# 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

#### 24. 10. 2017

Andreas Elben (IQOQI Innsbruck)

#### Renyi Entropies from Random Quenches in Atomic Hubbard and Spin Models

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)

#### 31. 10. 2017

Holiday (Reformationstag)

#### 07. 11. 2017

Workshop: Higgs Couplings (Heidelberg)

#### 14. 11. 2017

Johannes Lumma (Heidelberg University)

#### 21. 11. 2017

Eduardo Grossi (Heidelberg University)

TBA

#### 05. 12. 2017

Alexander Lehmann (Heidelberg University)

TBA

TBA

#### 09. 01. 2018

Igor Boettcher (Simon Fraser University, Vancouver)

#### 16. 01. 2018

Giovanni Rabuffo (DESY, Hamburg)

#### 23. 01. 2018

Alexander Stegemann (Goethe University, Frankfurt)

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

#### 17. 10. 2017

Nicolas Wink (Heidelberg University) Slides

#### Finite temperature n-point functions from analytic continuation

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

Simon Resch (Justus-Liebig-University Giessen)

#### Mass Sensitivity of the QCD Phase Diagram

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

Christian Steinwachs (Albert-Ludwigs-University Freiburg) Slides

#### Quantum UV properties of Lifshitz theories

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

Sebastian Schenk (Heidelberg University)

#### Perturbation Theory and Geometry

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

Bernhard Ihrig (Heidelberg University) Slides

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

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).

#### Modeling chiral criticality and its consequences for heavy-ion collisions

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

Walid Mian (University of Graz & Heidelberg University) Slides

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

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

Aline Ramires (ETH Zuerich) Slides

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

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

Nicolo Defenu (Heidelberg University) Slides

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

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

René Sondenheimer (Friedrich-Schiller University Jena) Slides

#### Rethinking flavor physics

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

Aaron Held (Heidelberg University) Slides

#### Viability of quantum-gravity induced ultraviolet completions for matter

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

Vladyslav Shtabovenko (Technical University of Munich) Slides

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

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

Niklas Müller (Heidelberg University) Slides

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

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

Sebastian Wetzel (Heidelberg University) Slides

#### Detecting Phase Transitions with Artificial Neural Networks

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

Fleur Versteegen (Heidelberg University) Slides

#### Quantum gravity signatures in the Unruh effect

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

Tobias Denz (Heidelberg University) Slides

#### Towards apparent convergence in asymptotically safe quantum gravity

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]