76th North British Mathematical Physics Seminar & EMPG seminar

Abstract: This is a student oriented preseminar in which some background material related to QFTs in AdS will be covered.

Abstract: We study QCD on AdS space with scalars or fermions in the fundamental representation, extending earlier results on pure Yang-Mills theory. In the latter, the Dirichlet boundary condition is conjectured to disappear via merger and annihilation, as signalled by the lightest scalar singlet operator approaching marginality as the coupling increases. With matter, there are two candidate operators for this mechanism. We compute their one-loop anomalous dimensions via broken conformal Ward identities and Witten diagrams. In the confining phase, with Dirichlet (Neumann) boundary condition, their anomalous dimensions are negative (positive), consistent with the disappearance (persistence) of the associated boundary CFT in the flat-space limit. In the conformal window, one of these operators becomes the displacement operator of the IR CFT, as signalled by the vanishing of its one-loop anomalous dimension in the perturbative Banks-Zaks regime. Possible scenarios for the lower edge of the conformal window are discussed. Finally, we consider general boundary conditions on fermions and discuss their relation to chiral symmetry breaking in flat space.

Abstract: We investigate the dynamics of linearised perturbations around the de Sitter static patch, in the presence of a timelike boundary with conformal boundary conditions. Inspired by the case of global AdS4, for which the limit in which the conformal boundary approaches the asymptotic boundary reveals a hierarchy of real modes, we consider the limit in dS4 in which the conformal boundary approaches the cosmological horizon. In this limit, potential perturbations that grow exponentially in time are attenuated. We find a hierarchy of modes that localise near the conformal boundary, suggesting a holographic interpretation. In particular, we uncover sound and shear fluid-dynamical modes that we interpret in terms of a conformal fluid with shear viscosity over entropy density ratio \eta/s=1/4\pi. 

Abstract: A local, covariant and model independent description of the measurement process in relativistic quantum field theory was recently developed by Fewster and Verch. In this framework, a target quantum field theory is measured indirectly by means of coupling to a probe field theory in a compact region of spacetime. For field theories admitting a description by a Lagrangian, the coupled theory differs from the uncoupled one the addition of an interaction term. To date, concrete implementations of the Fewster-Verch framework have been exemplified by an uncoupled theory of free scalar fields, with a coupled theory described by a linear interaction term. In this talk, we discuss how methods of perturbative algebraic quantum field theory can be applied to analyse measurement schemes for interacting quantum fields and how these techniques allow for explicit computation of quantities of interest.

Abstract: As pointed out by Freed and Witten, there are situations in which a brane cannot wrap a compact cycle in spacetime, depending on the topology of the cycle and the presence of fluxes. In this case, a brane is said to suffer from a Freed-Witten anomaly. The original analysis considered global anomalies on the open string worldsheet, making it perturbative result. However, it is desirable to have an understanding of Freed-Witten anomalies in the non-perturbative case as well, e.g. to study branes in general F-theory backgrounds. While results for many special cases have appeared in the literature, a general treatment was missing until recently. In this talk, I will present a systematic approach to non-perturbative Freed-Witten anomalies by relating them to a mixed 't Hooft anomaly of higher-form symmetries of the worldvolume theory. Based on progress made by Hsin, Tachikawa, and Yonekura, I will argue that they can be understood as the vanishing of the brane partition function. After showing that this reproduces the result by Freed and Witten, I will explain how this approach can be applied to more general cases: M5 branes, and D3 branes on S-folds, a class of non-geometric F-theory backgrounds. This talk is based on 2510.19935.

Abstract: Despite the archetypal status of the BTZ background in quantifying quantum aspects of black holes, several aspects at low temperatures remain imprecise and incomplete. In this talk, I will discuss the behaviour of the Euclidean path integral at low temperatures in the context of AdS_3 supergravity, encompassing an analysis of quantum fluctuations in the near-horizon and far region. I'll clarify and rectify aspects of the bosonic fluctuations, highlighting the role of boundary conditions in AdS_3. I'll also account in detail for the contributions of Chern-Simons fields and spin-3/2 modes at low temperature. And finally, I’ll discuss how these results also impact our understanding of Kerr/CFT.