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 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
24. 10. 2017 
Andreas Elben (IQOQI Innsbruck) Renyi Entropies from Random Quenches in Atomic Hubbard and Spin ModelsIn this talk, I discuss a technique for measuring nonlinear functionals of a manybody 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 manybody 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 twodimensional 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) 
31. 10. 2017 
Holiday (Reformationstag)

07. 11. 2017 
TBA

14. 11. 2017 
TBA

21. 11. 2017 
TBA

28. 11. 2017 
TBA

05. 12. 2017 
TBA

12. 12. 2017 
TBA

19. 12. 2017 
TBA

09. 01. 2017 
TBA

16. 01. 2017 
Giovanni Rabuffo (DESY, Hamburg) 
23. 01. 2017 
TBA

30. 01. 2017 
TBA

17. 10. 2017 
Nicolas Wink (Heidelberg University)
Slides Finite temperature npoint functions from analytic continuationA formalism for the self consistent calculation of general nonperturbative npoint 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 
Simon Resch (JustusLiebigUniversity Giessen) Mass Sensitivity of the QCD Phase DiagramThe 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 quarkmeson 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. Nonperturbative fluctuations are captured within the functional renormalization group framework and the results compared to mean field calculations. 
18. 07. 2017 
Christian Steinwachs (AlbertLudwigsUniversity Freiburg)
Slides Quantum UV properties of Lifshitz theoriesLifshitz 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 
Sebastian Schenk (Heidelberg University) Perturbation Theory and GeometryThere 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 nonperturbative information about higher nonperturbative 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 allorders 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 
Bernhard Ihrig (Heidelberg University)
Slides Chiral critical behavior of Dirac materials: GrossNeveuYukawa model at three loopsDirac and Weyl Fermions appear as quasiparticle excitations in many condensedmatter 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 GrossNeveu model  the GrossNeveuYukawa model  at threeloop 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 
Gabor Almasi (GSI Darmstadt)
Slides Modeling chiral criticality and its consequences for heavyion collisionsA central question in heavyion 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 netproton fluctuations measured in heavyion 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 
Walid Mian (University of Graz & Heidelberg University)
Slides Formulating electroweak pion decays in functional methods and the influence of CPviolationDuring 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 nonperturbative treatment of both sector is necessary. The functional methods in from of the DysonSchwingerEquations, BetheSalpeterEquations and the FunctionalRenormalizationGroup provide such a nonperturbative tool. The dominant process in the neutron star is the betadecay. 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 betadecay. We hereby take the special features of the electroweak interaction into account, namely C and Pviolation. Our studies at the level of the quark propagator indicates, that the dynamical backcoupling of the C and Pviolation 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 
Aline Ramires (ETH Zuerich)
Slides LargeN: from a theoretical tool to the laboratoryHeavy 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 nonperturbative approaches, at which point largeN techniques become useful. If one requires the largeN treatment to preserve the fundamental properties of the spin under time reversal, one needs to introduce what is called the symplecticN 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 largeN 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 
Nicolo Defenu (Heidelberg University)
Slides Nonperturbative RG treatment of amplitude fluctuations for phi^4 topological phase transitionsThe study of the BerezinskiiKosterlitzThouless (BKT) transition in twodimensional phi^4 models can be performed in several representations, and the amplitudephase (AP) Madelung parametrization is the natural way to study the contribution of density fluctuations to nonuniversal 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 nonuniversal 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 
René Sondenheimer (FriedrichSchiller University Jena)
Slides Rethinking flavor physicsGaugeinvariant perturbation theory is an extension of ordinary perturbation theory, which describes strictly gaugeinvariant states in theories with a BroutEnglertHiggs effect. Such gaugeinvariant 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 
Aaron Held (Heidelberg University)
Slides Viability of quantumgravity induced ultraviolet completions for matterWe highlight how the existence of an ultraviolet completion for interacting StandardModel type matter puts constraints on the viable microscopic dynamics of asymptotically safe quantum gravity within truncated Renormalization Group flows. A first constraint – the weakgravity bound – is rooted in the destruction of quantum scaleinvariance in the matter system by strong quantum gravity fluctuations. A second constraint arises by linking Planckscale 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 
Vladyslav Shtabovenko (Technical University of Munich)
Slides Relativistic O(alpha_s^0 v^2) corrections to e+ e to chi_{cJ} gamma in NRQCDPhysics 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 Pwave $c\bar{c}$ 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 
Niklas Müller (Heidelberg University)
Slides The chiral anomaly, Berry's phase and chiral kinetic theory, from worldlines in quantum field theoryWe outline a novel chiral kinetic theory framework for systematic computations of the Chiral Magnetic Effect (CME) in ultrarelativistic heavyion collisions. The real part of the fermion determinant in the QCD effective action is expressed as a supersymmetric worldline action of spinning, colored, Grassmanian point particles in background gauge fields, with equations of motion that are covariant generalizations of the BargmannMichelTelegdi and Wong equations. Berry's phase is obtained in a consistent nonrelativistic 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 worldline 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 classicalstatistical simulations of the CME at early times and anomalous hydrodynamics at late times. 
02. 05. 2017 
Sebastian Wetzel (Heidelberg University)
Slides Detecting Phase Transitions with Artificial Neural NetworksIn this talk we explore how it is possible to identify phase transitions in physical systems with artificial neural networks. The methods range from feedforward 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 
Fleur Versteegen (Heidelberg University)
Slides Quantum gravity signatures in the Unruh effectWe 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, Lorentzinvariant corrections to the twopoint 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 KaluzaKlein 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 
Tobias Denz (Heidelberg University)
Slides Towards apparent convergence in asymptotically safe quantum gravityThe 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 threepoint functions. In the present work this expansion scheme is extended to the dynamical graviton fourpoint 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 nonclassical 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 [hepth] [2] arXiv:1403.1232[hepth] [3] arXiv:1506.07016 [hepth] 