# Cold Quantum Coffee

The Cold Quantum Coffee brings together research students of the institute to discuss topics revolving around gauge theories, quantum gravity, cold quantum gases, solid state systems, and everything in between. The seminar is organized by students. In each seminar we have a talk of a member of the institute or an invited speaker. For further questions or in case you want to give a talk, please contact one of the organizers (Anton K. Cyrol, Manuel Reichert, Nicolas Wink and Felix Ziegler).

We are supported by the Heidelberg Graduate School of Fundamental Physics.

Date: Tuesday 16:15
Location: Institute for Theoretical Physics, Philosophenweg 16, seminar room

### Upcoming talks

 25. 07. 2018 Wednesday 16:15 SR, Philweg 16 Carlos Mauricio Nieto Guerrero (SISSA, Trieste) In search of a UV completion of the Standard Model 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. 31. 07. 2018 Yanick Volpez (University of Basel)

### Past talks (more can be found here)

 17. 07. 2018 Anders Eller Thomsen (CP3 Origins, Odense) Slides Beta functions at large N_f 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 Jan Maelger (Centre de Physique Théorique) Slides Generic features of the heavy quark QCD phase diagram in one-loop models and 2-loop Curci-Ferrari 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 Marc Schiffer (Heidelberg University) Slides Ultraviolet Dynamics of Fermions and Gravity 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 Sebastian Schmalzbauer (Goethe University, Frankfurt am Main) QCD with isospin asymmetry: phase diagram, extensions and applications 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 Manuel Scherzer (Heidelberg University) Slides Approaching the sign problem by complexification 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 Riccardo Martini (Friedrich Schiller University, Jena) Slides A curvature bound from gravitational catalysis 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 Bruno Faigle-Cedzich (Heidelberg University) Slides Dimensional crossover in ultracold Fermi gases from Functional Renormalisation 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 Marc Steinhauser (Friedrich Schiller University, Jena) N=1 supersymmetric Yang-Mills theory on the Lattice 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 Lukas Kades (Heidelberg University) Slides Langevin type dynamics for continuous and discrete systems 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.