Usual time: Wednesdays at 1:00pm (student session), 1:20pm (main seminar), except where stated. Online via Zoom.

Schedule:

07/10/20 - Pedro Vieira (Perimeter)

14/10/20 - Bidisha Chakrabarty (Southampton)

21/10/20 - Mritunjay Verma (Southampton)

28/10/20 - Horacio Casini (Bariloche)

04/11/20 - Stefano Cremonesi (Durham)

11/11/20 - Matthias Gaberdiel (ETH Zurich)

18/11/20 - Alice Bernamonti (Florence)

25/11/20 - Bogdan Stefanski (City)

02/12/20 - Ibrahima Bah (Johns Hopkins)

09/12/20 - Rishi Mouland (King's College)

[Winter break: 14 December 2020 through 15 January 2021]

20/01/21 - Xinan Zhou (Princeton) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

27/01/21 - Aleksey Cherman (Minnesota) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

03/02/21 - Clément Berthiere (Peking University) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

10/02/21 - Guilherme Pimentel (Amsterdam)

16/02/21 - Ofer Aharony (Weizmann) - *UNUSUAL DAY*

23/02/21 - Alok Laddha (Chennai) - *unusual day*

24/02/21 - David Meltzer (Caltech) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

03/03/21 - No talk this week (H0 tension workshop)

10/03/21 - Mario Martone (Stony Brook) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

16/03/21 - Gary Shiu (Wisconsin) - *unusual day and time* - 3:00 (student session), 3:20 (main seminar)

17/03/21 - Charlotte Sleight (Durham)

[Spring break: 22 March 2021 through 16 April 2021]

21/04/21 - Clay Cordova (Chicago) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

28/04/21 - Ivano Basile (Mons)

05/05/21 - Jim Gates (Brown) - *unusual time* - 3:00 (student session), 3:20 (main seminar)

12/05/21 - Stephan Stieberger (MPP Munich)

25/05/21 - Renann Lipinski Jusinskas (Prague)

Titles and Abstracts (reverse chronological order):

Renann Lipinski Jusinskas (Prague) L-infinity algebra in gauge theories with matter Slides

In this talk I will present some results involving L-infinity algebras in gauge theories coupled to matter. The first half will be a physicist's introduction to L-infinity algebras in field theory using the Batalin-Vilkovisky formalism.

In the second half I will move on to some applications of this underlying algebraic structure, in particular motivating perturbiner methods and presenting a generating function for tree level amplitudes.

Stephan Stieberger (MPP Munich) Celestial Amplitudes and Asymptotic Symmetries Slides

The celestial sphere is the target space for the proposal of flat space holography in which four-dimensional physics is encoded in two-dimensional celestial conformal field theory (CCFT). After giving some motivations we introduce the concepts and tools of physics on the celestial sphere.

We shall discuss the symmetries and properties of celestial amplitudes. By introducing the concept of soft theorems on the celestial sphere and their corresponding Ward identities we shall give an explicit field realization of the operators generating superrotations and supertranslations. Equipped with these results we construct the full extended BMS and extended super BMS algebras of CCFT. Finally, we shall present a conformal block expansion of a celestial four-gluon amplitude in terms of primary fields. Interestingly, higher spin states show up in this expansion.

Jim Gates (Brown) A New Hope To Resolve Foundational Problems of SUSY Representation Theory

Standard Lie algebras possess an elegant representation theory that allows complete access to all their manifestation. The tool of this theory consists of such concepts as root & weight lattices, Dynkin labels, etc. Though Gol'fand and Likhtman created the first supersymmetric (SUSY) field theories in 1971, currently - about fifty years later - no similar tools exist. An effort to solve this conundrum is discussed.

Ivano Basile (Mons) On string vacua without supersymmetry Slides

We discuss the existence and stability of vacua in string theories where supersymmetry is either absent or broken at the string scale. In particular we present no-go results regarding de Sitter compactifications, connecting them to some Swampland conjectures, thereby focusing on anti-de Sitter (AdS) flux vacua arising from the USp(32) and U(32) orientifold models and from the SO(16) x SO(16) heterotic model. We investigate perturbative and non-perturbative instabilities and frame the vacua in terms of brane stacks, analyzing their back-reacted geometry and reproducing AdS in the near-horizon limit. Then we describe the instabilities as branes separating from the stack, computing the associated decay rate matching a probe brane computation to the gravitational result. We conclude briefly discussing possible implications regarding the fate of unstable vacua in string theory beyond the semi-classical limit, connecting this scenario to the Weak Gravity Conjecture, holographic renormalization group flows and to a "de Sitter on a brane" construction that was outlined in the recent literature.

Clay Cordova (Chicago) Line Defect Quantum Numbers and Anomalies

I will describe a class of discrete anomalies of quantum field theories and explain how they are closely related to the statistics of heavy probe particles modeled by line defects.

Charlotte Sleight (Durham) A Mellin-Barnes Approach to Scattering in de Sitter Space Slides

The last decade has seen significant progress in our understanding of scattering in anti-de Sitter (AdS) space. Through the AdS/CFT correspondence, we can reformulate scattering processes in AdS in terms of correlation functions in Conformal Field Theory (CFT), which are sharply defined by the requirements of Conformal Symmetry, Unitarity and a consistent Operator Product expansion. Accordingly, numerous highly effective techniques for the study of scattering in AdS have been developed. This has been driven largely by the Conformal Bootstrap programme, which aims to carve out the space of consistent CFTs (and, in turn, quantum gravities in AdS space) principally through the three basic consistency requirements above. In this talk I will describe some steps towards extending some of these techniques and results to boundary correlators in de Sitter (dS) space. Compared to AdS, we have little grasp of the properties required of consistent correlation functions in Euclidean CFTs dual to physics in dS. The boundaries at infinity in dS are space-like with no standard notion of locality and time, so the basic criteria that underpin the Conformal Bootstrap programme do not directly apply to the corresponding programme in dS, the so-called Cosmological bootstrap. I will show how boundary correlators in AdS and dS can be placed on a similar footing by introducing a Mellin-Barnes representation in momentum space, providing a framework that could facilitate bridging the gap between the Conformal and Cosmological bootstrap programmes. I will then discuss how the Mellin-Barnes representation itself can be a useful tool to study boundary correlators both in AdS and dS.

Gary Shiu (Wisconsin) Quantum Gravity and the Swampland Slides

String theory seems to offer an enormous number of possibilities for low energy physics. The huge set of solutions is often known as the String Theory Landscape. In recent years, however, it has become clear that not all quantum field theories can be consistently coupled to gravity. Theories that cannot be ultraviolet completed in quantum gravity are said to be in the Swampland. In this talk, I’ll discuss these swampland criteria, focussing on one of the conjectured properties of quantum gravity known as the Weak Gravity Conjecture. I’ll present evidence for the Weak Gravity Conjecture and discuss its applications to cosmology.

Mario Martone (Stony Brook) Geometric constraints on the space of 4d theories Slides

The geometry of the moduli space of four dimensional superconformal theories is uniquely constrained by complex geometry and it thus represents an ideal set up for a bottom up classification program. I will describe these unique features, provide an update on the many new exciting results obtained and outline a variety of open questions.

David Meltzer (Caltech) Unitarity and Dispersion in AdS/CFT Momentum Space Slides

In this talk, I will present recent work on an AdS/CFT unitarity method for momentum space correlators. This unitarity method will be a direct generalization of the S-matrix unitarity method used to study amplitudes in flat-space. In the first part of the talk, I will review unitarity conditions on QFT four-point functions and their relation to the double-commutator. In the second part of the talk, I will use the QFT unitarity condition to derive the cutting rules for AdS Witten diagrams. In the last part of the talk, I will present new results on momentum space dispersion formulas for CFT correlators. The identities used to derive the cutting rules and dispersion formulas are valid in general theories and can be applied beyond AdS/CFT.

Alok Laddha (Chennai) The Infra-red triangles of Gravity Slides

In recent years, there has been a flurry of activity in analysing the infra-red (IR) sector of Quantum Gravity in asymptotically flat spacetimes . A remarkable structure appears to emerge from these studies which relates (classical) soft radiation, quantum soft graviton theorems and asymptotic symmetry of classical and quantum S Matrix.

First such structure was discovered by Pasterski and Strminger and is called infra-red triangle . It reveals the universality in the IR sector . In this talk, we will review some of the recent progress in identifying a hierarchy of these infra-red triangles and their implications for complete set of asymptotic symmetries of Quantum Gravity S matrix.

Ofer Aharony (Weizmann) A derivation of AdS/CFT for vector models

After discussing what it means to derive the AdS/CFT correspondence, I will describe our attempts (in collaboration with Shai Chester and Erez Urbach, 2011.06328) to make this correspondence explicit in the simplest case of the U(N) vector model. We explicitly rewrite the path integral for the free or critical U(N) (or O(N)) bosonic vector models in d space-time dimensions as a path integral over fields (including massless high-spin fields) living on (d+1)-dimensional anti-de Sitter space. Inspired by de Mello Koch, Jevicki, Suzuki and Yoon and earlier work, we first rewrite the vector models in terms of bi-local fields, then expand these fields in eigenmodes of the conformal group, and finally map these eigenmodes to those of fields on anti-de Sitter space. Our results provide an explicit (non-local) action for a high-spin theory on anti-de Sitter space, which is presumably equivalent in the large N limit to Vasiliev's classical high-spin gravity theory (with some specific gauge-fixing to a fixed background), but which can be used also for loop computations. Our mapping is explicit within the 1/N expansion, but in principle can be extended also to finite N theories, where extra constraints on products of bulk fields need to be taken into account.

Guilherme Pimentel (Amsterdam) Decoding and bootstrapping cosmological fluctuations

I will review our current understanding of the initial conditions of the universe, and describe what information is available from current and future measurements of cosmological correlation functions. Then I will describe a new method to compute and constrain the possible shapes of those correlation functions, assuming they were generated during inflation. This ``cosmological bootstrap” draws inspiration from the modern scattering amplitudes program in flat space, as well as the conformal bootstrap of phase transitions. After discussing primordial scalar fluctuations, I will also explain how the consistent propagation of gravitational waves imposes further constraints on the structure of spinning primordial correlators.

Clément Berthiere (Peking University) Entanglement in quantum Lifshitz theories Slides

In this talk, I will discuss entanglement in quantum Lifshitz theories. These are free (scalar) field theories with anisotropic (Lifshitz) scaling symmetry between space and time. Being closely related to CFTs, they provide simple models for non-relativistic critical theories where entanglement calculations can be performed analytically. I will first review how the entanglement entropy can be calculated in quantum Lifshitz theories. I will then introduce the logarithmic negativity (suitable to characterising quantum entanglement for mixed states), outline its computation in such theories and discuss the results.

Aleksey Cherman (Minnesota) Higgs-confinement phase transitions with fundamental representation matter Slides

I will discuss the conditions under which Higgs and confining regimes in gauge theories with fundamental representation matter fields can be sharply distinguished. It is widely believed that these regimes are smoothly connected unless they are distinguished by the realization of global symmetries. However, I will show that when a U(1) global symmetry is spontaneously broken in both the confining and Higgs regimes, the two phases can be separated by a phase boundary. The phase transition between the two regimes may be detected by a novel topological vortex order parameter.
I'll illustrate these ideas by explicit calculations in gauge theories in three spacetime dimensions, and then explain the generalization to four dimensions. One important implication of our results is that nuclear matter and quark matter are sharply distinct phases of QCD with an approximate SU(3) flavor symmetry.

Xinan Zhou (Princeton) Holographic correlators and emergent Parisi-Sourlas supersymmetry Slides (Seminar) Slides (student session)

In this talk, I will first review the recent progress in computing holographic four-point correlators in maximally supersymmetric CFTs. I will introduce the so-called Maximally R-symmetry Violating (MRV) limit, in which the Mellin amplitudes drastically simplify and become easy to compute. From the MRV limit, I will show that the full amplitudes can be reconstructed by using symmetries. This gives a complete answer to all tree-level four-point functions in all three maximally supersymmetric backgrounds AdS4xS7, AdS5xS5, and AdS7xS4. In the second part of my talk, I will point out that these results have surprising properties. The Mellin amplitudes exhibit an emergent dimensional reduction structure, which allows them to be expressed in terms of only scalar exchange amplitudes from lower dimensional spacetimes. I will explain that this dimensional reduction structure is closely related to a holographic realization of the Parisi-Sourlas supersymmetry.

Rishi Mouland (King's College): Exotic 5d Correlators and 6d CFTs Slides

I will discuss recent work investigating five-dimensional field theories with an SU(1,3) geometric symmetry. I will demonstrate the constraining power of the symmetry group—which includes a Lifshitz scaling—and in particular present the solutions to the conformal Ward identities for correlators. The utility of these theories in the construction of 6d CFTs will be discussed, along with an explicit Lagrangian example with a large degree of supersymmetry. Time permitting, I hope to contextualise these results within M-theory (in particular the DLCQ proposal for the M5-brane), and also within the AdS7/CFT6 correspondence.
Based on:
1) N. Lambert, A. Lipstein, R. Mouland & P. Richmond [2012.00626]
2) N. Lambert, A. Lipstein, R. Mouland & P. Richmond [1912.02638]

Ibrahima Bah (Johns Hopkins): Topological stars

In this talk, I will discuss a class of smooth spherically symmetric solutions in five dimensional Einstein-Maxwell theory that look like black holes upon a Kaluza-Klein reduction to four dimensions. These are simple prototypes of black hole microstate geometries that can be macroscopically large compare to the Kaluza-Klein circle. The smooth five dimensional space time contains topological cycles that are stabilized by magnetic flux. These topological cycle can also be placed behind black string horizons where they smooth out curvature singularities.

Bodgan Stefanski (City): Chern-Simons Origin of Superstring Integrability

I will review how four-dimensional Chern-Simons gauge theory provides a unified framework for a variety of integrable models. I will then show how the AdS5xS5 Green-Schwarz superstring can be obtained from a modification of 4d CS theory: the Beltrami-Chern-Simons theory. This reformulation of the superstring provides a new way of understanding integrable holography in terms of a (relatively) simple gauge theory as well as the relations between different worldsheet formulations of superstrings.

Alice Bernamonti (Florence): The First Law of Complexity

Based on Nielsen’s geometric approach, the variation of holographic complexity for two nearby target states only depends on the end point of the optimal trajectory, a result designated as the first law of complexity. As an example, we examine the variation of holographic and circuit complexity when the vacuum is perturbed by a scalar field excitation, which corresponds to a coherent state.

Matthias Gaberdiel (ETH Zurich): An exact AdS/CFT duality Slides

String theory on AdS3 x S3 x T4 with one unit of NS-NS flux is argued to be exactly dual to the symmetric orbifold of T4 in the large N limit. The string theory background can be described in terms of a solvable world-sheet theory. This allows one to compute the complete single-string spacetime spectrum and thereby demonstrate that it agrees with that of the symmetric orbifold of T4. Furthermore, the structure of the symmetric orbifold correlators can be reproduced from the world-sheet perspective .

Stefano Cremonesi (Durham): 3d N=4 gauge theories: vacua, low energy physics and correlators Slides

3d N=4 gauge theories are lower-dimensional cousins of 4d N=2 gauge theories, whose low energy physics on the Coulomb branch of the moduli space of vacua was famously determined in 1994 by Seiberg and Witten. Until a few years ago, much less was known about the physics and mathematics of their three-dimensional counterparts. In this talk I will give a biased overview of results on Coulomb branches of 3d N=4 gauge theories which have been obtained in recent years, and explain some of their uses within and beyond the realm of three-dimensional physics.

Horacio Casini (Bariloche): Entropic order parameters for symmetries in QFT

In QFT there is an algebra of operators attached to any spacetime region. Simple degradations of the most harmonious possible relation between algebras and regions are shown to encode generalized symmetries. Nets of algebras with these symmetries allow for a non uniqueness of the algebras that can be assigned to a given region. This non uniqueness suggests a simple geometrical order parameter in terms of a relative entropy. These satisfy a ''certainty relation'' connecting the statistics of the order and disorder parameters for complementary regions. We describe how the lore about phases of theories with generalized symmetries is seen in this new picture.

Mritunjay Verma (Southampton): Open Closed Superstring Field Theory

The open closed superstring field theory is necessary to study the field theory of an interacting system of open and closed strings in the presence of D branes. We construct the field theory of this system and describe its 1PI effective action and the BV master action. We also generalize our results to include the unoriented strings.

Bidisha Chakrabarty (Southampton): Out of time ordered effective dynamics of a quartic oscillator

Recently the connection of Out of Time Ordered Correlators (OTOCs) to scrambling, quantum chaos, thermalisation etc has been realised. With the increasing interest in OTOCs, it will be useful to develop a systematic prescription to compute them. In this talk I will discuss techniques of effective theory to compute OTOCs in open quantum systems. In particular, I will focus on the problem of a quantum Brownian particle interacting with a dissipative bath composed of two sets of harmonic oscillators via cubic coupling. Integrating out the bath’s degrees of freedom, we get an effective theory of the particle that is in general non-local in time. However for appropriate choice of distributions of bath oscillators, all the bath correlators decay exponentially fast at late times. Hence after the decay of the bath correlators has set in, one can write down a suitable 1-PI effective action of the particle which is temporally local. The correlators computed from the microscopic theory evolve the same way as the correlators computed from this 1-PI effective action. If the bath has microscopic time-reversal invariance and thermality, it imposes constraints on the 1-PI effective action. These constraint relations between the effective couplings of 1-PI action are OTO generalisations of the well-known Onsager-Casimir reciprocal relations and fluctuation-dissipation relations. Combining these relations, the non-Gaussianity of the thermal noise gets related to the thermal jitter in the damping constant of the Brownian particle.

Pedro Vieira (Perimeter): Multipoint bootstrap. Integrability and the Snowflake

We initiate an exploration of the conformal bootstrap for n > 4 point correlation functions. Here we bootstrap correlation functions of the lightest scalar gauge invariant operators in planar nonabelian conformal gauge theories as their locations approach the cusps of a null polygon. For that we consider consistency of the OPE in the so-called snowflake channel with respect to cyclicity transformations which leave the null configuration invariant. For general non-abelian gauge theories this allows us to strongly constrain the OPE structure constants of up to three large spin J_j operators (and large polarization quantum number l_j ) to all loop orders. In N = 4 we fix them completely through the duality to null polygonal Wilson loops and the recent origin limit of the hexagon explored by Basso, Dixon and Papathanasiou.