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# General info

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Journal clubs are on Tuesdays from 13 to 14 in Seminar Room 3 (building = 1).

Feel free to suggest papers in the comments section. Note that you can a= lso add attachments and comment on other people's suggestions.

**Important: You need to login to see and add comments.**

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# Spring semester 2020&=
nbsp;

## Journal club 07-07=
-2020

## Journal club 19-05=
-2020

## Journal club 21-04=
-2020

## Journal club 07-04=
-2020

## Journal club 09-03=
-2020

## Journal club 03-03=
-2020

## Journal club =
25-02-2020 (TBA)

## Journal club 18-02=
-2020

**
**## Journal club 11-02-2020 (Integrability school)

## Journal club 04-02=
-2020

## Journal club 28-01=
-2020

## Journal club 21-01=
-2020

## Journal club 10-12=
-2019

## Journal club 03-12=
-2019

## Journal club 26-11=
-2019

## Journal club 19-11=
-2019

## Journal club 12-11=
-2019

## Journal club 29-10=
-2019

## Journal club 22-10=
-2019

## Journal club 15-10=
-2019

# Summer 2019

## Journal club 08-10=
-2019

Journal club= 14-07-2020

- Alexander T. Kristensson (Niels Bohr Institute), "Thermodynamics of N=
=3D4 SYM at finite N"

Abstract: The maximally Supersymmetric Yang Mi= lls (SYM) theory is an SU(N) gauge theory that occurs many places in theore= tical high-energy physics -- most notably in the famous AdS/CFT duality. On= a compact space RxS^3, N=3D4 SYM theory exhibits a phase transition simila= r to the confinement/deconfinement phase transition of Quantum Chromo Dynam= ics (QCD). On the gravity side of the duality, this phase transition is bel= ieved to be dual to the Hawking-Page phase transition between a gas of grav= itons and a black hole. In the last twenty years, there has been much progr= ess in understanding the planar limit of the theory, where one takes N ->= ; infinity. This progress has shown strong hints of the theory being integr= able. The main focus of our research is to push this understanding beyond t= he planar limit to finite values of N. In this talk I will present our ongo= ing research on the thermodynamics of N=3D4 SYM at finite N. In particular,= I will discuss various sub-sectors of the full algebra and show how to com= pute the exact partition functions for small values of N. In the su(2) sect= or, I will present the claim that the Hagedorn behaviour at N -> infinit= y is replaced by a phase transition of Lee-Yang type at finite N, character= ized by a condensation of zeros in the complex fugacity plane.

- Matthias Volk (Niels Bohr Institute), "Geometry of Feynman Integrals in=
Twistor Space"

Abstract: To compute scattering amplitudes in quantu= m field theory perturbatively, one has to evaluate integrals over loop mome= ntum space known as Feynman integrals. In simpler cases (e.g. for low loop = order) the integrals can often be expressed in terms of so-called multiple = polylogarithms. However, explicit computations have shown that this fails f= or more complicated amplitudes and that new functions will be needed. One a= pproach to a better understanding of these functions has been to analyze th= e geometry associated to a Feynman integral and interesting geometric objec= ts such as elliptic curves, K3 surfaces and Calabi-Yau manifolds have been = found. In this talk, we construct and analyze the geometry of a certain cla= ss of Feynman integrals known as traintracks that occur for example in N = =3D 4 SYM or \phi^4 theory. In contrast to previous work, we describe the g= eometry directly in momentum twistor space. At two loops, we obtain an elli= ptic curve as the intersection of two quadrics. At three loops, we find a K= 3 surface as a branched surface over two elliptic curves in P^1 x P^1. Base= d on arXiv:2005.08771 together with C. Vergu.

- Junchen Rong, "Bootstrap and deconfined p=
hase transition""

Abstract:&nbs= p; I will discuss how using conformal bootstrap to study the deconfined qua= ntum

critical point (DQCP), which describes the phase transition between= anti-ferromagnetic Neel phase and the valence bond solid state phase. The = continuum limit of the model is described by the three dimensional Abelian = Higgs model.

- Niklas Henke, How tropical are seven-, eight- and nine-particle scatter=
ing amplitudes?

Abstract,

Using the symbol bootstrap, loop amplit= udes of planar N=3D4 SYM theory can be obtained from their alphabet, the li= st of their singularities, whose letters coincide with the variables (ratio= nal functions) of certain cluster algebras. However, at eight and more part= icles these cluster algebras become infinite, whereas it is believed that t= he amplitudes only have finitely many distinct singularities. First discuss= ing the application of cluster algebras to the seven-particle (N)MHV amplit= ude, I will show how the recently introduced mathematics of tropical geomet= ry gives rise to a selection rule and present the thus obtained finite alph= abets for eight and nine particles. Furthermore, I will discuss how infinit= e mutation sequences in cluster algebras also give rise to the algebraic si= ngularities at eight particles.

- Fabrizio Neri: Physics and geometry of knots-quivers corresp= ondence

- Matteo Parisi, Amplituhedra: Scattering Amplitudes from Geometry

Abstract:The Amplituhedra A(n,k,m) are generalisations of polytopes= introduced as a geometric construction encoding scattering amplitudes in p= lanar N=3D4 supersymmetric Yang-Mills theory (SYM). These are extracted fro= m a differential form, the canonical form of the Amplituhedron, which emerg= es from a purely geometric definition.

Following my recent works, I w= ill explain how the Jeffrey-Kirwan residue, a powerful concept in symplecti= c and algebraic geometry, computes the canonical form for whole families of= objects, namely for Amplituhedra of type A(n,1,m), which are cyclic polyto= pes and for their conjugates A(n,n-m-1,m) for even m, which are not polytop= es.

This method connects to the rich combinatorial structure of trian= gulations of Amplituhedra, captured by what we refer to as =E2=80=98Seconda= ry Geometry=E2=80=99. For polygons, this is the `Associahedron', explored b= y Stasheff in the sixties; for polytopes, it is the `secondary polytope' co= nstructed by the Gelfand's school in the nineties. Whereas, for Amplituhedr= a, we are the first to initiate the studies of what we called the =E2=80=98= Secondary Amplituhedra=E2=80=99. The latter encodes all representations of = scattering amplitudes, many not obtainable with any physical method, togeth= er with their algebraic relations produced by global residue theorems.

<= p>Finally, I will briefly illustrate some of the recent geometric direction= s in my work on the Amplituhedron in momentum space and new exciting develo= pments connecting the (secondary geometry of) m=3D2 amplituhedron with the = positive tropical Grassmannian. This object has been appearing in dozens of= papers in the physics community in the last year, both in bootstrapping lo= op amplitudes in planar N=3D4 SYM and in computing (a generalisation of) bi= adjoint scalar amplitudes.

- Lorenzo Quintavalle, Celestial Sphere Amplitudes

Abstract:

Ov= er the recent years there has been many developments in the study of four-d= imensional Quantum Field Theories through a CFT description on the Celestia= l sphere. The aim of this Journal club is to review the basic ideas behind = Celestial sphere amplitudes. We will start from the discussion of asymptoti= c symmetries in electrodynamics, to then describe soft theorems taking the = case of Scalar QED as an example. We will see how this suggests a 2D CFT in= terpretation, and therefore introduce the concept of Celestial sphere ampli= tudes.

- Madalena Lemos (CERN), Surface defects in 4d superconformal theori=
es and chiral algebras (This seminar will take place in
**= Building 3 Seminar room BAH 2<= /strong>****)**Abstract:

We study symmetry constraints on= BPS surface defects in four-dimensional superconformal field theories, sho= wing how supersymmetry imposes relations on anomaly coefficients. Turning t= o dynamics, we analyze a protected subsector of N=3D(2,2) surface defects t= hat is captured by a two-dimensional chiral algebra. We discuss how to comp= ute observables of interacting defects from the chiral algebra, including t= he aforementioned anomaly coefficients.

- Taro Kimura (Bourgogne U.), Yet another affinization of geometric Langl=
ands correspondence
Abstract:

One of the implications of the geom= etric Langlands correspondence is the isomorphism between conformal blocks = of W-algebra and affine Lie algebra. This correspondence has a natural q-de= formation, providing a relation between q-deformation of W-algebra and quan= tum affine algebra. In this talk, I'll discuss yet another affinization of = this correspondence between doubly affine W-algebra and quantum toroidal al= gebra, based on the formalism of quiver W-algebra. I'll also mention its po= ssible physical interpretation in gauge theory with the surface defects.

- Federico Carta (DESY), Supersymmetry Enhancement

- Lorenzo Quintavalle (DESY), Celestial sphere amplitudes

- Fabrizio del Monte (SISSA), Class S theories and isomonodromic deformat=
ions on the torus.

Abstract:In the last f= ew years there have been many new results connecting (linear quiver) N=3D2 = class S theories, and the topological strings that engineeer them, to the t= heory of isomonodromic deformations on the sphere and their q-deformations.= These gauge theories are constructed by compactifying the 6d N=3D(0,2) SCF= T on a punctured Riemann Sphere, whose moduli, which are the marginal defor= mations of the gauge theory, are the times of the isomonodromic flows. The = aim of this talk is to show how this connection can be extended beyond the = case of genus zero, for more general (asymptotically superconformal) class = S theories. We will discuss in detail the case of circular quiver gauge the= ories, that are obtained from the N=3D(0,2) SCFT on punctured tori, and see= how the genus one case displays new qualitative features that are absent o= n the sphere, due to the possibility of various inequivalent vector bundles= , and how this actually provides new interesting relations satisfied by the= gauge theory partition function.

Jack Foster (Southampton University): Cluste= r Adjacency, Tropical Geometry, and Scattering Amplitudes

Abstract: I will discuss two new areas of inte= rest in scattering amplitudes: cluster adjacency and tropical geometry. The= former describes how the analytic structure of planar amplitudes in N=3D4 = Super Yang-Mills is controlled by mathematical objects called cluster algeb= ras. The latter has been used to calculate amplitudes in the biadjoint phi^= 3 theory, which I will discuss briefly, but it also has implications for cl= uster adjacency.

- Francesco Galvano (Torino): =
;Emitted radiation and geometry <=
/span>(This seminar will take place =
in
**Building 3 Seminar ro= om BAH 2**)&nbs= p;

Abstract: We discuss the computation of the radiated energ= y by an accelerated heavy particle. This quantity is captured by the one-po= int function of the stress energy tensor in presence of a Wilson line. In a= N=3D2 superconformal theory we prove that this observable is exactly relat= ed to a small geometric deformation of the background geometry. In a four d= imensional case, supersymmetric localization allows to express the emitted = energy in terms of a matrix model on a squashed sphere.

- No seminar scheduled.

- Zhengwen Liu: Scat=
tering Equations and Multi-Regge Kinematics

Abstract**:**The scattering equations, a system of alg= ebraic equations connecting the space of kinematic invariants and the modul= i space of punctured Riemann spheres, provide a novel w= ay to construct scattering amplitudes. In this framework, the tree-level S-= matrix in many quantum field theories can be reformulated as a multiple int= egral that is entirely localized on the zeroes of the scattering equations.= After presenting a very minimal introduction to the scattering equations, = I will discuss the asymptotic behavior of the scattering equations in the s= o-called Multi-Regge kinematic regime and the corresponding factorizations = of amplitudes in gauge theory and gravity.

References**:**Multi-Regge kinematics and the scattering equations<= /a>

Gravitationa= l Scattering in the High-Energy Limit

Journal club= 05-11-2019

- Yuta Sekiguchi (U. Bern, AEC), O(d,d) transformations preser=
ve classical integrability.
Abstract: We stud= ied the classical integrability of O(d,d) transformations including not onl= y O(d,d;Z) duality but also global O(d,d;R) deformation. The latter is know= n in the traditional literature to generate so-called current-current (JJba= r) deformations of 2D CFTs. In this talk, I first plan to give brief review= s Yang-Baxter deformations of string backgrounds as well as the doubled sig= ma model. Then I will present how to construct the Lax pairs in the O(d,d) = deformed WZNW models via O(d,d) map through very easy examples. The resulti= ng Lax connections are in general non-local because they depend on the wind= ing modes. Finally I will briefly comment on open questions under considera= tion in relation to the recent irrelevant integrable deformation. This talk= is based on the recent work [1907.03759] with Domenico Orlando, Susanne Re= ffert and Kentaroh Yoshida.

Till Bargheer: Exact Four-Point Functions: G= enus Expansion and Strong Coupling (This seminar will take place in

**= Building 3 Seminar room BAH 2**)

Using integrability, correlation functions of local = operators in planar N=3D4 super Yang-Mills theory can be computed by recons= tructing the dual worldsheet from a set of "hexagon" patches via "gluing" (= performing complete state sums over mirror Bethe states). After reviewing t= his general procedure, I will explain how it can be used to compute a certa= in class of four-point correlators as an exact function of the coupling at = any order in the 1/Nc genus expansion, and how the resulting series can be = re-summed (via a suitable matrix model) to recover the full Nc dependence. = If time permits, I will also comment on the strong-coupling limit of these = correlators.

Gleb Kotousov: "Reflection operators in inte= grable QFT".

Sylvain Lacroix, Gauge Theory And= Integrability, III (focusing on the second part about "disorder defect= s").

Alessandro Pini, Extremal Correla= tors and Random Matrix Theory.

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