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Cold Quantum Coffee

The Cold Quantum Coffee brings together research students at the Institute for Theoretical Physics to discuss topics revolving around phenomenology, quantum gravity, cold quantum gases, solid state systems, and everything in between. The seminar is organised by students, for students. For further questions or in case you want to give a talk, please contact one of the organisers (Marc Bauer, Viktoria Noel, Jonas Wessely and Fabian Zhou).

We are supported by the SFB 1225 ISOQUANT.
Date: Tuesday 16:15
Location: Seminar Room of Philosophenweg 16, ITP Heidelberg

Upcoming Talks
06.02.2024 {$\hspace{0.5cm}$} Eduardo Ferreira (Univerität Graz)

Title: Towards TMDs with contour deformations

Abstract: Hadrons are strongly interacting particles composed of quarks and gluons and described by Quantum Chromodynamics (QCD). Their internal structure can be described in terms of structure functions that encode, for example, the momentum and spin distributions of their constituents. Parton distribution functions (PDFs) and Transverse Momentum Distributions (TMDs), for example, describe the quark and gluon momentum distributions inside a hadron. These distribution functions are, however, not easy to calculate, because they are defined on the light front, whereas most hadron calculations are performed in a Euclidean metric. The main problem is then to go from Euclidean onto the light front.

We are developing a new method to compute the parton distributions (TMDs and PDFs) from hadronic matrix elements using contour deformations. We will illustrate the method for a simple system of two interacting scalar particles of equal mass, using an handbag approximation to the matrix element, that includes the two-body Bethe-Salpeter amplitude as input (calculated from its Bethe-Salpeter Equation). Afterwards, the projection onto the light front is done through a combination of contour deformations and analytic continuation methods. We then explore ways of extending the handbag approximation by adding "quark-quark" interactions via the introduction of the four-point function in the diagram, which, in turn, is calculated self-consistently, from its own scattering equation.

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Winter Semester 2023/24 Schedule
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DateSpeaker
17.10.2023 
24.10.2023 
31.10.2023 
07.11.2023 
14.11.2023Ido Siovitz (KIP Heidelberg)
21.11.2023Nikolas Liebster (KIP Heidelberg)
28.11.2023 
05.12.2023Tara Butler (LIX Paris)
12.12.2023Yannick Deller (KIP Heidelberg)
19.12.2023Xinru Wang (Jilin University)
09.01.2024Yadikaer Maitiniyazi (Jilin University)
16.01.2024Friederike Ihssen (ITP Heidelberg)
23.01.2024 
30.01.2024Jan Philipp Klinger (Goethe University Frankfurt)
06.02.2024Eduardo Ferreira (Univerität Graz)

Past talks
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30.01.2024 {$\hspace{0.5cm}$} Jan Philipp Klinger (Goethe University Frankfurt)

Title: QCDs thermal phase transition: About massless many-flavour QCD from the lattice

Abstract: The talk addresses the chiral phase transition of QCD with massless quarks. This is a challenging problem for lattice computations as the chiral limit of QCD is not directly simulable. Its study, however, provides constraints on the phase diagram of QCD with physical masses. We show that the chiral limit is approached via tricritical scaling relations which let us determine the order and temperature of the phase transition as a function of the number of quark flavours $N_f$. Based on simulations of lattice QCD with standard staggered fermions, it was found that QCD with massless quarks has a second order phase transition for $N_f \leq 7$. Additionally, we confirm an expected decrease in the critical temperature for increasing number of flavours. Running simulations on finer lattices and larger $N_f$ will allow us to resolve the question whether massless QCD approaches the conformal window by a first or a second order phase transition.

16.01.2024 {$\hspace{0.5cm}$} Friederike Ihssen (Heidelberg University)

Title: Flowing fields and optimal RG-flows

Abstract: Renormalisation group approaches are tailor made for resolving the scale-dependence of quantum and statistical systems, and hence their phase structure and critical physics. Usually this advantage comes at the price of having to truncate the full theory at hand, which asks for optimal expansion schemes. In the present talk I introduce a functional renormalisation group (fRG) approach for the effective action which includes general scale-dependent reparametrisations of the theory. This approach is used in an O(N)-theory to set up adaptive RG-flows that correspond to an optimal systematic expansion of the theory about the ground state or rather its full covariance or propagator. These parametrisations are induced by flowing fields that encode the differential reparametrisation steps. The approach is put to work for an investigation of the thermal phase transition in the O(4)- theory in view of applications to QCD. This talk is based on [arXiv:2305.00816], but I will also discuss other possible applications.

09.01.2024 {$\hspace{0.5cm}$} Yadikaer Maitiniyazi (Jilin University)

Title: Irreversible vierbein postulate: Emergence of spacetime from quantum phase transition

Abstract: We formulate a model for quantum gravity based on the local-Lorentz symmetry and diffeomorphism. A key idea is the irreversible vierbein postulate that a tree-level action for the model at a certain energy scale does not contain inverse vierbein. Under this postulate, only the spinor becomes a dynamical field and no gravitational background field is not introduced in a tree-level action. In this paper, after explaining the transformation rules of the local-Lorentz and diffeomorphism transformation in detail, a tree-level action is defined. We show that fermionic fluctuations induce a non-vanishing gravitational background field.

19.12.2023 {$\hspace{0.5cm}$} Xinru Wang (Jilin University)

Title: Baryogenesis through QCD preheating

Abstract: Our universe is mainly made of matter rather than anti-matter with the asymmetry of 10^-10 detected by CMB, which leaves us an open question of the mechanism to generate this amount of asymmetry called baryogenesis. It is believed that QCD can not play major role to account for cosmological matter abundances observed in the Universe today including the baryon asymmetry of the Universe. However, in this talk, I would like to share a new picture of the thermal history called "QCD preheating". The dynamic motion of light quark condensate should have the potential to explosively produce the number densities for nucleon and antinucleon by nonadiabatic processes, similarly to the preheating induced by the nonadiabatic-varying vacuum. And this dynamic aspect of the QCD vacuum opens a new frontier to explore low-scale matter generation such as baryogenesis.

12.12.2023 {$\hspace{0.5cm}$} Yannick Deller (KIP Heidelberg)

Title: Towards the experimental realization of real-time instantons in a quenched spinor Bose condensate

Abstract: Many-body systems far from equilibrium can exhibit self-similar dynamics characterized by universal exponents. Numerical studies of a quenched ferromagnetic spinor BEC have revealed the appearance of extreme wave events on the way to the universal regime. Furthermore, as a result of these caustics, real-time instanton defects are generated, which take on the form of space-time vortices in the transversal spin order parameter. However, the random appearance of real-time instantons in space and time makes it experimentally challenging to study these excitations in a controlled way. Thus we aim for deterministic preparation of a single instanton event. We employ local spin-dependent phase imprints, which lead to excitations in the transversal spin length. We probe their time evolution and characterize their structure with spatially resolved detection of all relevant spin observables.

05.12.2023 {$\hspace{0.5cm}$} Tara Butler (École Polytechnique Paris)

Title: Creative AI: Towards a Smart 3D Authoring System for the Visual Exploration of Scientific Models

Abstract: This talk aims to transform scientific visualization by developing a user-centered Creative AI-driven 3D authoring system. The goal is to empower scientists to create and manipulate 3D content as easily as using a pen. It addresses challenges in expressing complex phenomena, enabling user-controlled motion and deformation, and facilitating versatile behavior specification. The methodology involves progressive creation, intuitive interaction, narration, and AI integration. The outcome will provide scientists with a dynamic and intuitive tool for expressing complex ideas and fostering creativity in 3D scientific modeling.

21.11.2023 {$\hspace{0.5cm}$} Nikolas Liebster (KIP Heidelberg)

Title: Emergence of a Crystalline Steady State in a Driven BEC

Abstract: The spontaneous emergence of structures from initially homogenous systems belongs to the most striking topics in natural science. Systems driven into deeply nonlinear regimes are theoretically difficult to describe and can produce states that do not exist in equilibrium. We observe the emergence of a stable square lattice density modulation from an initially homogenous, two-dimensional, radially symmetric Bose-Einstein condensate when periodically driving the two-particle interaction. We show theoretically that this state can be understood as an attractive fixed point of coupled nonlinear amplitude equations, which result from phonon-phonon interactions. As a self-stabilized state characterized by spontaneously broken translational symmetry, our results establish a novel quantum material related to supersolids.

14.11.2023 {$\hspace{0.5cm}$} Ido Siovitz (KIP Heidelberg)

Title: Universal dynamics of rogue waves in a quenched spinor Bose condensate

Abstract: Universal scaling dynamics of a many-body system far from equilibrium is a phenomenon documented both in theory and experiment, the mechanisms of which are not yet fully understood. Here, we connect the universal dynamics of a spin-1 gas with rogue-wave like events in the mutually coupled magnetic components of the gas, which propagate in an effectively random potential governed by the nonlinear spin-changing interaction. As a result, real-time instanton defects appear in the Larmor phase of the spin-1 system as vortices in space and time. We investigate the spatial and temporal correlations of these events to find two mutually related scaling exponents defining the coarsening evolution of length and time scales, respectively.

25.07.2023 {$\hspace{0.5cm}$} Louis Jussios (ITP, Heidelberg University)

Title: Early-time dynamics with non-Gaussian initial states in nonequilibrium quantum fields

Abstract: Recent breakthroughs in ultracold atom experiments enable scientists to construct, control, and explore isolated quantum systems, presenting an unprecedented opportunity for investigating the dynamics of interacting quantum systems out of equilibrium. A key challenge in their theoretical description and simulation lies in establishing a suitable initial state with correlations beyond quadratic order. Such non Gaussian initial states play an essential part in the renormalization of the nonequilibrium quantum field theory. Moreover, they are indispensable when studying the early-time evolution, as their presence exerts the most significant influence during this initial period. I explore how higher-order initial interactions influence the evolution of scalar quantum fields close to thermal equilibrium and look at both the general out-of-equilibrium evolution equations and discuss the results of numerical simulations.

11.07.2023 {$\hspace{0.5cm}$} Marc Winstel (Goethe University Frankfurt)

Title: Inhomogeneous phases and non-monotonic dispersion relations in strongly-interacting models

Abstract: In this talk, we discuss results about inhomogeneous chiral phases, i.e., phases where in addition to chiral symmetry also translational symmetry is broken, in strongly-interacting four-fermion and Yukawa models at finite density. We show that inhomogeneous phases are highly dependent on the used regularization scheme and regulator values in the NJL model. The absence of inhomogeneous condensates is then shown using a general analysis for a variety of models in 2+1 spacetime dimensions, where four-fermion models are renormalizable. We also present preliminary results including additional vector meson interactions and discuss the relevance of these model calculations with respect to inhomogeneous chiral condensates in QCD. In the second part of the talk, negative bosonic wave function renormalization Z is studied using an O(N) model. This phenomenon is often related to the existence of inhomogeneous regimes and has recently been observed in a FRG study of QCD. Building on existing large-N results, we study the effects of such a non-monotonic dispersion relation on bosonic correlations functions using lattice field theory.

04.07.2023 {$\hspace{0.5cm}$} Yunxin Ye (Bielefeld University) {$\hspace{0.2cm}$} Slides

Title: Dynamic critical behavior of chiral transition from the functional renormalization group of model G

Abstract: In the chiral limit the complicated many-body dynamics around the second order chiral phase transition of two-flavour QCD can be understood by appealing to universality. We present a novel formulation of real-time functional renormalization group that describes the stochastic hydrodynamic equations of motion for systems in the same dynamic universality class, which correspond to Model G in the Halperin-Hohenberg classification, and preserves all the relevant symmetries of such systems with reversible mode couplings. We show that the calculations indeed produce the non-trivial value z = d/2 for the dynamic critical exponent, where d is the number of spatial dimensions. From the momentum and reduced temperature dependency of the diffusion coefficient of the conserved charge densities, we extracted the dimensionless universal scaling function.

27.06.2023 {$\hspace{0.5cm}$} Jonas von Milczewski (MPQ München)

Title: Boson-induced superconductivity from strong exciton-electron coupling

Abstract: The idea of using bosons to mediate an attractive interaction between electrons is an established pillar of the theory of superconductivity. Due to recent advances in two-dimensional semiconductors and ultracold atoms, it is now possible to fabricate Bose-Fermi mixtures in which the coupling between fermions and bosons is so strong that it features a bound state, which competes with the formation of Cooper pairs. In my talk, I will present our latest work on Bose-induced superconductivity in two-dimensional semiconductor heterostructures in which electrons and excitons can bind into trions, competing with the formation of Cooper pairs. Our theory takes into account the bound state physics between electrons and excitons, as well as the Polaron physics observed for large population imbalances. As a result of the strong Polaron dressing a picture of a BCS-BEC crossover from weakly bound cooper pairs to Bipolaron-condensation arises. We find critical temperatures of around 10% of the Fermi temperature, rendering such TMD structures a promising candidate for high Tc superconductivity as the excitons and trions are stable even at room temperature due to the strong coupling.

20.06.2023 {$\hspace{0.5cm}$} Ruben Kuespert (Heidelberg University)

Title: Small Kinetic Mixing in String Theory

Abstract: Kinetic mixing between gauge fields of different U(1) factors is a well-studied phenomenon in 4d EFT and represents one portal to a hidden sector. In this talk, we will discuss kinetic mixing from the perspective of string theory. Thus, string theory will be considered as a UV completion of the 4d EFT and therefore provides an explanation for origin of kinetic mixing. Surprisingly, kinetic mixing is absent in many cases due to a non-trivial cancellation. Finally, despite the cancellation, we will identify modifications and generalizations to obtain small kinetic mixing in 4d. We will assume no prior knowledge of string theory and explain the necessary ingredients to understand these phenomena.

07.06.2023 {$\hspace{0.5cm}$} Aiman Al-Eryani (Bochum University) {$\hspace{0.2cm}$} Slides

Title: fRG Flow Equations for Extended Interactions

Abstract: The functional renormalisation group has played an important role in providing a tool for unbiased investigation of strongly correlated systems in condensed matter. To aid for a quantitative investigation, a full, yet efficient, momentum and frequency treatment of the vertex and the self-energy is needed. Such methods have been developed for local Hubbard interactions, but face challenges upon the introduction of more extended interactions. We show how the extended interactions can be implemented in a multi-channel partially bosonised fRG flow equations (the so called Single Boson Exchange fRG flow equations), which nonetheless avoids bias resulting from the so called "Fierz ambiguity". We find that even with extended interactions, the part of the vertex responsible for the multi-boson exchanges is quantitatively negligible - similar to what has been found for local interactions in. This paves the road for the future investigation of extended Hubbard Models (in the form applicable to Moirė materials), and the effect of electron-phonon coupling. Finally, we present an analysis of the extended Hubbard model on square and triangular geometries at van Hove Fillings;

30.05.2023 {$\hspace{0.5cm}$} Fabian Zhou (Heidelberg University) {$\hspace{0.2cm}$} Slides

Title: Mini-jet quenching in non-equilibrium quark-gluon plasma

Abstract: The pre-equilibrium stage in heavy ion collions can be described by an effective kinetic theory of QCD (EKT). Within this framework we study the energy deposition of a high-momentum parton travelling through a quark-gluon plasma. We show that the energy is first transported to the soft sector by collinear cascade and then isotropised by elastic scatterings. In case of a thermal plasma, we find that, remarkably, the jet wake can be well described by a thermal distribution function with angle-dependent temperature. For more realistic scenarios where the background is expanding, we observe signatures of hydrodynamisation of the mini-jet.

23.05.2023 {$\hspace{0.5cm}$} Lillian de Bruin (Heidelberg University)

Title: Sphaleron damping and anomalous charge transport in high-temperature QCD plasmas

Abstract: We modify the hydrodynamic equations of a relativistic chiral plasma to account for damping sourced by sphaleron transitions. We show that sphaleron damping leads to nontrivial effects on transport phenomena. Notably, a wavenumber threshold emerges that characterizes the hydrodynamic behavior of coupled charge modes. Sphaleron damping also significantly impacts the time evolution of coupled charge modes. The dependence of charge separation on the rate of sphaleron transitions has intriguing implications for the experimental search for chiral phenomena in heavy ion collisions.

16.05.2023 {$\hspace{0.5cm}$} Jonas Turnwald (TU Darmstadt) {$\hspace{0.2cm}$} Slides

Title: Real-time dynamics via spectral reconstruction - introducing a general framework based on Gaussian process regression

Abstract: Reliably extracting spectral functions from Euclidean data is an important task for the calculation of a wide variety of dynamical properties in QCD, such as transport properties or the hadronic resonances. However, the inversion of the Källén-Lehmann spectral representation is an ill-conditioned inverse problem that remains notoriously hard to solve. We introduce Gaussian process regression as a general framework for solving linear inverse problems. This method is applied to different problems in QCD, ranging from the calculation of the timelike strong coupling to thermal photon rates in the quark gluon plasma.

07.02.2023 {$\hspace{0.5cm}$} Paul Hotzy (TU Wien) {$\hspace{0.2cm}$} Slides

Title: Stabilizing complex Langevin for real-time gauge theories with an anisotropic kernel

Abstract: The complex Langevin (CL) method is a promising approach to overcome the sign problem that occurs in real-time formulations of quantum field theories. Using the Schwinger-Keldysh formalism, we study SU(N) gauge theories with CL. We observe that current stabilization techniques are insufficient to obtain correct results. Therefore, we revise the discretization of the CL equations on complex time contours, find a time reflection symmetric formulation and introduce a novel anisotropic kernel that enables CL simulations on discretized complex time paths. Applying it to SU(2) Yang-Mills theory in 3+1 dimensions, we obtain unprecedentedly stable results that we validate using additional observables and that can be systematically improved. For the first time, we are able to simulate non-Abelian gauge theory on time contours whose real-time extent exceeds its inverse temperature. Thus, our approach may pave the way towards an ab-initio real-time framework of QCD in and out of equilibrium with a potentially large impact on the phenomenology of heavy-ion collisions.

31.01.2023 {$\hspace{0.5cm}$} Stefan Blücher (TU Berlin) {$\hspace{0.2cm}$} Slides

Title: PredDiff: Explanations and Interactions from Conditional Expectations

Abstract: In this talk I will give a short overview on Explainable AI (XAI) and present PredDiff as a model-agnostic, local attribution method that is firmly rooted in probability theory. Its simple intuition is to measure prediction changes while marginalizing features. I will discuss the main properties of PredDiff and its close connection to Shapley values. Further, I will introduce our new, well-founded measure for interaction effects between arbitrary feature subsets. The study of interaction effects represents an inevitable step towards a comprehensive understanding of black-box models and is particularly important for science applications. Equipped with our novel interaction measure, PredDiff is a promising model-agnostic approach for obtaining reliable, numerically inexpensive and theoretically sound attributions. See https://arxiv.org/abs/2102.13519 for more details.

24.01.2023 {$\hspace{0.5cm}$} Thorben Frank (TU Berlin) {$\hspace{0.2cm}$} Slides

Title: Symmetries in Neural Networks - Path a way towards large-scale and long time-scale dynamics simulations of atomistic systems

Abstract: QCD with massive quarks contains a CP-odd rephasing invariant, commonly called $\bar \theta$. Potential effects of this parameter may be mediated to hadrons (e.g. to the neutron's electric dipole or $eta^\prime\to\pi\pi$) in the effective theory following the pattern by which axial symmetry is broken by the 't Hooft vertex. Calculating the latter reveals that CP-violating effects are absent in the Green's functions for the fermions---the phases from instanton effects and quark masses are aligned. Yet, one must still interfere between the different topological sectors. In the path integral quantization, integer topological sectors only follow when Euclidean time (or real time on a contour with ${\rm i} \epsilon$ prescription) is taken to infinity. This implies that the spacetime volume must be taken to infinity before interfering among the sectors. Taking this into account, CP-violating correlations are absent despite nonvanishing $\bar\theta$. Some remarks concerning canonical quantization will be made toward the end of the seminar.

17.01.2023 {$\hspace{0.5cm}$} Daniel Alvestad (University of Stavanger) {$\hspace{0.2cm}$} Slides

Title: Kernel controlled real-time Complex Langevin simulation

Abstract: We present a novel strategy aimed at restoring correct convergence in complex Langevin simulations. The central idea is to incorporate system-specific prior knowledge into the simulations, in order to circumvent the NP-hard sign problem. In order to do so, we modify complex Langevin using kernels and propose the use of modern auto-differentiation methods to learn optimal kernel values. The optimization process is guided by functionals encoding relevant prior information, such as symmetries or Euclidean correlator data. Our approach recovers correct convergence in the non-interacting theory on the Schwinger-Keldysh contour for any real-time extent. For the strongly coupled quantum anharmonic oscillator we achieve correct convergence up to three-times the real-time extent of the previous benchmark study. We also shed light on the fact that for correct convergence not only the absence of boundary terms, but in addition the correct Fokker-Plank spectrum is crucial. This also tie together the use of kernel with the relation between compelx Langevin and Lefschetz thimbles.

10.01.2023 Johannes Roth (Giessen University) {$\hspace{0.2cm}$} Slides

Title: Causality and critical dynamics in the real-time functional renormalization group

Abstract: Real-time quantities such as spectral functions and transport coefficients can serve to examine the real-time evolution of a system close to equilibrium, as they encode the possible excitations in the medium and show universal static and dynamic scaling behaviour near a critical point. The functional renormalization group (FRG) formulated on the Schwinger-Keldysh closed-time path provides an excellent calculational tool for such real-time correlations. In this talk I will present a novel approach for the systematic construction of causal regulators for the FRG, which comply with the analytic structure of the propagators, and demonstrate that they can be interpreted as a coupling to a fictitious external heat bath with FRG scale dependent spectral distribution. As particular applications, I will discuss the relaxational Models A, B and C according to the classification scheme by Halperin and Hohenberg, and show how they can be implemented in the real-time FRG. With this setup I will then present results which demonstrate the generation of dynamic scaling behaviour in spectral functions obtained from one and two-loop self-consistent truncation schemes. Our results for the different dynamic critical exponents z in both two and three spatial dimensions compare favorably with existing results from the literature.

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