# Workshop on Strongly-Interacting Field Theories

**November 29 - December 1**

This workshop is founded by the DFG research training unit "Quantum and Gravitational Fields" (GK1523/1) and aims to bring together researchers from different areas of theoretical physics who study phases of strongly-interacting field theories. The main goal of the workshop is to provide a platform of the communication between people working on different aspects of strongly-interacting field theories, in order to allow for a transfer of knowledge and methods. The main topics are:

**gauge theories****fermionic systems****quantum gravity**

Speakers include:

- Reinhard Alkofer, Graz
- Dario Benedetti, Potsdam
- Falk Bruckmann, Regensburg
- Michael Buballa, Darmstadt
- Simon Catterall, Syracuse
- Shailesh Chandrasekharan, Duke
- Christian S. Fischer, Giessen
- Christof Gattringer, Graz
- Andrzej Goerlich, Krakow
- Lukas Janssen, Jena
- Daniel Körner, Jena
- Jack Laiho, Glasgow
- Kurt Langfeld, Plymouth
- Biagio Lucini, Swansea
- Axel Maas, Jena
- Jan Pawlowski, Heidelberg
- Owe Philipsen, Frankfurt
- Frank Saueressig, Mainz
- Urs Wenger, Bern

## Program

Program as PDF## Talks

#### Reinhard Alkofer

"Aspects of confinement and the center symmetry phase transition from QCD correlation functions"- The status of some studies of Landau gauge QCD Green functions within functional continuum methods is reported. The renormalization of two-loop terms in a Dyson-Schwinger equation and the corresponding result for including a gluonic sunset diagram in a self-consistent calculation are given. Results for the propagators of adjoint quarks are shown. On the other hand, to introduce matter not only quarks but also scalar fields in the fundamental representation are considered. Some preliminary numerical results on the gluon-matter-antimatter vertex functions are presented. Starting from the hypothesis that the linearly rising static confinement potential is contained in the matter-antimatter four-point function the consequences of a corresponding $1/k^4$ infrared divergence on all other correlation functions are studied. Restrictions on the general color tensor structure of the infrared divergent part of correlation functions are derived. It is shown that (i) possible singular contributions to the three-point functions vanish due to cancellations within the color algebra and (ii) a straightforward explanation of Casimir scaling follows then directly from these constraints. The propagators of fundamentally charged scalars and quarks at non-vanishing temperatures and their properties with respect to center symmetry are investigated. Novel dual order parameters are extracted from the propagators. For quarks the newly suggested order parameter compares well with the dual chiral condensate but has the advantage that no regularization is necessary even at non-zero quark masses.

#### Falk Bruckmann

"QCD probed by strong magnetic fields"#### Michael Buballa

"Inhomogeneous phases in strong interaction physics"#### Simon Catterall

"A supersymmetric lattice theory: N=4 Yang-Mills"#### Shailesh Chandrasekharan

"Fermion-bag approach to strongly interacting massless fermions"- We discuss a new Monte Carlo method to solve some strongly interacting fermionic lattice field theories containing massless fermions. Instead of integrating over all fermion degrees of freedom as is conventionally done, we group fermion degrees of freedom and integrate within each group separately. Each group is made up of regions of space time that resembles a bag and hence we call the new approach the fermion-bag approach. The new approach helps solve new sign problems and allows us to formulate efficient Monte Carlo algorithms in the massless limit. We discuss results for critical exponents obtained at a quantum critical point in a four-fermion field theory using the new method. References: Phys.Rev.Lett. 108 (2012) 140404; PoS LATTICE2011 (2011) 058; Phys.Rev. D85 (2012) 091502; Phys.Rev. D86 (2012) 021701.

#### Christian S. Fischer

"Phase transitions in strong QED"#### Christof Gattringer

"Lattice field theories with dual variables"#### Andrzej Goerlich

"Causal dynamical triangulations: transfer matrix"- In this talk I will give an introduction to the model of causal dynamical triangulations (CDT) which is a background independent approach to quantum gravity. Next I will present recent developments in this field. I will show that the measurements of the scale factor of the (CDT) universe are well described by an effective transfer matrix which elements are labeled only by the scale factor. I will discuss how to determine the effective action and give evidence that it agrees with the 'minisuperspace' model.

#### Lukas Janssen

"Critical Phenomena in the (2+1)-dimensional Thirring model"- On the one hand, relativistic fermion models in 2+1 dimensions have very interesting applications as effective theories for novel low-dimensional condensed matter systems, such as high-Tc superconductors or graphene. On the other hand, they can serve as prototypes for strongly-interacting fermion field theories. In this talk, I review recent work on chiral symmetry breaking and its accompanying critical behavior as a function of the number of fermion flavors. Previous analytical and numerical calculations point to the existence of a critical flavor number, above which a chiral phase transition is prohibited. However, the results have been quantitatively and qualitatively inconsistent. I will show that the functional renormalization group approach allows to give (1) in a purely fermionic description a microscopic explanation for the occurrence of a critical flavor number and (2) in a dynamically bosonized formulation detailed quantitative predictions for the critical flavor number and the universal critical exponents. References: L. Janssen and H. Gies, arXiv:1208.3327. H. Gies and L. Janssen, Phys. Rev. D 82 (2010) 085018, arXiv:1006.3747.

#### Daniel Körner

"Supersymmetric lattice models with and without gauge fields"#### Jack Laiho

"Results from Euclidean Dynamical Triangulations"#### Kurt Langfeld

"New methods for the simulation of cold and dense matter"- Simulating matter at finite densities and low temperatures is an outstanding problem in particle physics. In my talk, I review some of the "traditional" methods and then summarise recent promising methods to address this issue. I point out that for a direct simulation using the fundamental degrees of freedom of the theory an exponential error suppression is needed. I then show that the "density of states approach" is just delivering that 1. Subsequently, I illustrate the potential of methods which make use of a reformulation of the theory. Among those, I address the role of the duality transform and the infamous "worm algorithms" in the context of the quantum O(2) model. References: K. Langfeld, B. Lucini and A. Rago, Phys. Rev. Lett. 109 (2012) 111601, arXiv:1204.3243

#### Biagio Lucini

"Mass-deformed infrared conformal gauge theories"- I review general expectations for the spectrum of an infrared conformal gauge theory after soft-breaking conformality with a small mass term for the fermions. In the case of SU(2) gauge theory coupled with two flavours of adjoint Dirac fermions, results from lattice simulations will be compared with general arguments. I show how mass deformation can be exploited to compute the anomalous dimension of the chiral condensate, which is a quantity of phenomenological relevance for strongly interacting dynamics beyond the standard model. Finally, I will discuss the current status of lattice calculations and possible future developments.

#### Axel Maas

"G2 gauge theories"#### Jan Pawlowski

"Thermodynamics & transport coefficients from QCD correlationfunctions"

- In this talk I give an update of the approach to QCD correlation functions with functional continuum methods. At vanishing temperature meson masses, decay constants and the constituent quark masses are determined. At finite temperature and density the correlation functions are used for the computation of the phase structure of QCD, the thermodynamics and transport coefficients, i.e. the shear viscosity-entropy ratio.

#### Owe Philipsen

"Finite density QCD from an effective lattice theory"#### Frank Saueressig

"A functional renormalization group equation for foliated space-times"#### Urs Wenger

"Spontaneous supersymmetry breaking on the lattice"- Simulations of supersymmetric models on the lattice with (spontaneously) broken supersymmetry suffer from a fermion sign problem related to the vanishing of the Witten index. We discuss a novel approach which evades this problem in low dimensions by formulating the path integral on the lattice in terms of fermion loops. In this context we present exact results on the spectrum and the Witten index for N=2 supersymmetric quantum mechanics and results from simulations of the N=1 Wess-Zumino model.

## Posters

#### Natalia Alkofer

"Dimensional Reduction in Asymptotically Safe Gravity"#### Maximilian Demmel and Omar Zanusso

"The flow of the geometric effective average action and nonperturbativeresults for the nonlinear sigma model"

#### Christian Fitzner

"New solutions to the massless Gross-Neveu-Model in the 't Hooft limit:Scattering kinks, baryons and breathers"

#### Markus Hopfer

"The role of the quark-gluon vertex in the QCD phase transition"#### Markus Huber

"The influence of three-point functinos on the propagators of Landau gauge Yang-Mills theory"#### Andreas Rodigast

"Fixed Points and Infrared Completion of Quantum Gravity"#### Bjorn Wellegehausen

"TBA"#### Andreas Windisch

"The analytic structure of scalar glueball operators at Born-level"## List of Participants

- Natalia Alkofer, Graz
- Reinhard Alkofer, Graz
- Dario Benedetti, Potsdam
- Georg Bergner, Frankfurt
- Alexander Blinne, Jena
- Falk Bruckmann, Regensburg
- Michael Buballa, Darmstadt
- Simon Catterall, Syracuse
- Shailesh Chandrasekharan, Duke
- Maximilian Demmel, Mainz
- Christian S. Fischer, Giessen
- Christian Fitzner, Erlangen
- Dvin Galstian, Jena
- Christof Gattringer, Graz
- Holger Gies, Jena
- Andrzej Goerlich, Krakow
- Tobias Hellwig, Jena
- Markus Hopfer, Graz
- Markus Huber, Darmstadt
- Lukas Janssen, Jena
- Daniel Korner, Jena
- Felix Karbstein, Jena
- Jack Laiho, Glasgow
- Stefan Lippoldt, Jena
- Kurt Langfeld, Plymouth
- Biagio Lucini, Swansea
- Axel Maas, Jena
- Tajdar Mufti, Jena
- Jan Pawlowski, Heidelberg
- Owe Philipsen, Frankfurt
- Andreas Rodigast, Heidelberg
- Frank Saueressig, Mainz
- Rene Sondenheimer, Jena
- Bjorn Wellegehausen, Jena
- Urs Wenger, Bern
- Andreas Windisch, Graz
- Andreas Wipf, Jena
- Omar Zanusso, Mainz

The workshop will be held at:

Haus "Zur Rosen"

Johannisstraße 13

07743 Jena

Größere Kartenansicht

Arrival by plane:

Airport Halle/Leipzig (+ 1h train ride, one-way ticket ~35 Euros )

Airport Berlin (Tegel or Schoenefeld) (+ 3h train ride, one-way ticket ~65 Euros)

Airport Frankfurt (+ 3h train ride, one-way ticket ~75 Euros)

Airport Munich (+4h train ride, one-way ticket ~85 Euros)

Arrival by train:

There are two train stations in Jena: Jena-Paradies (directions Berlin or Munich), and Jena-West (direction Frankfurt). The hotels as well as the conference site are in walking distance from both stations (taxi ride ~ 5-10 Euros)