UK-QFT XI

Monday 9 Jan 2023, 09:30 → 22:45 Europe/London

Cambridge Centre for Mathematical Sciences (University of Cambridge)

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Important update: change of date for UK-QFT XI

 

The recently announced strike action affecting the UK rail network on the planned date of UK-QFT will make it difficult for many to travel to Cambridge. In order to allow the greatest fraction of participants to attend and interact in person, we have decided to postpone the meeting to Monday 9^th January 2023.

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UK-QFT XI will take place on Monday, 9th January 2023 in a hybrid format: both in person in the Cambridge Centre for Mathematical Sciences, and streamed online via Zoom. 

 

The UK-QFT meetings provide a unique opportunity to bring together researchers from the UK communities and overseas, who are working at the forefronts of quantum field theory (QFT) and quantum gravity research across high-energy physics, cosmology and astro-particle physics.  Past topics have covered both perturbative and non-perturbative aspects of QFT, and included, for example: scattering amplitudes, the quantum effective action, functional renormalization group approaches, cosmology, non-equilibrium phenomena, phase transitions and topological defects (all from the perspective of the underlying QFT description).  The history of the meetings can be found at https://sites.google.com/view/uk-qft.

 

Postgraduate students and postdoctoral researchers are particularly encouraged to attend and present their work through short accessible talks, providing an inspiring and informal environment for members of the community at all career stages to engage with one another.  The meetings are held on a single day, and there are no registration fees, so as to ensure the widest possible participation.

 

Follow @uk_qft or join the mailing list at https://sites.google.com/view/uk-qft for information from the UK-QFT community.

 

We gratefully acknowledge the support of the IOP HEPP Group.

Talks

1 Black hole entropy from quantum mechanics

I will discuss a holographic duality between a family of seven dimensional supersymmetric black holes and superconformal quantum mechanics on the moduli space of SU(N) Yang-mill instantons. The duality predicts a particular exponential growth of degeneracy of BPS states of the quantum mechanics which must reproduce the black hole horizon formula. This exponential growth has been proven rigorously in the case of U(1) instantons and we have good evidence for the growth in the SU(N) case.

Speaker: Boan Zhao (University of Cambridge)

2 The analytic wavefunction

In recent years there has been an increasing interest in the interplay between amplitudes and cosmological correlators, in particular in the use of amplitudes techniques to constrain cosmological correlators. In this talk, I will give an overview of the formalism of the wavefunction of the universe and how it relates to cosmological correlators. After this, I will review the success of the S-matrix programme in studying the analytic properties of the amplitude of scattering A(s,t) in a gapped theory. Among the many results, I will focus on the sum rules for Wilson coefficients, which relate the coefficients in an EFT expansion with an integral of the UV completion of the theory. To incorporate the sum rules to the wavefunction of the universe framework, we define off-shell wavefunction coefficients whose analytic structure is fixed by its tree-level diagrams. The resulting sum rules encapsulate a larger set of Wilson coefficients than those from amplitudes. Finally, I will address future research in the rich interplay between amplitudes and cosmology.

Speaker: Santiago Agui Salcedo (University of Cambridge)

3 Causality in the EFT of gravity

Effective field theory (EFT) is a theory-agnostic approach to understanding how high-energy phenomena would manifest in our low-energy universe. It is an expansion in higher-dimension operators built out of light fields with unknown coefficients. These coefficients can either be constrained through experiment or by demanding consistency with theoretical expectations, such as causal wave propagation. In dynamical gravitational EFTs, where there is no globally defined lightcone, defining causality is a nuanced problem. In this talk, using Gauss-Bonnet gravity as an example, I will explain why the recently introduced "infrared" causality is the correct criterion for determining consistency of low-energy EFTs. The crucial ingredient will be properly identifying the "regime of validity" of the EFT expansion, and recognising that it is only sensible to ask whether it is causal within that regime. Based on arXiv:2112.05031.

Speaker: Aoibheann Margalit

Aoibheann - UKQFT notes.pdf

11:15 Break

Talks

4 On Graviton non-Gaussianities in the Effective Field Theory of Inflation

In cosmology we measure correlation functions (cosmological correlators) which we can trace back to the boundary at the end of inflation. In the spirit of the S-matrix in flat space and holography in AdS, the cosmological bootstrap allows us to compute these boundary observables by sidestepping cumbersome Lagrangians, and instead using dS isometries and fundamental principles with no explicit reference to time evolution, i.e. “bootstrapping time”. Recently, progress has been made on bootstrapping correlators in the case where dS boosts are broken, to make more phenomenologically relevant observational predictions. In this talk I will explain how we can use locality and unitarity to bootstrap the graviton three-point function within the Effective Field Theory of Inflation (EFToI). I will first present the full class of operators that contribute to the on-shell two- and three-point functions of gravitons, and show how our analysis also captures graviton bispectra with a perturbative correction to the graviton two-point function.
Based on:
https://link.springer.com/article/10.1007/JHEP10(2022)154

Speaker: Ayngaran Thavanesan (University of Cambridge)

5 Introduction to the worldline formalism of QFT

This talk will provide an introduction to the first quantised "worldline approach" to quantum field theory. Here, physical information is encoded in path integrals over relativistic point particle trajectories. The path integrals typically lead to "Master Formulae" for scattering amplitudes that combine multiple Feynman diagrams.

I will review the historic development of the worldline formalism before presenting some of its recent successes, specifically in the context of non-perturbative gauge transformations and applied to QED in external background fields.

Speaker: James Edwards (Univeristy of Plymouth)

Discussion

13:00 Lunch

Talks

6 NEC violation: Tunnelling versus Casimir effect

Tunneling between degenerate vacua is allowed in a finite volume, and leads to a non-extensive symmetric ground state. This talk displays how this leads to a violation of the Null Energy Condition for small enough temperatures, assuming a continuous set of momenta in the finite volume containing the field. Taking into account discrete momenta can modify this picture, and is achieved by adding the Casimir energy to the tunneling-induced ground state energy. Focusing on zero-temperature, it is shown that these non-trivial effects compete when the typical size of the box containing the field is of the order of the particle's Compton wavelength.

Speaker: Drew Backhouse

7 Scale separation in AdS vacua and holography

The existence of AdS vacua in string theory with a parametric separation between the Hubble scale and the Kaluza-Klein scale of the extra dimensions is an open question, and holography is a promising tool to tackle this. I will discuss some remarkable features of the holographic CFT duals of the DGKT vacua, which are candidate AdS vacua with parametric scale separation.

Speaker: Fien Apers (University of Oxford)

8 A dynamical formulation of ghost-free massive gravity

The ghost-free massive gravity theory of de Rham, Gabadadze and Tolley (dRGT) has attracted a lot of attention since its formulation over a decade ago. Many studies have looked at its consequences for cosmology, and explored various limits in which the theory simplifies. However, until now few attempts have been made at numerically simulating its full non-linear equations, as an explicit dynamical formulation, analogous to the ADM formulation of GR, was not known.

In this talk, based on work with de Rham, Tolley and Wiseman, I will introduce the history and nuances of the formulation of massive gravity. I will then outline a dynamical formulation for the minimal and next-to-minimal dRGT models with a flat reference metric, explicitly identifying the phase-space variables, their associated momenta, as well as the evolution and constraint equations. I will go over the construction of initial data, which, like in GR, must still obey the Hamiltonian and momentum constraints. Finally, the techniques developed will be applied to perform numerical spherically symmetric gravitational collapse of scalar field matter for the minimal model, finding generically that this model breaks down before any large curvatures can appear.

Speaker: Jan Kożuszek

9 Integrability treatment of AdS/CFT orbifolds

The integrability program provides powerful tools to determine the spectrum of holographic theories. The development of these tools and the generalisation to a wider scope of theories go hand in hand. In particular, non-supersymmetric versions of AdS/CFT can be accessed, which is a first step towards more realistic models of QCD and could shed some light on SUSY-breaking mechanisms needed for phenomenology.
We discuss the simple case of orbifolds of $AdS_5\times S^5$ and the dual $\mathcal{N}=4$ super Yang-Mills theory. We explain how the orbifolding features in the various integrability techniques and demonstrate this for two simple $\mathbb{Z}_2$-orbifolds. When we break supersymmetry completely, we expect tachyons to appear on both sides of the duality and we comment on their implications and potential matching through AdS/CFT.

Speaker: Torben Skrzypek (Imperial College London)

15:30 Break

Talks

10 How to find the Feynman Rules from any scalar-tensor theory and not collapse in the process

The ability to represent perturbative expansions of interacting quantum field theories in terms of simple diagrammatic rules has revolutionised calculations in particle physics. However, in the case of extended theories of gravity, deriving this set of rules requires linearization of gravity, perturbation of the scalar fields and multiple field redefinitions, making this process very time-consuming and model dependent. In this talk, I will motivate and present FeynMG, a Mathematica extension of FeynRules that automatizes this calculation, allowing for the application of quantum field theory techniques to scalar-tensor theories.

Speaker: Sergio Sevillano

11 Unified description of corpuscular and fuzzy scalar dark matter

We derive coupled equations for self-interacting scalar dark matter, which can include both a condensed, low momentum ''fuzzy'' component and one with higher momenta that may be described as a collection of classical particles. We do this from first principles, using the Schwinger-Keldysh path integral and the corresponding Feynman diagrams in a perturbative expansion. The resulting coupled equations consist of a modified Gross-Pitaevskii equation describing the condensate, a kinetic equation describing the higher momentum modes (the ''particles''), and the Poisson equation for the gravitational potential sourced by the two components. We show that this model contains known models of dark matter and cold atom physics in some limits.

Speaker: Alex Soto (Newcastle University)

12 From the tabletop to the Big Bang: Quantum simulators of false vacuum decay

False vacuum decay (FVD) plays a vital role in many models of the early Universe, with important implications for inflation, the electroweak vacuum, and gravitational waves. However, it is also a highly non-perturbative and non-equilibrium process which is challenging to model theoretically, with existing Euclidean methods relying on numerous assumptions that have yet to be empirically tested.

An exciting route forward is to use laboratory experiments which undergo transitions analogous to FVD, allowing nature to simulate all of the relevant physics for us. In this talk, I will discuss ongoing work to develop such analogue FVD experiments within the Quantum Simulators for Fundamental Physics (QSimFP) program. In particular, I will present numerical lattice simulations of ultracold atom systems undergoing FVD, and discuss outstanding challenges in using upcoming experimental data to understand the early Universe.

Speaker: Alexander C Jenkins (University College London)