Welcome

Bin Zhu (Edinburgh) -- Eikonal probe scattering on the celestial sphere In this talk, I will motivate the recent studies of celestial holography on backgrounds. I will review eikonal amplitudes and their relations to probe amplitudes on a fixed background. This would allow us to compute celestial probe amplitudes on shockwaves, Schwarzchild, and Kerr backgrounds to all orders in Newton's constant G. I will also discuss the dispersion relations of the celestial probe amplitudes on the shockwave backgrounds and celestial eikonal amplitudes.

Fernando Alday (Oxford) -- The AdS Virasoro Shapiro Amplitude

Jan de Boer (Amsterdam) -- The Statistical Interpretation of Semi-Classical Gravity In this talk I will describe a picture which has emerged over the past few years regarding the statistical interpretation of semiclassical gravity and how this relates to wormholes, averaging and the so-called factorization puzzle, the information paradox, and a combinatorial description of 3d gravity.

Kasia Budzik (Perimeter Institute) -- Non-perturbative effects in twisted holography Twisted holography relates the 2d chiral algebra subsector of N=4 SYM to the B-model topological string theory on the deformed conifold SL(2,C). Operators of order N in the chiral algebra insert Giant Graviton branes in the dual geometry. In this talk, I will present a match between the large N saddles of determinant correlation functions with semiclassical D-brane configurations in the dual theory, using a spectral curve construction. The match can be extended to saddles in non-trivial Coulomb branch vacua. The talk is based on work in collaboration with Davide Gaiotto.

Sonja Klisch (Edinburgh) -- The double copy in twistor space The double copy is a powerful tool connecting gauge theoretic and gravitational scattering amplitudes. It was originally derived from string theory, relating the tree level amplitudes of closed string amplitudes to two copies of open string amplitudes. In the field theory limit, this reduces to being able to obtain tree-level graviton amplitudes from the "square" of tree-level gluon amplitudes. At the same time, these field theory amplitudes have miraculously simple expression as map moduli space integrals in twistor space for any number of external legs and helicity configuration of gluons and gravitons. However, the double copy relation between these formulae has historically been extremely non-obvious. In this talk, we will use basic concepts from graph theory to demonstrate the derivation of a double copy based in twistor space, including a helicity graded KLT kernel integrand.

Ian Moult (Yale) -- Conformal Colliders Meet the LHC I will discuss recent theoretical progress in the study of a class of asymptotic observables called energy correlators. I will show that higher point energy correlators can be computed using modern integration techniques, and present recent measurements from the LHC illustrating key physical properties of the correlators, namely their scaling behavior in OPE limits, and their shape dependence.

Prahar Mitra (Amsterdam) -- Celestial Conformal Primaries in Effective Field Theories Scattering amplitudes in $d+2$ dimensions can be recast as correlators of conformal primary operators in a putative holographic CFT$_d$ by working in a basis of boost eigenstates instead of momentum eigenstates. In this talk, we show that completeness, normalizability, and consistency with CPT implies that we must restrict the boost eigenvalues of the operators to either $\Delta \in \frac{d}{2} + i {\mathbb R}$ or $\Delta \in {\mathbb R}$. Unlike those with $\Delta \in \frac{d}{2} + i {\mathbb R}$, operators with $\Delta \in {\mathbb R}$ can be constructed without knowledge of the UV and can therefore be defined in effective field theories. With additional analyticity assumptions, we can restrict $\Delta \in 2 - {\mathbb Z}_{\geq0}$ or $\Delta \in \frac{1}{2}-{\mathbb Z}_{\geq0}$ for bosonic or fermionic operators, respectively.

Frank Coronado (ETH Zurich) -- Determinant operators and massive amplitudes in N=4 SYM In planar N=4 SYM, massless scattering amplitudes are dual to null polygonal Wilson loops (T-duality) or the same as the four-dimensional null limit of stress-tensor correlators. I will present a (conjectured) generalization of this duality which equates correlators of determinant operators, in a special ten-dimensional null limit, with massive scattering amplitudes in the Coulomb branch of N=4. This determinant operator is a generating function of all half-BPS single-traces operators. By taming it on twistor space I will show its correlators have ten dimensional poles which combine 4d space-time and 6d R-charge kinematics.

Per Kraus (UCLA) -- Actions and S-matrices I discuss the formulation of the S-matrix as the evaluation of a path integral with specified asymptotic boundary conditions, along with applications to asymptotic symmetries and the flat space limit of AdS/CFT

Romain Ruzziconi (Oxford) -- Carrollian Amplitudes and the Flat Space Limit of AdS Carrollian amplitudes are massless scattering amplitudes expressed in position space at null infinity. These are to be re-interpreted as correlation functions in a putative dual Carrollian CFT. I will discuss the properties of Carrollian amplitudes and their relations with celestial amplitudes. I will explain how the collinear limit unveils a notion of Carrollian OPE and deduce the action of celestial symmetries at null infinity. Finally, I will argue that Carrollian amplitudes are natural objects obtained in the flat limit of AdS Witten diagrams.

Massimo Porrati (NYU) -- Supertranslations, Angular momentum, and Covariance in 4d Asymptotically Flat Space I will present a supertranslation-invariant and Lorentz-covariant definition of angular momentum in asymptotically flat 4D spacetime. This definition uses only asymptotic metric data and reproduces the flux necessary to obtain known radiation reaction effects. The formula has an appealing physical interpretation, it extends to Lorentz boost charges and integrated fluxes, and agrees with other existing definitions in appropriate reference frames.

David Nichols (U. Virginia) -- Gravitational-wave memory effects as an observational window into celestial holography The spectrum of gravitational-wave sources is rapidly broadening. At the higher-frequency end, the LIGO-Virgo-KAGRA collaboration detects black-hole mergers at a rate of roughly one every other day when the detectors are operating. At the lower-frequency end, pulsar timing arrays have found strong evidence for a stochastic gravitational-wave background. The intermediate frequencies will be measured by the LISA detector in the 2030s. These observatories are measuring the properties of curved spacetime from the most strongly gravitating and dynamically evolving astrophysical sources that have been discovered. General relativity predicts that the gravitational waves from these sources will have distinctive features that arise from nonlinear interactions of gravitational waves themselves as they propagate away from an isolated source. After reviewing the latest status of gravitational-wave observations, I will focus on one of the more distinctive nonlinear phenomena: the gravitational-wave memory effect. Its observational feature is a lasting change in the gravitational-wave strain before and after a burst of gravitational waves pass through a detector. I will discuss the features of the memory effect that gravitational-wave detectors can measure and discuss the measurement prospects for the effect, which has yet to be detected. I will briefly discuss the connection between the memory effects, soft theorems, Bondi-Metzner-Sachs symmetry, and the conserved charges. Finally, I will also describe a hierarchy of higher memory effects that are more challenging to measure, but which could be related to aspects of celestial holography.

Adam Ball (Perimeter Institute) -- Dynamical Edge Modes and Entanglement in Maxwell Theory At least intuitively, edge modes in gauge theory are the finite-boundary analogues of the soft and Goldstone modes familiar from asymptotic symmetries. However, most treatments of edge modes are either limited to kinematics or view them as superselection labels. I show how to treat edge modes as dynamical degrees of freedom. I find that their contribution to the partition function resolves certain discrepancies in the literature, and allows the entanglement entropy to be written as a genuine trace over a density matrix.

Kevin Costello (Perimeter Institute) -- QCD scattering amplitudes in self-dual backgrounds from large N chiral algebras I will show how certain loop level scattering amplitudes and form factors in QCD can be realized as correlators of a large N chiral algebra. The construction goes via twisted holography for the type I string on a Calabi-Yau 5-fold related to twistor space. This is joint work with Roland Bittleston and Keyou Zeng.