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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 (Maruice Beringuier, Zois Gyftopoulos, Renzo Kapust, and Fabian Zhou). We are supported by the SFB 1225 ISOQUANT. Upcoming Talks Saswato Sen, 23.06.2026: Critical Phenomena on the Bethe Lattice We investigate the critical behavior of a family of {$\mathbb{Z}_2$}-symmetric scalar field theories on the Bethe lattice (the tree limit of regular hyperbolic tessellations) using both the non-perturbative Functional Renormalization Group and lattice perturbation theory. The family is indexed by the parameter {$\zeta \in (0,1]$}, which determines the range of the theory via the kinetic term constructed from the graph Laplacian raised to the power {$\zeta$}. Specifically, {$\zeta=1$} is the short-range theory, while {$0<\zeta<1$} defines the long-range model. Due to the hyperbolic nature of Bethe lattices, the Laplacian lacks a zero mode and exhibits a spectral gap. We find that upon closing this spectral gap by a modification of the Laplacian, the scalar field theories exhibit novel critical behavior in the form of non-trivial fixed points with critical exponents governed by {$\zeta$} and the spectral dimension {$d_s=3$}. In particular, our analysis indicates the presence of a Wilson-Fisher fixed point for the short range {$\zeta =1$} theory. In contrast, the nearest‐neighbor Ising model on the Bethe lattice is known to exhibit mean‐field critical exponents. To the best of our knowledge, this work provides the first evidence that a scalar {$\phi^4$} theory and the discrete Ising model on the same underlying lattice may lie in distinct universality classes. Simran Singh, 30.06.2026: Normalising flows for the generalised density of states: from gauge theories to the doped Hubbard model The normalising-flow-based generalised density-of-states (NF-gDoS) method has been shown to successfully reproduce the Lee-Yang zeros of scalar field theory up to two spacetime dimensions. In this work we extend the framework in two directions: (i) (1+1)D U(1) gauge theory with and without a topological theta-term, where we employ gauge-equivariant flows and demonstrate controlled generation of configurations at fixed topological charge; and (ii) the doped Hubbard model, where the gDoS provides a natural route to sign-problem-affected regimes at finite chemical potential. Together, these applications establish NF-gDoS as a flexible tool spanning gauge theories and strongly correlated electron systems, and identify the model expressivity in the tails of constrained distributions as the central direction for further development. Summer Semester 2026 Schedule
Past Talks Summer Semester 2026 Alexander Wowchik, 26.05.2026: Universal Kardar-Parisi-Zhang scaling in non-equilibrium magnon condensates Extensive theoretical study in the past decade has unveiled how driven-dissipative condensates in one and two dimensions can form a unique non-equilibrium phase of matter characterized by a Kardar-Parisi-Zhang (KPZ) equation for the emergent Goldstone modes. We investigate whether the condensation of magnons, as demonstrated in Yttrium Iron Garnet (YIG) films when driven with microwave radiation in the presence of an external magnetic field, is a suitable candidate to study such phenomena. Via micromagnetic simulations of a corresponding system, we show that the dynamical correlation function of the magnon field exhibits the universal behaviour predicted by the KPZ equation. Pierre-Louis Taillat, 19.05.2026: Exact perturbative expansion of the transport coefficients of a normal low-temperature Fermi gas with contact interactions We compute the shear viscosity, thermal conductivity and spin diffusivity of a Fermi gas with short-range interactions in the Fermi liquid regime of the normal phase, that is at temperatures T much lower than the Fermi temperature TF and much larger than the superfluid critical temperature Tc. Given recent advances in the precision of cold atom experiments, we provide exact results up to second-order in the interaction strength. We extend the Landau-Salpeter equation to compute the collision amplitude beyond the forward-scattering limit, covering all collisions on the Fermi surface. We treat the collision kernel exactly, leading to significant corrections beyond relaxation-time or variational approximations. The transport coefficients, as functions of the s-wave scattering length a and Fermi wavenumber kF, follow (1+γkFa)/a2 up to corrections of order O(a0), with a positive coefficient γ for the viscosity and negative one for the thermal conductivity and spin diffusivity. Shahram Vatani, 28.04.2026: Chiral Dynamics: Do Symmetries Have To Break? Gauge-fermion theories and their IR fate remain puzzling mysteries in QFT, even after decades of study. Beyond their theoretical interest, they may play a natural role in extensions of the Standard Model, such as grand unified theories, dynamical symmetry breaking, and models of quark and lepton substructure. Yet our limited understanding of their nonperturbative dynamics severely hampers their application to realistic theories of nature. It is therefore of utmost importance to gain insight into their IR behavior. Using functional methods based on the Effective Average Action, we investigate from first principles the dynamics of a class of chiral gauge theories. Our results reveal a rich structure, ranging from IR conformality to novel patterns of chiral symmetry breaking. In addition, we find evidence for confinement without symmetry breaking, a phenomenon that may provide a natural realization of Symmetric Mass Generation and offer new perspectives on lattice regularization of chiral fermions. Pietro Benzoni, 21.04.2026: Open quantum system approach to the transverse momentum broadening of a colour dipole Using the open quantum formalism, we study the propagation of a quark-antiquark pair propagating through a dense QCD plasma and we derive the Lindblad evolution equation for the density matrix of the system. We essentially focus on the boosted regime where the opening angle of this effective colour dipole is small. In the back-to-back limit where the quark-antiquark relative transverse momentum p_\perp is much larger than the imbalance q_\perp, we demonstrate that the resulting Wigner distribution displays quasi factorisation between a hard factor describing the hard splitting producing the quark-antiquark pair and the transverse momentum imbalance q_\perp-distribution encoding the broadening induced by the medium. The factorisation is violated by a "colour decoherence'' factor that controls the q_\perp-distribution dependence on the opening angle through the ratio between the latter and the characteristic angle \theta_c. The open quantum approach enables us to clarify the role of this critical angle \theta_c and its associated critical time t_c in the genuine quantum decoherence of the density matrix in colour and kinematic space: in particular, t_c controls both the suppression of the off-diagonal elements of the density matrix in colour space and the transition between singlet and octet states. We find, however, that colour decoherence sets in earlier than the full decoherence of the density matrix, thereby marking the onset of classical behaviour in the system. Finally, we investigate the corrections beyond the quasi-factorised picture due to the quantum diffusion term in p_\perp of the Lindblad equation and we find that these corrections are mild. Past Talks Winter Semester 2025/2026 Diego Buccio, 28.10.2025: Momentum running of Quadratic Gravity Running couplings were introduced in quantum field theory to preserve perturbativity in scattering amplitudes, despite the appearance of large logs of external momenta. It is commonly believed that such logarithms are directly related to UV divergencies in one-loop perturbation theory. However, this is not completely true in higher derivative theories: on the one hand, large logs can also emerge from UV finite loop integrals due to IR effects; on the other hand, some UV divergent diagrams do not depend on external momenta. We define a new set of beta functions for quadratic gravity based on the explicit computation of large logs of momenta and discuss their features concerning the asymptotic UV behaviour of the theory and their gauge dependence. Fabian Wagner, 04.11.2025: Quantum field theory with minimal spacetime volume It is common lore that phenomenological quantum-gravity models with a fundamental scale, such as a minimal length, must break Lorentz invariance. In my talk, I show that a Lorentz-covariant cutoff yields a fundamental scale, a minimal spacetime volume, while preserving Lorentz invariance. Thus, such a cutoff may serve as a continuum analogue to causal sets. I construct an interacting QFT with Lorentz-covariant cutoff, and show that its correlation functions can be mapped to those of a (possibly nonlocal) QFT, which makes the phenomenology much more tractable. Peter Lowdon, 18.11.2025: Goldstone bosons at finite temperature Temperature has a significant effect on the properties of QFTs with spontaneously broken symmetries, in particular for the massless Goldstone bosons that exist in the vacuum state. In this talk I will discuss recent results which indicate that Goldstone modes persist at high temperatures, even if the symmetry is restored, and that they have the properties of screened massless excitations, so-called thermoparticles. This has important implications for the phase structure of QFTs at finite temperature. Maximilian Neumann, 13.01.2026: Applications of TDA to Viral Evolution Understanding viral evolution in the context of rapidly growing genomic datasets requires mathematical frameworks that extend beyond classical phylogenetic approaches. Topological Data Analysis (TDA) offers a scalable and principled approach for extracting structural information from high-dimensional genomic datasets, without relying on the reconstruction of explicit phylogenetic trees. Motivated by large-scale genomic surveillance efforts during the COVID-19 pandemic, this work leverages TDA to investigate viral evolution through the detection of topological signatures associated with adaptation, convergent evolution, and selective pressures. By capturing recurrent and non-tree-like evolutionary patterns, TDA enables efficient analysis of hundreds of thousands of genomes and facilitates high-resolution temporal tracking of emerging mutations. Ankur Singha, 27.01.2026: Scalable Lattice Simulations Near Criticality via Renormalisation-Inspired Generative Models Studying lattice systems near a second-order phase transition is both fascinating and challenging. A central difficulty is the scalability of lattice simulations: as the correlation length grows near criticality, long-range correlations emerge and conventional simulation algorithms suffer from critical slowing down, leading to rapidly increasing computational costs. Closely related challenges arise in lattice field theory when approaching the continuum limit, where fine lattices and large physical volumes must be handled simultaneously. Recent advances in Machine Learning (ML) offer new ways to address these problems. In this talk, I will discuss how generative AI models inspired by renormalisation-group ideas can improve the scalability and efficiency of lattice simulations near criticality, and will present our Renormalisation-Inspired Generative Critical Sampler (RiGCS) framework for the 2D Ising model and scalar phi4 theory, which uses a hierarchical, multiscale generative approach to efficiently generate configurations across scales and mitigate critical slowing down. |
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This page was last modified on June 05, 2026, at 07:27 AM