. More generally, starting from the fundamental, Lorentz-covariant electromagnetic wave equation in materials as used in plasma physics, we rederive a well-known, three-dimensional form of the wave equation in materials and thereby clarify the connection between the covariant fundamental response tensor and the various cartesian tensors used to describe optical properties. Finally, we prove a general theorem by which the fundamental, covariant wave equation can be reformulated concisely in terms of the microscopic dielectric tensor.

24.01.2017 {$\hspace{0.5cm}$} Ouraman Hajizadeh (University of Graz)

Title: A G(2)-QCD Neutron Star

Abstract: The determination of the properties of neutron stars from the underlying theory, QCD, is still an unsolved problem. This is mainly due to the difficulty to obtain reliable results for the equation of state for cold, dense QCD. As an alternative route to obtain qualitative insights, we determine the structure of a neutron star for a modified version of QCD: By replacing the gauge group SU(3) with the exceptional Lie group G2, it is possible to perform lattice simulations at finite density, while still retaining neutrons. Here, results of these lattice simulations are used to determine the mass-radius relation of a neutron star for this theory. The results show that different regimes express themselves in this relation. Also, the radius of the most massive neutron stars is found to vary very little, which would make radius determinations much simpler if this would also be true in QCD.

17.01.2017 {$\hspace{0.5cm}$} Alessia Platania (INAF, Catania & Radboud University, Nijmegen)

Title: Quantum Einstein Gravity on foliated spacetime

Abstract: In this talk I will discuss the non-perturbative renormalization group flow of gravity within the ADM-formalism. The decomposition of the metric degrees of freedom into a lapse function, shift vector and spatial metric equips spacetime with a natural foliation structure which permits the Wick-rotation from Euclidean to Lorentzian signature. As an application, I will discuss the properties of the renormalization group flow projected onto the Einstein-Hilbert action and evaluated on a Euclidean Friedmann-Robertson-Walker background. The resulting phase diagram is similar to the one obtained within the metric formalism: it exhibits the ultraviolet non-Gaussian fixed point (UV-NGFP) responsible for Asymptotic Safety. The interesting novel feature of the foliated computation is the existence in D=3 of a second NGFP, connected to the UV-NGFP by a crossover. In particular, I will show that the existence of this second NGFP provide a natural mechanism to regularize the flow in the IR limit.

10.01.2017 {$\hspace{0.5cm}$} Felipe Attanasio (Swansea University)

Title: An Exploration of the different Phases of (HD)QCD using the Complex Langevin Method

Abstract: The interaction of quarks and gluons under different thermodynamical conditions is still an open problem. Questions in this area arise when studying, e.g., the stability of neutron stars or looking for condensates that would characterise certain phases of hadronic matter. Quantum Chromodynamics (QCD), the theory that describes such interactions, cannot be treated perturbatively in the entire temperature-density phase diagram and so numerical methods, such as lattice QCD must be used. Adding to that, a finite quark density in QCD gives rise to the infamous sign problem, which hinders results from usual Monte Carlo techniques. In this talk I will review the motivation for studying the QCD phase diagram and an alternative method that allows the circumvention of the sign problem in this context. I will then show our results for the phase diagram in the simpler model of QCD with heavy quarks, discuss difficulties that arose during the simulations and introduce a new method we have proposed to deal with them.

19.12.2016 {$\hspace{0.5cm}$} Daniel Becker (Radboud University, Nijmegen)

Title: Asymptotic Safety, unitarity and the theory space

Abstract: On the search for finding a suitable mathematical description of Nature, we are confronted with an infinite variety of possible theories. Ultimately, experiments will tell us which of them gives rise to the construction plan for our universe and its evolution. However this is only promising if the infinite number of free parameters parameterizing the space of theories is constraint by further insight based on symmetries or other physically motivated mathematical principles. In this talk, I will give a brief overview on two such (very powerful) concepts: asymptotic safety (renormalizability and uv-completion) and stability (unitarity and vacuum stability). Some general remarks on their compatibility will conclude the talk.

13.12.2016 {$\hspace{0.5cm}$} Ralf-Arno Tripolt (ECT, Trento)

Title: The "Resonances Via Padé" (RVP) Method

Abstract: The RVP method in principle allows to obtain the analytic continuation of a function given in numerical form. It only requires real input in order to reconstruct the underlying function not only along the real axis but also in the complex plane. It is applied to experimental data in order to locate complex resonance poles as well as decay thresholds. Moreover, it is applied to numerical data on a Euclidean (imaginary-time) propagator in order to obtain the real-time propagator and the corresponding spectral function in momentum space. This procedure in principle represents an alternative to techniques like the Maximum Entropy Method (MEM) and to inverting the associated Laplace transform. arXiv:1610.03252 [hep-ph]

06.12.2016 {$\hspace{0.5cm}$} Jordi Paris Lopez (University of Graz & Heidelberg University)

Title: Properties of Bound States using the FRG

Abstract: Predictions of observables and properties of bound states in the low energy regime require alternative methods to perturbation theory due to the non-perturbative behaviour of QCD. Some of these methods are the Dyson-Schwinger equations (DSE) and the Bethe-Salpeter equations (BSE), which can successfully predict properties of hadrons using given approximations. For complex systems, these approximations can be numerically very demanding. The goal of my thesis is to use the FRG as an alternative method to the DSE-BSE to recover successful results, to use different approximations to solve these complex systems and to analyse its viability. In this talk I will give an introduction to the DSE-BSE and the techniques used to derive observables and then I will mostly talk about the project we are working on here in Heidelberg using the FRG in the Quark-Meson model. Techniques such as the dynamical hadronization, analytical continuation and explicit derivation of flow equation will be shown.

29.11.2016 {$\hspace{0.5cm}$} Tobias Kühn (Research Centre Jülich)

Title: Field theory and (functional) renormalisation group in theoretical neuroscience

Abstract:

22.11.2016 {$\hspace{0.5cm}$} Lukas Holicki (JLU Gießen)

Title: QCD-like theories at finite density and diquark condensation in two-colour matter

Abstract: Lattice Monte Carlo simulations of QCD-like theories without sign problems give insight in the high density features of the respective phase diagram, and allow a direct comparison to effective theories. This talk gives a short introduction in methods to study such theories and present results obtained in G2-QCD and QC2D. Also lattice artifacts will be discussed, such as the condensation of Z(2) monopoles and effects arising from the fermion matrix discretization. The diquark condensation transition in two-colour matter is studied and compared to results from a quark-meson-diquark model calculation and chiral perturbation theory.

15.11.2016 {$\hspace{0.5cm}$} Anton K. Cyrol (Heidelberg University)

Title: FormTracer - A Mathematica Tracing Package Using FORM

Abstract: I present our recently published [1] high-performance, general purpose, easy-to-use Mathematica tracing package dubbed FormTracer. It allows to evaluate (Euclidean) Lorentz/Dirac traces in arbitrary dimensions and includes support for finite temperature and density applications. Furthermore, it supports traces over an arbitrary number of Lie group product spaces. This talk is tailored towards those (considering) using it. Hence, it focuses on capabilities and usage (instructions). [1] arXiv:1610.09331 [hep-ph]

08.11.2016 {$\hspace{0.5cm}$} Stefan Lippoldt (Heidelberg University & FSU Jena)

Title: The early Time Expansion of the Heat Kernel

Abstract: In this talk I will give an Introduction to heat kernel methods as they are used within the FRG in curved space. The aim is to demonstrate the advantages (applicability to curved spacetimes), while also pointing to the problems (field insertions). I will show how the heat kernel naturally arises within the evaluation of the flow equation. As this is a very technical subject, I try to put the emphasis on the concepts and ideas, working with quite some examples.

25.10.2016 {$\hspace{0.5cm}$} Masatoshi Yamada (Heidelberg University & Kanazawa University)

Title: Solving Dynamical Chiral Symmetry Breaking at Finite temperature and density with Weak Renormalization Group

Abstract: We discuss the dynamical chiral symmetry breaking (DCSB) using the functional renormalization group with the local potential and large-N approximations. The Nambu-Jona-Lasinio (NJL) model is used as the chiral effective model of QCD. When the DCSB takes place the renormalization group equation becomes singular. Therefore, we cannot follow the equation after the symmetry breaking. To overcome this situation, we introduce the weak solution method to the equation. In this talk, we see the basic idea of the weak solution method and an application to the DCSB at finite temperature and density. The chiral phase diagram of the NJL model is shown.

13.09.2016 {$\hspace{0.5cm}$} Igor Boettcher (Simon Fraser University)

Title: Quasi-Long-Range Order and Vortex Proliferation in trapped two-dimensional Bose Gases

Abstract:

19.07.2016 {$\hspace{0.5cm}$} Patrick J. Wong (Uni Koeln)

Title: Shape Dynamics in Loops

Abstract: Loop quantum gravity is currently one of the most well-developed formulations of quantum gravity. In spite of this, there are some unresolved complications regarding the choice of canonical variables, namely their lack of inherent Lorentz symmetry and the presence of the anomalous Immirzi parameter. This talk presents a possible resolution of these outstanding issues by shifting the context of the variables from general relativity to that of shape dynamics, a new formulation of gravity whose symmetries are 3d spatial diffeomorphism and conformal symmetry rather than the 4d spatiotemporal diffeomorphism symmetry of GR.

28.06.2016 {$\hspace{0.5cm}$} David Sanchez de la Pena (RWTH Aachen)

Title: Competing electronic instabilities of extended Hubbard models on the honeycomb lattice: A functional Renormalization Group calculation with high wavevector resolution

Abstract: We investigate the quantum many-body instabilities for electrons on the honeycomb lattice at half-filling with extended interactions, motivated by a description of graphene and related mate- rials. We employ a recently developed fermionic functional Renormalization Group scheme which allows for highly resolved calculations of wavevector dependences in the low-energy effective inter- actions. We encounter the expected anti-ferromagnetic spin density wave for a dominant on-site repulsion between electrons, and charge order with different modulations for dominant pure n-th nearest neighbor repulsive interactions. Novel instabilities towards incommensurate charge density waves take place when non-local density interactions among several bond distances are included simultaneously. Moreover, for more realistic Coulomb potentials in graphene including enough non-local terms there is a suppression of charge order due to competition effects between the dif- ferent charge ordering tendencies, and if the on-site term fails to dominate, the semi-metallic state is rendered stable. The possibility of a topological Mott insulator being the favored tendency for dominating second nearest neighbor interactions is not realized in our results with high momentum resolution.

21.06.2016 {$\hspace{0.5cm}$} Alexander Rothkopf (Heidelberg)

Title: A gauge invariant Debye mass from the complex heavy-quark potential

Abstract: We discuss our recent proposal [1] to extract a gauge invariant Debye mass parameter mD from the complex in-medium heavy quark potential. To this end we derive an analytic parametrization of both Re[V] and Im[V] whose temperature dependence is solely determined by mD. It is based on combining a generalized Gauss law [2] for the zero temperature potential of Cornell form with a weak-coupling medium permittivity \epsilon. The resulting linear response-like integro-differential equations not only reproduce the HTL in-medium potential [3] for the Coulomb part but provide a genuinely new contribution to Re[V] and Im[V] arising from the remnants of the confining linear part of the T=0 potential. We show that by tuning mD, one is able to reproduce the lattice QCD determined values for Re[V] and Im[V] in a purely gluonic medium. [1] Y. Burnier, A.R., Phys.Lett. B753 (2016) 232 [2] V. V. Dixit, Mod. Phys. Lett. A 5, 227 (1990) [3] M. Laine et. al. JHEP 0703 (2007) 054

07.06.2016 {$\hspace{0.5cm}$} Stefan Flörchinger (Heidelberg)

Title: Variational principle for theories with dissipation

Abstract: The analytic continuation from the Euclidean domain to real space of the one-particle irreducible quantum effective action is discussed in the context of generalized local equilibrium states. Discontinuous terms associated with dissipative behavior are parametrized in terms of a conveniently defined sign operator. A generalized variational principle is then formulated, which allows to obtain causal and real dissipative equations of motion from the analytically continued quantum effective action. Differential equations derived from the implications of general covariance determine the space-time evolution of the temperature and fluid velocity fields and allow for a discussion of entropy production including a local form of the second law of thermodynamics.

17.05.2016 {$\hspace{0.5cm}$} Philipp Scior (TU Darmstadt)

Title: Effective Polyakov Loop Models for QCD-like Theories at Finite Density

Abstract: We study the heavy quark limit of QCD-like theories by using a three-dimensional Polyakov theory. This theory can be derived from the full QCD-like theory by a combined strong coupling expansion. In particular we investigate the cold and dense regime of the phase diagram where we expect to find the Silverblaze property realized as Bose-Einstein-condensation of diquarks or a fist order liquid-gas transition, depending on the gauge group of the theory. We find evidence for the Silverblaze property when the quark chemical potential reaches half the diquark mass. For even higher chemical potential we find the deconfinement transition indicated by the rise of the Polyakov loop as well as the quark number density.

16.02.2016 {$\hspace{0.5cm}$} Sebastian Wetzel (Heidelberg)

Title: Physics and the choice of regulators in functional renormalisation group flows

Abstract: The Functional Renormalisation Group is a versatile tool for the study of many systems where scale-dependent behaviour is important. The functional RG flow for the scale-dependent effective action depends explicitly on the choice of a regulator, while the physics does not. I will illustrate this dependency and show how one can responsibly treat possible issues to obtain reliable physical results.

12.01.2016 {$\hspace{0.5cm}$} Sören Lammers (Heidelberg)

Title: Dimensional crossover of non-relativistic bosons

Abstract: Experiments with ultracold atoms are able to probe systems consisting of bosons or molecules over a wide range of parameters, like temperature, density and interaction strength. In addition the dimensionality of the system can be changed using strongly anisotropic trapping potentials. In this talk I discuss the dimensional crossover from three to two dimensions, which is of relevance for ultracold atom experiments. Using the Functional Renormalization Group I investigate how confining a transverse spatial dimension influences the few and many body properties of non-relativistic bosons. In particular I show how the phase transition temperature changes as a function of the spatial extent of the transverse dimension.

16.12.2015 {$\hspace{0.5cm}$} Igor Boettcher (Simon Fraser University)

Title: Superconductivity in Semi-Metals with Quadratic Band Touching

Abstract:

08.12.2015 {$\hspace{0.5cm}$} Felix Ziegler (Heidelberg)

Title: Stochastic Quantization with Colored Noise

Abstract: Lattice field theories provide an essential framework in high energy physics for the determination of physical quantities from first principles. Short-distance fluctuations on the lattice may cause falsifying effects on observables. In order to extract the correct continuum physics, smoothening approaches such as cooling or the Wilson flow are applied. In this work we construct a method based on stochastic quantization that sets a momentum scale and tries to create smoothened configurations from the beginning. The associated Langevin equation contains a modified noise term whose momentum modes are UV-regulated. This is also called colored noise. The method is tested at the example of a real scalar field theory in two and four dimensions. We find that a smooth transition between the Wilson flow and the Langevin equation with white noise is possible. Depending on the hopping and coupling parameter of the theory a momentum scale can be extracted above which certain observables do not depend on larger momentum scales. However, further investigations and the extension of the method to non-abelian gauge theories are necessary to compare with existing results.

01.12.2015 {$\hspace{0.5cm}$} Maxime Guilleux (APC Université Paris)

Title: NPRG techniques in de Sitter space

Abstract: We investigate scalar field theories in de Sitter space by means of nonperturbative renormalization group techniques. We compute the functional flow equation for the effective potential of O(N) theories in the local potential approximation and we study the onset of curvature-induced effects as quantum fluctuations are progressively integrated out from subhorizon to superhorizon scales. This results in a dimensional reduction of the original action to an effective zero-dimensional Euclidean theory. We show that the latter is equivalent both to the late-time equilibrium state of the stochastic approach of Starobinsky and Yokoyama and to the effective theory for the zero mode on Euclidean de Sitter space. We investigate the immediate consequences of this dimensional reduction: symmetry restoration and dynamical mass generation.

01.12.2015 {$\hspace{0.5cm}$} Andréas Tresmontant (APC Université Paris)

Title: Yang-Mills propagators close to the phase transition

Abstract: We propose to study thermal Yang-Mills theories close to the confinement/deconfinement phase transition in a recently proposed massive extension. This massive extension turned out to be very interesting already in the vacuum since it allows one to use simple perturbation theory down to zero momentum. In particular, it was shown in the peculiar case of the Landau gauge that one-loop computations are in very good agreement with nonperturbative lattice results. In the present work, we extend this massive extension to the non zero temperature case in presence of a background field for the gauge field (corresponding to the so-called Landau-DeWitt gauge). Prior works have shown that within this approach the phase transition can be investigated in perturbation theory. We will present some of the implications of the presence of the background field and detail how in this case, perturbative calculations can be done in order to compute the propagators.

26.11.2015 {$\hspace{0.5cm}$} Vladimir Juricic (KTH Royal Institute of Technology)

Title: Quantum superconducting criticality in graphene-like systems

Abstract:

27.10.2015 {$\hspace{0.5cm}$} Stefan Weßel (RWTH Aachen University)

Title: Spin Dynamics in Two-dimensional Quantum Spin Systems

Abstract:

15.4.2015 {$\hspace{0.5cm}$} David Mesterhazy (Chicago)

Title: From quantum to classical dynamics: Dynamic crossover in the relativistic O(N) model

Abstract: We investigate the crossover from quantum to classical dynamics in the relativistic O(N) vector model using the nonperturbative functional renormalization group in the real-time formalism. In thermal equilibrium, the theory is characterized by two scales, the interaction range for coherent scattering of particles and the mean free path determined by the rate of incoherent collision with excitations in the thermal medium. Their competition determines the renormalization group flow and the effective dynamics of the model which is quantified in terms of the dynamic critical exponent z. Here, we determine the crossover and the dynamic properties of the model in the vicinity of the continuous phase transition for arbitrary temperatures and in 2 < d < 4 spatial dimensions.

31.3.2015 {$\hspace{0.5cm}$} Lukas Janssen (Simon Fraser University)

Title: Quadratic Band Touching in 3D: NFL phase, nematic quantum criticality, and the merging of fixed points

Abstract: We will discuss the effects of the long-range Coulomb interaction in three-dimensional systems in which conduction and valence bands touch quadratically at the Fermi level. Such band structure is realized in various strongly spin-orbit-coupled materials, such as HgTe, $\alpha$-Sn, and some pyrochlore iridates. We will argue that these systems may be unstable towards spontaneous formation of the strong topological Mott insulator already at weak long-range Coulomb interaction. The mechanism of the instability can be understood as the collision of a non-Fermi-liquid fixed point, discovered by Abrikosov in the '70s and revisited recently, with another, critical, fixed point, which approaches it in the coupling space as the system's dimensionality approaches a certain ``critical dimension'' from above. Some universal characteristics of this scenario, the width of the non-Fermi-liquid crossover regime, the nematic nature of the quantum critical point, and the observability of the topological Mott phase will be discussed. Reference: I. F. Herbut and L. Janssen, Phys. Rev. Lett. 113, 106401 (2014); L. Janssen and I. F. Herbut, arXiv:1503.04242

3.2.2015 {$\hspace{0.5cm}$} Jan Meibohm (Heidelberg)

Title: Gravity-induced fermion self-interaction in the asymptotic safety scenario of quantum gravity

Abstract: In this talk we will discuss whether gravity has the potential of dynamically driving fermions to chiral criticality. In other words, if there is a way in which gravitational interactions at Planck-scale energies could lead to chiral symmetry breaking and thus to mass generation at lower energies. Fermion self- interactions induced by gauge fields are known to be responsible for the fermion-mass generation in the strong sector of the Standard Model. A similar mechanism in gravity would lead to very heavy fermions in contrast to observation. Hence, a unification of the Standard Model and gravity would have to avoid this phenomenon in some way. For a simple gravity-fermion model we will discuss under which circumstances gravity-induced chiral-symmetry breaking is possible.

27.1.2015 {$\hspace{0.5cm}$} Manuel Reichert (Heidelberg)

Title: Inclusion of fermionic matter in the Asymptotic Safety Scenario of Quantum Gravity

Abstract: In this talk I will give an introduction to the Asymptotic Safety Scenario of quantum gravity. I will argue why it is necessary to include matter in a theory of quantum gravity and present results of the inclusion of fermions. The work is based on a vertex expansion of the Einstein-Hilbert action and the flow of Newton's coupling is taken from the graviton three-point function.