## Seminars

We host an online seminar series discussing recent papers related to celestial holography. The seminars take place every other Tuesday at noon (EST). To join the mailing list for seminar announcements, please send an email to d.b.skinner@damtp.cam.ac.uk

**Upcoming seminars:**

**28 Nov 2023**–**Niklas Garner (U Washington)**

**Twistorial Monopoles & Chiral Algebras**

Twistor methods are remarkably powerful for studying four-dimensional scattering problems, but a description in terms of a local QFT on six-dimensional twistor space is often obstructed by an anomaly. As observed by Costello and Paquette, when such a description exists the chiral algebra of asymptotic symmetries can also be described as the degrees of freedom living on the boundary of a three-dimensional QFT obtained by reduction from twistor space. In this talk I will describe some recent work with N. Paquette studying non-perturbative aspects of these celestial chiral algebras by combining the above observation with techniques available to three-dimensional, holomorphic-topological gauge theory.**12 Dec 2023**–**Diksha Jain (TIFR, Mumbai)****The S-Matrix and Boundary Correlators in Flat Space**

**Past seminars:**

**14 Nov 2023**–**Adam Ball (Perimeter)****Multicollinear Singularities in Celestial CFT**

After reviewing the celestial OPE, soft current algebras, and the Jacobi identity, I will introduce the holomorphic multicollinear limit and its celestial avatar, the multi-OPE. This new tool provides more information than the usual 2-OPE, and I will use it to show that from the perspective of hard celestial amplitudes the failure of Jacobi is due to the presence of certain branch cuts in the z-dependence. This puts to rest some concerns in the literature that the hard OPE is sick in theories failing Jacobi. Finally, I will use the multi-OPE to show the existence of a new singular term in the OPE between two massless particles, and I will argue it is associated with multi-particle operators.**17 Oct 2023**–**Hofie Hannesdottir (IAS)****What can be measured asymptotically?**In this talk, we consider asymptotic observables in quantum field theories in which the S-matrix makes sense. We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time ordering is relaxed. These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes. We explain how to calculate them in different ways: by relating them to amplitudes and products of amplitudes and by using a generalization of the LSZ reduction formula. Finally, we discuss how to relate them to one another through new versions of crossing symmetry. As an application, we discuss one-loop contributions to gravitational radiation in the post-Minkowski expansion, emphasizing the role of classical cut contributions and highlighting the infrared physics of in-in observables.

**3 Oct 2023**–**Noah Miller (Harvard)****Integer modes and Bulk $w_{1+\infty}$ symmetry**In celestial holography, integer modes with Δ ∈ ℤ have the special importance of generating soft symmetries. In this talk (based on the paper arxiv.org/abs/2302.04905) we prove the completeness of these modes and their descendants (and their shadow pairs) for the massless scalar by reconstructing the positive frequency Wightman function as a sum of these modes. Interestingly, this proof utilizes a novel CFT-like inner product called the RSW inner product.We then discuss how integer modes and descendant wave functions can be used to the bulk spacetime symmetry which satisfies the commutation relations of the (loop algebra of) the S algebra and the w_(1+infinity) algebra in Self Dual Yang Mills and Self Dual Gravity. In these self dual theories, we will find that there is essentially a complete correspondence between radiative modes and “gauge transformations,” once interpreted appropriately.

**19 Sept 2023**–**Wei Bu (Edinburgh)****Infra-red structures of scattering on self-dual radiative backgrounds**

The scattering of gluons and gravitons in trivial backgrounds is endowed with many surprising infrared features which have interesting conformal interpretations on the two-dimensional celestial sphere. However, the fate of these structures in more general asymptotically flat backgrounds is far from clear. In this paper, we consider holomorphic infrared structures in the presence of non-perturbative, self-dual background gauge and gravitational fields which are determined by freely specified radiative data. We make use of explicit formulae for tree-level gluon and graviton scattering in these self-dual radiative backgrounds, as well as chiral twistor sigma model descriptions of the classical dynamics. Remarkably, we find that the leading holomorphic part of tree-level collinear splitting functions – or celestial OPEs – and infinite-dimensional chiral soft algebras are undeformed by the background. We also compute all-order holomorphic celestial OPEs in the MHV sectors of gauge theory and gravity.