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# General info

Comprehensive DESY hep-th seminar calendar

Journal clubs take place online on Tuesdays from 13:00 to 14:00.

We collect potentially interesting papers to discuss here: link to paper list
Feel free to add papers to the list!
You can also use the comments section to suggest papers. Note that you can also add attachments and comment on other people's suggestions.

# Summer Semester 2022

### 2022-08-23: Kaushlendra Kumar (Uni Hannover): Yang–Mills solutions on Minkowski space via non-compact coset spaces

Abstract: We find a two-parameter family of solutions of the Yang-Mills equations for gauge group SO(1,3) on Minkowski space by foliating different parts of it with non-compact coset spaces with SO(1,3) isometry. The interior of the lightcone is foliated with hyperbolic space $//$, while the exterior of the lightcone employs de Sitter space $//$. The lightcone itself is parametrized by SO(1,3)/ISO(2) in a nilpotent fashion. Equivariant reduction of the SO(1,3) Yang-Mills system on the first two coset spaces yields a mechanical system with inverted double-well potential and the foliation parameter serving as an evolution parameter. Its known analytic solutions are periodic or runaway except for the kink. On the lightcone, only the vacuum solution remains. The constructed Yang-Mills field strength is singular across the lightcone and of infinite action due to the noncompact cosets. Its energy-momentum tensor takes a very simple form, with energy density of opposite signs inside and outside the lightcone.

### 2022-06-15: EXCEPTIONAL DATE: Enrico Olivucci (Perimeter Institute):  Null Polygons in Conformal Gauge Theory

Abstract: We consider correlation functions of single trace operators approaching the cusps of null polygons in a double-scaling limit where so-called cusp times $//$ are held fixed and the t'Hooft coupling is small. With the help of stampedes, symbols and educated guesses, we find that any such correlator can be uniquely fixed through a set of coupled lattice PDEs of Toda type with several intriguing novel features. These results hold for most conformal gauge theories with a large number of colours, including planar N=4 SYM.

The talk is based on 2111.12131.pdf  and 2205.04476.pdf

### 2022-06-07: Junchen Rong: Rydberg atoms, quantum dimer models and Wilson Fisher CFTs

Abstract: Recently, experimentalists have been using Rydbery atoms arranged on different lattices to realize exotic phases (and phase transitions) of condensed matter systems, such as the Z2 spin liquid phase. The systems can sometimes be mapped onto quantum dimer  models, which can be simulated using quantum monte carlo techniques. What's more, the low energy excitations of the system are described by scalar phi^4 theory. Some of the phases transitions of Rydbery atoms arrays are therefore described by the famous Wilson-Fisher CFTs. The talk is based on 2205.04472.pdf

### 2022-05-31: Zhenjie Li (CAS Beijing): Kinematics, cluster algebras and Feynman integrals

Abstract: In recent years, it has been found that cluster algebra plays important roles in analyzing and predicting singularities of amplitudes and Feynman integrals. In this talk, we identify cluster algebras for planar kinematics of conformal Feynman integrals in four dimensions, as sub-algebras of that for top-dimensional G(4,n) corresponding to n-point massless kinematics. We provide evidence that they encode information about singularities of such Feynman integrals, including all-loop ladders with symbol letters given by cluster variables and algebraic generalizations. As a highly-nontrivial example, we apply the method to an n=8 three-loop wheel integral, which contains a new square root, and bootstrap its result. By sending a point to infinity, our results have implications for non-conformal Feynman integrals. 2112.11842.pdf

### 2022-05-03: Alessandro Pini (Torino): Exact results in a N=2 SCFT at strong coupling

Abstract: We consider the N=2 SYM theory with gauge group SU(N) and a matter content consisting of one multiplet in the symmetric and one in the anti-symmetric representation of the gauge group. This theory is conformal and it admits a large-N 't Hooft expansion and a gravity dual given by a particular orientifold of AdS_5 x S^5. We analyze this theory relying on the matrix model provided by localization à la Pestun. Even if this matrix model has very non-trivial interactions, by exploiting the full Lie algebra approach to the matrix integration, we show that a large class of observables can be expressed in a closed form in terms of an infinite matrix depending on the 't Hooft coupling lambda. These exact expressions can be used to generate the perturbative expansions at high orders and also to analytically study the leading behavior at strong coupling. Finally we compare these predictions to a direct Monte Carlo numerical evaluation of the matrix integral and to the Padé resummation derived from very long perturbative series. Depending on time we also discuss the generalization of these results for a circular quiver gauge theory and the corresponding holographic interpretation

Three-point functions in a superconformal gauge theory and their strong-coupling limit: 2202.06990

Strong-coupling results for superconformal quivers and holography: 2109.0055

Exact results in a N=2 SCFT at strong coupling: 2105.15113

### 2022-04-19 (15:00): Joshua Cork (ITP, Hannover): On the instanton approximation of skyrmions

Abstract: The Skyrme model is a nonlinear field theory of nuclei which acts as a low-energy effective theory for QCD. The Skyrme model admits topological soliton solutions called skyrmions, which are physically identified with baryons. The Skyrme field equations do not allow for exact solutions, so over the years this has prompted several approximate descriptions to complement numerical simulations, and to allow a quantum treatment. One attempt (originally due to Atiyah and Manton) approximates skyrmions with the holonomy of Yang-Mills instantons. Although seemingly ad hoc, this approach is remarkably accurate. Recently, an holographic understanding due to Sutcliffe has explained its accuracy, putting the instanton approximation into a framework which is controlled, and not ad hoc, and allows for generalisation. In this talk, I will provide a survey of the instanton approximation. In particular I shall discuss some recent work of myself (joint with various collaborators) on how the instanton approximation can be generalised to study electromagnetic effects in the Skyrme model in a more realistic way to earlier attempts, and how to push the ordinary instanton approximation further to gain greater insights into quantum properties of skyrmions.

Inspire papers Joshua Cork

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