Square Seminars (S2)
Welcome to the Square Seminars portal.
The Square Seminars (S2) are intended for, and delivered by, postgraduate research students in theoretical physics and related areas.
Contact details: click here to join the mailing list. Click here to view the recordings of previous seminars.
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Season S2 Seminars
The flow of critical phenomena
30/11/2022 17:30: Emma Albertini from Imperial College London
Location: Hybrid (Imperial College London + Zoom)
Abstract: In this seminar, we will focus on the Ising model, to go back to the ’60s reviewing the work of Kadanoff, Michael Fisher and, most importantly, Kenneth Wilson. Firstly, we will introduce the Landau theory and define first and second order phase transitions. As the failure of the model is evident at the critical point, we will move on with Landau Ginzburg theory. As a spatial dependence is introduced, it is possible to define a correlation length and predict critical exponents with mean field approach in agreement with experiment for d>4. However, to understand what happens at the critical point for d<=4, renormalisation group technique is required. Does this word ring a bell? How is statistical field theory related to QFT?
Get slides -- Recording coming soon.
4d N=1 SQCD
16/11/2022 17:30: Mitchell Woolley from Queen Mary University of London
Location: Hybrid (Queen Mary + Zoom)
Abstract: We present a low-brow tour through the world of 4d minimally supersymmetric gauge theories with matter. We set the scene by discussing the role of Wilsonian renormalization and conformal field theories in the landscape of QFTs. We then introduce 4d N=1 superQCD by describing its field content and various global symmetries. We go on to map out its phase structure and discover Seiberg duality. We will also comment briefly on superconformal field theories.
Instantons and Spheres
02/11/2022 17:30: Tancredi Schettini Gherardini from Queen Mary University of London
Location: Hybrid (Imperial College + Zoom)
Abstract: We will show how the geometry of the famous SU(2) Yang-Mills instanton corresponds to a seven-sphere, constructed via Hopf fibration. No prior knowledge on instantons is assumed, no prior knowledge on fibre bundles is assumed. Only familiarity with the definition of a manifold and with Yang-Mills theory is expected.
Braneworlds (&) Holography
26/10/2022 17:30: Emanuele Panella from University College London
Location: Hybrid (Imperial College + Zoom)
Abstract: We introduce the topic of braneworld from the ground up, starting from Israel junction conditions and briefly analysing failures and successes of the RSI and RSII models. We will top it off with recent developments, showing how braneworlds have turned out to be extremely powerful calculation tools once paired with holography. No knowledge of string theory or holography will be assumed.
Introduction to TQFT and application to Chern-Simons Theory
19/10/2022 17:30: Benjamin Suzzoni from the University of Southampton
Location: online (Teams)
Abstract: We will briefly present an axiomatic approach to TQFT as presented in work by Atiyah. Following this, a crash course in knot theory will be presented. This will allow us to understand better the work of Witten, which we will look at in the final part of this seminar.
Season S1 Seminars
A Lazy graviton theory
11/06/2022 10:00: Emma Albertini from Imperial College London
Location: Imperial College London + online (Zoom)
Abstract: This is the first of a S1 x S1 modified gravity seminar series. I will start introducing the linearized action of a Poincare invariant spin 2 massless field theory and show how considering non linearities brings to the equivalence with general relativity. Then, I will introduce a mass term for gravity in the linear Fierz-Pauli action and show the presence of VdVZ discontinuity and ghosts. Then, adding non-linearities, I will explain why this theory is still pathological. Is this enough to rule out massive gravity?
Branewashing you into string theory
05/06/2022 10:00: Pietro Capuozzo from the University of Southampton
Location: online (Teams)
Abstract: This seminar provides an introduction to branes, extended objects in superstring theories and M-theory. We introduce the two standard pictures -- branes as fundamental objects in generalised electromagnetism, and branes as charged black hole solutions to supergravity -- and attempt to reconcile these two points of view. We then build the "web of dualities" and review D-branes, focussing on their low-energy worldvolume theories. We end with two applications: first, we use "branengineering" to realise mirror symmetry and the moduli space of instantons in string theory; second, we take a brief excursion on curved branes and branes probing singularities.
Particles: do they really exist? Let’s find out!
30/05/2022 17:30: Emanuele Panella from University College London
Location: Imperial College London + online (Zoom)
Abstract: A brief excursion through the Unruh effect, Hawking radiation and Bogoliubov transformations.
Gravity Wars Episode 2: A new (finite) Hope
14/05/2022 15:00: Tancredi Schettini Gherardini from Queen Mary University of London
Location: online (Zoom)
Abstract: 't Hooft and Veltman 1974's article brought hope to the scientific community: maybe gravity CAN be quantised, it's just a tedious (but rewarding) business. The talk will present the proof that quantum gravity is divergences-free at one loop, after covering the basic QFT techniques involved.
Simple introduction to the descent equations and anomaly cancellation
12/02/2022 12:00: Benjamin Suzzoni from the University of Southampton
Location: online (Zoom)
Abstract: The process of quantising a classical theory doesn't come without its drawbacks. Some theories, upon quantisation, exhibit loss of symmetries known as anomalies. In this talk we briefly look at how such anomalies can appear in simple chiral theories. We also give an exposition of the beautiful link between the anomaly function and the geometry of our manifold of interest (principal bundle). This geometric interpretation is given by the Stora-Zumino descent equations for which a couple of examples are given.
Field theory on Causal Sets
05/02/2022 11:00: Emma Albertini from Imperial College London
Location: online (Zoom) -- a Screen Sharing Square Seminar (S4)
Abstract: This seminar focuses on Causal Set Theory as an approach to Quantum Gravity that replaces the idea of continuum spacetime with a discrete collection of events ordered by causality. As a consequence, the road to quantization is taken through a Sum-over-Histories framework, abandoning the canonical way. After investigating some properties of the free decoherence functional, I deal with interactions, formulating an expression for the interacting k-point function in ϕ4 theory on a fixed background causal set. To conclude, I propose modified Feynman rules to construct the allowed diagrams, estimating all terms at any order in perturbation expansion.
Spacetime, quantum entanglement, and holography
29/01/2022 15:30: Pietro Capuozzo from the University of Southampton
Location: Imperial College London + online (Zoom)
Abstract: This seminar will present a holographic description of the entanglement entropy of a conformal field theory. Along the way, we will briefly discuss the notions of entanglement, holography, and black hole entropy, leading us to the Ryu-Takayanagi formula. We will show how this formula can be proved for a particular geometry, and discuss some of its applications, namely the study of the worldvolume theory of an M5-brane. We conclude with some more philosophical/speculative remarks on the relationship between entanglement and gravitation. Warning: the words "path integral Monte Carlo method" will be spoken (most likely inappropriately) at some point during this seminar.
The sphere (weirdly) maximises the free energy of quantum field theories
22/01/2022, 12:00: Emanuele Panella from University College London
Location: Imperial College London + online (Zoom)
Abstract: Ever since the discovery of the Casimir effect, the vacuum energy of quantum fields has been at the centre of many discussions in theoretical physics, with an eye to the possible roles that it can play in various cosmological models. We will first review the origin of Casimir energy for a scalar field in the famous original setup, before proceeding to discuss the weird effects that arise when these fields are placed on curved spacetimes. With these tools in mind, we will discuss techniques to analyse the free energy of QFTs specifically on (2+1)-dimensional curved spacetimes, and suggest experiments that might verify the surprising result that QFTs seem to reject the spherical geometry in favour of more crumpled configurations.
(This Square Seminar (S2) will be followed by a Square Snack (S2) with the Speaker (S1)).
Gravity Wars Episode 1: Return of the Einstein
22/11/2021, 17:30: Tancredi Schettini Gherardini from Queen Mary University of London
Location: online
Abstract: This talk is the first of a series of three presentations on quantum gravity. Which quantum gravity? The first one. Yes, the one that did not work. This first seminar is just an invitation to why the procedure of quantising gravity with the usual prescription hints at beautiful results from the very beginning. We will take the viewpoint of a Venutian scientist: we will construct gravity starting only from our knowledge of QFT's, such as QED, and very reasonable assumptions on how such "force" should behave. We will illustrate how a linear action can be obtained via simple considerations, and sketch how to correct this action by including higher order terms. The result will be a familiar one...
