spring 2022

KCL Strings Journal Club Spring 2022

Included page "clone:kclstrings" does not exist (create it now) Spring 2022 Journal Club Here's a summary of all the activity of our Journal Club during the Spring 2022 term.

Summary

Wednesday June 29th: Simon Ekhammar Time: 13:45h LIVE seminar. Room: K0.20 Title: Analytic Q-systems and AdS3/CFT2 Quantum Spectral Curve Abstract: The Quantum Spectral Curve (QSC) is a powerful integrability-based method capable of computing the spectrum of planar N=4 SYM. It has also been generalised in many directions, for example to cusped Wilson lines and various deformations. The success of the QSC motivates trying to extend the formalism beyond N=4 to other theories. This requires the study of the underlying structure of the QSC, a so called analytic Q-system. To construct an analytic Q-system it is necessary to specify both its algebraic structure, usually encoded into QQ-relations, and its analytic properties. I will talk about recent work to study Q-systems beyond the ones relevant for N=4, discussing both their algebraic and analytic properties. In particular I will discuss the recent conjecture of a QSC for AdS3/CFT2 which non-trivially couples two different Q-systems. While the curve shares many features with the N=4 QSC it also offers new surprises and challenges. If this new curve can be brought under full control and further tested many interesting applications and generalisations are within reach. Wednesday June 22nd: Ines Varela Aniceto Time: 13:45h LIVE seminar. Room: K0.20 Title: Hydrodynamization, asymptotics and the early to late time interpolation in relativistic hydrodynamics Abstract: Dissipative relativistic hydrodynamics is expected to describe the late times, thermalised behaviour of strongly coupled fluids such as a strongly coupled super Yang-Mills plasma. These systems are then accurately described by a hydrodynamic series expansion in small gradients. Surprisingly, this hydrodynamic expansion is accurate even when the systems are still quite anisotropic: the non-hydrodynamic modes governing the non-equilibrium behaviour at very early-times become exponentially close to the hydrodynamic solution in an early process called hydrodynamization. This early success is intimately related with the fact that the hydrodynamic expansion is asymptotic. The theory of transseries and resurgence explicitly shows how the non hydrodynamic modes are in fact encoded in this late-time expansion. In this talk we will focus on a MIS-type model and use exponentially accurate summations of the the late-time resurgent transseries to recover the behaviour of the fluid before hydrodynamisation, and effectively match it to any given initial non-equilibrium condition. We will further show that such summations can provide analytic predictions beyond the late time regime. Wednesday June 15th: Vladimir Kazakov Time: 13:45h LIVE seminar. Room: K0.20 Title: Bootstrap for matrix models and lattice Yang-Mills theory Abstract: I will speak about my recent work with Zechuan Zheng where we study the SU(Nc) lattice Yang-Mills theory in the t Hooft limit Nc -> infinity, at dimensions D=2,3,4, via the numerical bootstrap method. It combines the Makeenko-Migdal loop equations, with the cut-off L on maximal length of wilson loops, and the positivity conditions on certain correlation matrices. We thus obtain rigorous upper and lower bounds on plaquette average at various couplings. The results are quickly improving with the increase of the cutoff L. In particular, for D=4 and L=16, the upper bound data in the most interesting weak coupling phase are not far from the Monte-Carlo results and they reproduce well the 3-loop perturbation theory. We also attempt to extract the information about the gluon condensate from this data. Our results suggest that bootstrap can provide a tangible alternative to, so far uncontested, Monte Carlo approach. I will also mention our bootstrap results for an "unsolvable" two-matrix model in the large N limit, where this method appears to be superior in efficiency over Monte Carlo. Wednesday May 18th: Pietro Ferrero Time: 13:45h LIVE seminar. Room: K0.20 Title: Bootstrapping the Maldacena Wilson line at strong coupling Abstract: The insertion of local operators along a straight Maldacena Wilson line in planar N=4 super Yang Mills defines a defect supersymmetric conformal field theory in one dimension. This is a simple but interesting setup where one can combine field theory techniques such as bootstrap, integrability and localization, aiming at a full solution of a non-trivial quantum mechanical system. I will adopt a bootstrap approach and study correlation functions of local operators for large t Hooft coupling, where the system is dual to an open superstring in AdS5×S5. I will present results for the four-point function of the displacement multiplet of the 1d defect CFT corresponding to three-loop diagrams in AdS, which are obtained using a suitable position-space ansatz and after considering a large system of mixed correlators. The problem is made particularly hard by the large degeneracy of operators at strong coupling, which we solve by taking into account four-point functions with external unprotected operators. The simple 1d kinematics is an ideal toy model for bootstrap techniques of interest for higher-dimensional cases as well. The seminar will be based on published as well as ongoing work with Carlo Meneghelli (see arxiv:2103.10440). Wednesday May 11th: Elizabeth Himwich Time: 13:45h LIVE seminar. Room: K0.20 Title: Celestial Operator Product Expansions and w(1+infinity) Symmetry for All Spins Abstract: The operator product expansion of massless celestial primary operators of arbitrary spin is investigated. Poincaré symmetry is found to imply a set of recursion relations on the operator product expansion coefficients of the leading singular terms at tree-level in a holomorphic limit. The symmetry constraints are solved by an Euler beta function with arguments that depend simply on the right-moving conformal weights of the operators in the product. These symmetry-derived coefficients are shown not only to match precisely those arising from momentum-space tree-level collinear limits, but also to obey an infinite number of additional symmetry transformations that respect the algebra of w(1+infinity). In tree-level minimally-coupled gravitational theories, celestial currents are constructed from light transforms of conformally soft gravitons and found to generate the action of w(1+infinity) on arbitrary massless celestial primaries. Results include operator product expansion coefficients for fermions as well as those arising from higher-derivative non-minimal couplings of gluons and gravitons. Friday May 6th: Adolfo Cisterna Time: 1pm LIVE seminar. Room: K0.20 Title: (A)dS Taub-NUT and exact black bounces with scalar hair Abstract: We present a new family of exact four-dimensional Taub-NUT spacetimes in Einstein-Λ theory supplemented with a conformally coupled scalar field exhibiting a power-counting super-renormalizable potential. Our configurations are constructed in the following manner: A solution of a conformally coupled theory with a conformal potential, henceforth the seed (gμν,φ), is transformed by the action of a specific change of frame in addition with a simultaneous shift of the seed scalar field. The conformal factor of the transformation and the shift are both affine functions of the original scalar φ. The new configuration, (ḡμν , φ̄), solves the field equations of a conformally coupled theory with the extended aforementioned super-renormalizable potential, this under the presence of an effective cosmological constant. The new spectrum of solutions is notoriously enhanced with respect to the original seed containing regular black holes, wormholes, and bouncing cosmologies. We highlight the existence of two types of exact black bounces given by de Sitter and anti-de Sitter geometries that transit across three different configurations each. The de Sitter geometries transit from a regular black hole with event and cosmological horizons to a bouncing cosmology that connects two de Sitter Universes with different values of the asymptotic cosmological constant. An intermediate phase, which might be represented by two different configurations, takes place. These configurations are given by a de Sitter wormhole or by a bouncing cosmology that connects two de Sitter Universes, both under the presence of a cosmological horizon. On the other hand, the anti-de Sitter geometries transit from a regular black hole with inner and event horizons to a wormhole that connects two asymptotic boundaries with different constant curvatures. The intermediate phase is given in this case by an anti-de Sitter regular black hole with a single event horizon. This regular black hole might appear in two different configurations. As a regular anti-de Sitter black hole inside of an anti-de Sitter wormhole or as an anti-de Sitter regular black hole with a cosmological bounce in its interior. All these geometries are shown to be smoothly connected by the mass parameter only. Other standard stationary black holes, bouncing cosmologies and wormholes are also identified. Wednesday May 4th: Jakob Salzer LIVE seminar. Room: K0.20 Title: w-infinity Symmetry of Quantum Self-Dual Gravity Abstract: General Relativity in asymptotically flat spacetimes gives rise to an infinite number of symmetries which form the celebrated BMS group comprising superrotations and supertranslations. These symmetries are closely related to soft theorems of gravitational scattering amplitudes. Recently it was shown that supertranslations and superrotations are only the lowest levels of a whole tower of symmetries of tree level gravitational scattering amplitudes that form a w-infinity algebra. The fate of this symmetry once loop effects are taken into account is currently unknown. In this talk I will review the emergence of this symmetry algebra based on the celestial CFT program and argue that the w-infinity algebra persists quantum corrections in self-dual gravity. This talk is based on 2111.10392 with A.Ball, S. Narayanan, and A. Strominger. Wednesday April 20, @ 13:45h: José Figueroa-O'Farrill ONLINE seminar. Title: Possible kinematics for flat space holography Abstract: I will discuss recent and ongoing work with Emil Have, Stefan Prohazka and Jakob Salzer on possible kinematics for flat space holography. I will discuss how a seemingly novel projective compactification of Minkowski spacetime reveals a rich asymptotic geometry homogeneous under the Poincaré group and including the blow-ups at spatial and timelike infinities as well as a novel four-dimensional space intimately associated to null infinity. This allows for novel geometric descriptions of the Minkowski asymptotic geometries and gives us a glimpse of the asymptotic geometry of asymptotically flat spaces. Wednesday April 13, @ 13:45h: Irene Valenzuela LIVE seminar. Room: K0.20 Title: Swampland Conjectures from Finiteness of Black Hole Entropy Abstract: Consistency with quantum gravity can impose non-trivial constraints at low energies, even if the Planck scale is at very high energy. The Swampland program aims to determine the constraints that an effective field theory must satisfy to be consistent with a UV embedding in a quantum gravity theory. One of the most important swampland conditions is the presence of infinite towers of states becoming massless at the weak coupling/large field limits. This has been extensively tested in string theory compactifications, but a bottom-up explanation was missing. In this talk I will provide a possible explanation based on finiteness of black hole entropy. I will also explain how several wampland criteria, including the Weak Gravity Conjecture, Distance Conjecture and bounds on the finiteness of the quantum gravity vacua, may be more fundamentally a consequence of the finiteness of quantum gravity amplitudes. Wednesday March 30, @ 13:45h: Eleni Kontou LIVE seminar. Room: K0.20 Title: Singularity theorems in semiclassical gravity Abstract: The classical singularity theorems predict the existence of singularities, defined using incomplete geodesics, under a set of general assumptions. One of those assumptions, namely the energy condition, is always violated by quantum fields and thus the realm of semiclassical gravity is outside the scope of these theorems. However, quantum fields do obey weaker conditions which can also be used to predict singularities. In this talk, I will present derivations of such semiclassical singularity theorems both in the timelike and the null case and discuss the challenges and open questions for each case. Wednesday March 23, @ 13:45h: Avner Karasik LIVE seminar. Room: K0.20 Title: Anomalies for anomalous symmetries. Abstract: 4d gauge theories with massless fermions typically have axial U(1) transformations that suffer from the ABJ anomaly. One can modify the theory of interest by adding more fields in a way that restores the axial symmetry, and use it to derive rigorous't-Hooft anomaly matching conditions. These conditions are not valid for the original theory of interest, but for the modified theory. I will show that the modification can be done in a specific way that allows us to relate the dynamics of the modified theory to the dynamics of the original theory. In this way, the anomaly matching conditions of the modified theory can be used to learn new things on the original theory even though they involve axial transformations which are not a symmetry of the original theory. In the talk I will describe this method and discuss some applications to various examples. Wednesday March 16, @ 13:45h: Sean Hartnoll LIVE seminar. Room: K0.20 Title: Entanglement in the quantum Hall matrix model Abstract: Quantum mechanical theories describing large N by N matrices of oscillators can lead to an emergent space as N -> infinity. In the most fully fledged version, the emergent space is dynamical and gravitating. However, there are also simpler, lower dimensional versions of this phenomenon. One of the simplest occurs in the so-called quantum Hall matrix model, in which a 2 dimensional space emerges and supports Chern-Simons dynamics. I will describe how this solvable model leads to insights about the emergence of space from matrices. In particular, I will describe how the emergent spatial locality is reflected in the entanglement structure of the ground state of theory. Wednesday March 9, @ 13:45h: Michele del Zotto LIVE seminar. Room: K0.20 Title: Global structures from the infrared Abstract: Quantum field theories with identical local dynamics can admit different choices of global structure, leading to different partition functions and spectra of extended operators. Recent work has determined the structure of such choices via geometric methods for various classes of non-Lagrangian theories obtained from stringy geometric engineering techniques. In this talk I will discuss a purely field theoretical counterpart of this analysis, showing that global structures can be captured from a careful analysis of the infrared Coulomb-like phases. Our results confirm and extend the many results obtained within geometric engineering about the global structures of Argyres-Douglas theories, 5d SCFTs and 6d SCFTs. Wednesday March 2, @ 13:45h: Kamran Salehi Vaziri LIVE seminar. Room: K0.20 Title: Towards non-perturbative cosmological bootstrap: 1st gear Abstract: We are going to cover some motivations and basics behind this study from QFT in AdS and the conformal bootstrap. Then we develop the basic tools needed to deal with QFT in dS from a conformal bootstrap perspective. Then we recover the Källen-Lehmann decomposition of bulk two-point functions and derive an inversion formula for the spectral density and its relation to boundary CFT data. Next, we focus on the conformal partial wave decomposition of four-point functions of boundary operators and find their positivity properties. As a proof of principle, we will conclude with a concrete bound for the case of d=1. Wednesday February 23, @ 13:15h: Johan Henriksson Room: K0.20 Title: Higher-Genus Partition Functions from Error-Correcting Codes Abstract: In this talk I will present some recent and ongoing work with Ashish Kakkar and Brian McPeak, where we describe a very explicit construction of a class of two-dimensional conformal field theories, denoted code CFTs. In the chiral case, code CFTs are constructed by compactifying n free bosons on a lattice, which in turn is defined from a classical error-correcting code via Construction A by Leech and Sloane. We show that constraints from higher-genus modular invariance on code CFTs can be recast into simple linear relations on the the higher-weight enumerator polynomial, which is a natural object from the code perspective. With this machinery at hand, we show that higher-genus modular invariance greatly reduces the number of seemingly consistent code CFT partition functions that were found by demanding modular invariance at genus one only. I will also cover some upcoming work, on the relation between quantum error-correcting codes and non-chiral (Narain) CFTs, and on averaging over code CFTs. Wednesday February 16, @ 13:15h: Vasco Gonçalves ONLINE seminar, No screening Title: Old and new results on higher point correlation functions Abstract: In this talk, I will review a duality between correlation function and null polygon Wilson loops and present this duality from a conformal bootstrap perspective. In the process I will also work out, in detail, a new duality between spinning three point functions in large N conformal gauge theories and null polygonal hexagonal Wilson loop. Wednesday February 09, @ 15:15h: Justin Kaidi ONLINE seminar: Screening room K0.19 Title: Kramers-Wannier-like duality defects in (3+1)d gauge theories Abstract: I will introduce a class of non-invertible topological defects in (3+1)d gauge theories whose fusion rules are the higher-dimensional analogs of those of the Kramers-Wannier defect in the (1+1)d critical Ising model. As in the lower-dimensional case, the presence of such non-invertible defects implies self-duality under a particular gauging of the discrete (higher-form) symmetries. I will illustrate this by means of the example of SO(3) Yang-Mills (YM) at θ=π, as well as SU(2)N=4 SYM at τ=i. Wednesday January 26, @ 13:15h: Mao Zeng LIVE seminar: room k0.20 Title: Multi-loop scattering amplitudes and gravitational binary dynamics Abstract: Next-generation gravitational wave detectors require highly precise predictions for the waveforms from inspiraling black holes and neutron stars. We present advances in binary inspiral dynamics by taking classical limits of scattering amplitudes in perturbative quantum gravity. The amplitudes are calculated efficiently using modern methods for scattering amplitudes, including double copy and generalized unitarity, and loop integration techniques borrowed from collider physics. Classical physics can be extracted by several complementary approaches, including effective field theory, eikonal exponentiation, and observables in wavepacket scattering. For both conservative and dissipative dynamics of binary systems, we obtain new terms in the post-Minksowskian expansion beyond the best previous results from purely classical methods. Wednesday January 19, @ 13:15h: Diego Delmastro Online seminar, screening in room k0.20 Title: Infrared phases of 2d QCD Abstract: We will study some aspects of Quantum Chromodynamics (QCD) in d=1+1 spacetime dimensions. The theory presents many of the same challenges as d=3+1 dimensional QCD (e.g., strong interactions where perturbation theory breaks down, chiral quarks which are hard to put on the lattice, etc.). But, in 2d, there are also some special features that make the problem more tractable. We will see that one can effectively solve the system at strong coupling, revealing interesting connections to other well-studied theories such as 2d rational CFTs (minimal models, WZW models, etc.).