# HEP/GR Informal Seminar Series for Graduate Students

**Place:** CMS, Potter Room (B1.19)

**Time:** Wednesdays, 4.15 - 5.15pm (unless otherwise stated.)

**Organisers: Alec Barns-Graham & Chris King **

A series of seminars aimed at graduate students in the High Energy Physics and General Relativity groups. It will contain talks by the students both on original research and as pedagogical introductions to more specialised topics.

All are welcome.

### Michaelmas 2015

#### 11-Nov-2015 Spin Chains and Bethe Ansatz

Alec Barns-Graham, DAMTP

*Abstract:* In this talk I'll define the ferromagnetic XXX-1/2 spin chain, solving it for one and two magnons. We'll then define the monodromy and show briefly how it leads to integrability and energy eigenstates.

### Lent 2015

#### 11-Feb-2015: Well-Posedness for the Einstein Equations

Joe Keir, DAMTP

*Abstract: *tbc

#### 18-Feb-2015: Massive Gravity

Adam Solomon, DAMTP

*Abstract: *I will discuss the fascinating and winding history of massive gravity, from the development of the linear theory in 1939, through the pitfalls that arise at higher orders, to the development of the full nonlinear theory only in 2010.

#### 25-Feb-2015: The ins and outs of extremal black holes

Dejan Gajic, DAMTP

*Abstract:* In order to gain a complete understanding of black holes in classical general relativity, we must first resolve both the issue of stability of the exterior region of Kerr black holes and the nature of the singularity in the interior of a perturbed Kerr black hole. The aim of this talk is to give an overview of some recent developments towards these issues, by considering the toy problem of a wave equation both outside and inside unperturbed Kerr black holes. In particular, I will highlight the geometric differences between subextremal and extremal Kerr, and describe how the analysis in the extremal limit suggests more instability in the exterior, yet more stability in the interior.

#### 4-Mar-2015: String theory, T-duality and non-geometry

Chris Blair, DAMTP

*Abstract:* A remarkable feature of string theory is the existence of powerful and unexpected duality symmetries. One such is T-duality, which in its simplest form relates a string wrapped around a large circle to a string moving on a small circle. In this talk, I will give an introduction to T-duality, and explain how one can define so-called "non-geometric" backgrounds, which are only globally defined if one allows patching by duality transformations in addition to the usual geometric coordinate and gauge transformations. No prior exposure to string theory will be assumed.

#### 11-Mar-2015: Twistor Theory, Self-Dual Spacetimes, and Newton-Cartan Geometry

James Gundry, DAMTP

*Abstract:* Twistor theory can be thought of as a framework in which spacetime is derived from supposedly more fundamental complex geometry. Remarkably, spacetimes derived in this way often come equipped with physically-relevant structures like metrics, connections, and gauge fields. I will give a minimalist introduction to this central idea of twistor theory, assuming no prior knowledge of the subject. Two examples will demonstrate how this idea works in practice: I'll describe the twistor theory of self-dual Ricci-flat spacetimes and the nascent twistor theory of Newton-Cartan spacetimes.

#### 1-Apr-2015: Geometry in superconformal quantum mechanics

Andrew Singleton, DAMTP

*Abstract: *The framework of discrete light-cone quantisation (DLCQ) offers a potential window into superconformal field theories via models with finitely many degrees of freedom. These models exhibit a superconformal invariance whose details are closely tied to the existence of extra geometrical structure in the target space. Of particular relevance to DLCQ are those with a scale-invariant special Kaehler target. We will describe a construction of a broad class of these models and discuss their application to N=4 SUSY Yang-Mills.

### Michaelmas 2014

#### 22-Oct-2014: Instanton Effects in Quantum Gravity

Carl Turner, DAMTP

*Abstract:** *Quantum gravity is hard. So when we figure out how to compute something in it, we celebrate. This is such an occasion. In this talk, I'll describe a very concrete instanton-induced instability (computed with David Tong) to the dynamics of d=4, N=1 supergravity compactified on a circle. I will take time to explain how it arises by analogy with gauge theories, and give a general overview of the strategies required to tackle such a problem, in order to highlight points of conceptual interest like the non-cancellation of supersymmetric determinants, and the existence of a potential hidden IR scale in quantum gravity.*
*

#### 29-Oct-2014: A Skyrme model approach to the spin-orbit force

Chris Halcrow, DAMTP

*Abstract: *The spin-orbit force is a vital tool in describing finite nuclei and nucleon interactions; however its microscopic origin is not fully understood. The Skyrme Model provides a classical explanation for the force based on the pion field structure of separated Skyrmions. In this talk I will review the Skyrme Model and the spin-orbit force before setting up and solving a precise, simplified model of interacting Skyrmions.

#### 12-Nov-2014: Averaging the average: new insights on the dynamics of precessing black-hole binaries

Davide Gerosa, DAMTP

*Abstract: *The dynamics of precessing black-hole binaries in the post-Newtonian regime is deeply characterized by a timescale hierarchy: the orbital timescale is very short compared to the spin-precession timescale which, in turn, is much shorter than the radiation reaction timescale on which the orbit is shrinking due to gravitational-wave emission. The binary dynamics is typically studied in an orbit-averaged fashion: one only cares about the orbit itself, not the instantaneous position of each black hole. Here we also average over the precessional time, thus considering the precessional cones "as a whole", without tracking the spin's secular motion. These solutions improve our understanding of spin precession in much the same way that the conical sections for Keplerian orbits provide additional insights beyond Newton's 1/r^{^2} law. More on arXiv:1411.0674. * *

#### 26-Nov-2014: Quantising a perfect fluid

Dave Sutherland, Cavendish Laboratory

*Abstract: *A perfect fluid is one of the simplest fluid systems imaginable, and it can be described classically by an effective field theory. However, quantising this theory - i.e. putting said action in a path integral and computing correlators - has proved difficult, as, owing to the unrestrained excitations of transverse modes, correlators diverge at late times. After reviewing the above, I will show how to extract useful information from the path integral, providing evidence that a "quantum perfect fluid" is a consistent theory (with interesting behaviour).

#### 10-Dec-2014: Unitarity in Holographic, Massive 3D Gravity

Alex Arvanitakis, DAMTP

*Abstract: *The recent model of three-dimensional massive gravity known as "Minimal Massive Gravity" (MMG) is the simplest known (hence "minimal") example of a gravitational theory that satisfies basic requirements of both bulk and boundary unitarity in the semi-classical approximation while propagating bulk degrees of freedom. In this talk we will outline how MMG evades the "bulk/boundary unitarity clash" on asymptotically AdS backgrounds and discuss features of the flat-space limit.

### Michaelmas 2013

#### 09-Oct-2013: The dynamics of BPS vortices

Kenny Wong, DAMTP

*Abstract:
*

#### 16-Oct-2013: Extended supersymmetry and the Seiberg-Witten solution

Andrew Singleton, DAMTP

*Abstract: *Strongly coupled phenomena in realistic QFTs are notoriously difficult to say anything very precise about, owing to the lack of theoretical control we have over quantum corrections. Supersymmetry offers the tantalising possibility of 'miraculous cancellations', allowing us to make exact quantitative statements valid for all values of the coupling. I will describe the 1994 work of Seiberg and Witten which led the way in this area, computing the exact low energy effective action of N=2 Super-Yang-Mills theory. Along the way we'll see an electromagnetic duality between fundamental particles and solitons, an explicit mechanism for confinement and remarkable links to algebraic geometry, integrable systems and more.

#### 23-Oct-2013: The Singularity Theorems

Alec Graham, DAMTP

*Please note Alec's talk is in MR9*

*Abstract: *The singularity theorems of Hawking and Penrose are one of the most beautiful and important results in relativity. The aim of this talk will be to describe the theorems, and to explain how they are proved. Time permitting we will also discuss some modern extensions of these results.

#### 30-Oct-2013: The Interior of Black Holes and the Strong Cosmic Censorship Conjecture

Dejan Gajic, DAMTP

Abstract: Resolving the strong cosmic censorship conjecture is one of the biggest open problems in classical general relativity. The conjecture essentially says that general relativity is deterministic. In this talk I will discuss how this relates to the nature of singularities inside black holes, by means of "toy model" spacetimes that are solutions to the spherically symmetric Einstein-Maxwell-scalar field equations. In particular, I will motivate why one should ignore the extension of the Kerr solution beyond the Cauchy horizons. If time permits, I will also discuss the weak cosmic censorship conjecture briefly.

#### 06-Nov-2013: The O(D,D) Geometry of String Theory

Emanuel Malek, DAMTP

*Abstract:* I will briefly review the T-duality of string theory and how double field theory gives a manifestly T-duality invariant action for 10-d SUGRA at the cost of introducing new coordinates related to the winding modes of strings. Double field theory not only combines the NS-NS sector of 10-d SUGRA into a generalised metric but also combines the infinitesimal symmetries into a generalised Lie derivative. In order to define a covariant derivative with respect to this Lie derivative requires a new geometry which can be built using a flat but torsionful connection. The geometric information is now fully contained in the torsion and I will show how to use this torsion to construct the double field theory action. If time permits I will discuss how this relates to the action of gauged supergravities using the Scherk-Schwarz procedure.

#### 26-Nov-2013: LHC constraints on a light baryon number violating sbottom

Sophie Renner, DAMTP

*Abstract: *Supersymmetry (SUSY) is a beyond the Standard Model theory that answers many of the unsolved questions of the Standard Model. Most of the LHC searches for SUSY have focussed on looking for "R-parity conserving" models, in which the lightest SUSY particle is stable and neutral and so will escape the detector undetected and without decaying. The conclusions of these searches don't apply to "R-parity violating" models, which are hence currently less constrained.

I will start by giving an introduction to SUSY and physics at the LHC, and then go on to talk about some work I have recently done with my supervisor Ben Allanach on a model of R-parity violating SUSY with the right-handed sbottom as the lightest SUSY particle. Ben and his collaborator K. Sridhar had previously found that this model could explain an anomalous effect (the ttbar asymmetry) seen at the Tevatron collider, if this sbottom has a large coupling to top and down quarks. I will show that large regions in the mass-coupling parameter space of the sbottom are ruled out using recent LHC measurements.

### Lent 2013

#### 13-March-2013: The wave equation as a poor man's linearisation of the Einstein

equations

Damon Civin, DAMTP

*Abstract:* The Einstein field equations (EFE) can be written in harmoniccoordinates as a system of quasilinear wave equations. This allows for thestudy of the (EFE) within the theory of hyperbolic PDE. In particular, this can be used to study well-posedness and stability of the (EFE) for a large class of initial data.

I'll focus on the linear wave equation, which is the prototype hyperbolic PDE. It can also be viewed as a "poor man's linearisation" of the (EFE). Therefore the study of boundedness and decay of solutions of the wave equation on a fixed black hole background are a first step towards stability of the background as a solution of the (EFE).

I'll start off by discussing this "poor man's linearisation" and recalling what well-posedness means. We'll then move onto the highlights of the proof of well-posedness the Cauchy problem for the wave equation.

Along the way, we'll run into some Sobolev spaces and energy estimates, and I will try to convince you of their power and naturality. Time permitting, I'll discuss the heuristics of my work on the linear stability of

subextremal Kerr-Newman spacetimes.

#### 06-March-2013: D-branes

Arnab Rudra, DAMTP

*Please note Arnab's talk is 4pm in MR9*

*Abstract:* String Theory contains a lot of objects other than the fundamental strings. For example, D Branes which are extended objects in space-time, is one of the key ingredients in string theory. They break half of the space-time supersymmetry, ( ie. BPS states) and also play a crucial role in understanding dualities in string theory. In this talk, I will try to discuss the basics D-Brane physics and their interpretation in perturbative string theory.

#### 04-Feb-2013: Modified Gravity - Great or Graveyard?

Jeremy Sakstein, DAMTP

**note start time is 4pm**

*Abstract: * The increasing need for dark phenomena to explain the cosmic acceleration and clumping of matter has lead to a recent resurgence of interest in modified gravity theories. Modified theories can help with these phenomena to some extent, however when one looks at our own solar system these theories are in blatant violation of our experiments. In this talk I will introduce these problems and show how it is possible to construct theories with interesting dynamics that can satisfy solar-system bounds. These theories posses "screening mechanisms" which have deep roots in the underlying symmetries of the field theories describing spin-2 particles.

#### 30-Jan-2013: Quantum electrodynamics and the on-shell renormalization scheme

Kenny Wong, DAMTP

**note start time is 4pm**

*Abstract:* Renormalization gets too much bad press. My Cavendish friends feel uneasy about it. My pure mathmo friends say it's a complete joke. The problem is that renormalization can come across as a desperate attempt to rescue a sick theory - by "chopping off" infinities at will. That's nonsense. There's nothing artificial, unnatural or contrived in renormalization. Renormalization is a subtlety, not a fix. Field theory has no infinities, and if you calculate an infinity, you should question your physical reasoning rather than tweak the theory itself. I'm going to talk about something called the "on-shell renormalization scheme". It's conceptually different from the minimal subtraction renormalization scheme that we learned in AQFT. What's great about the on-shell scheme is that it makes the whole renormalization business feel natural and inevitable. I'll also talk about the famous precision tests of QED, including the anomalous magnetic moment of the electron that QED gets right to 11 significant figures. Finally I'll explain how to deal with the IR divergences in QED - and trust me, this is really, really beautiful.

### Michaelmas 2012

#### 10-Oct-2012: An AdS/CFT - Boson/Fermion Double Duality

Kenny Wong, DAMTP

*Abstract:* It is possible for a physical system to have two equivalent mathematical descriptions, each providing a different perspective on its properties. The AdS/CFT duality, for example, tells us that strongly-coupled systems can be described in terms of classical fields in one higher dimension. The dual description gives us an opportunity to answer questions about these systems that could be answered using conventional techniques in QFT. Bosonization is a duality between bosons and fermions in 1+1 spacetime dimensions. Since bosons are easier to handle than fermions, this duality is a valuable tool in string theory and condensed matter. The subject of this talk is a strongly-interacting system with a finite density of fermions. Using both AdS/CFT and bosonisation, one discovers that it has an elegant, intuitive description as a star of solitons in anti-de Sitter space.

#### 17-Oct-2012: Large N Gauge Theory

Mike Blake, DAMTP

*Abstract:* At low energies QCD is strongly coupled and so an expansion in the coupling constant is not valid. In 1973 't Hooft proposed an imaginative alternative - one can study the theory via an expansion in the number of colors, an expansion in '1/N'. The study of the first terms of this expansion - large N gauge theory - lies at the heart of modern theoretical physics. In particular it underpins the AdS/CFT correspondence of Maldecena. In this talk I shall provide a pedagogical introduction to large N QCD following the classic approaches of Coleman and Witten. I shall then briefly review modern research into a web of large N equivalences that connect different gauge theories.

#### 24-Oct-2012: A low energy effective introduction to string theory (and generalised geometry)

**note the unusual location of MR4**

Chris Blair, DAMTP

*Abstract:* My original hope for this seminar was to talk about the notion of "generalised geometry" and how it is applied in string and M-theory. Though this is still roughly going to be the goal of the talk, the principal focus will be to give a very basic but hopefully helpful introduction to some aspects of string theory (the closed bosonic string only, supersymmetry, branes etc will not be mentioned). The thinking behind this is to make the talk accessible to graduate students who don't work in string theory and haven't done the Part III course. The talk will be "low energy" in that you will not have to think very hard during it, and "effective" in that I will be giving an outline and ignoring subtleties of higher order. Time permitting I will be able to present enough stringy ideas to show why generalised geometry is interesting and useful.

#### 31-Oct-2012: 3d Strings

Alasdair Routh, DAMTP

*Abstract:* String Theory is usually studied in D=26 (or 10), as in other dimensions Lorentz invariance is generally lost after quantisation. Recently, Paul Townsend and Luca Mezincescu showed that quantum theories of Lorentz invariant strings also exist in 3 dimensions and have interesting properties, and I have worked with them this year to extend these results.

I will discuss why traditional arguments about Lorentz invariance fail for D=3, why this case might be interesting, and what happens.

Very basic knowledge of bosonic string theory will be needed to get the most out of the talk, the quantisation of the bosonic string in D=26 should be enough.

#### 07-Nov-2012: Quantum theory of massless spin 1 & spin 2 particles

**note the unusual location of MR4**

Arnab Rudra, DAMTP

*Abstract:* The role of symmetry in physics is to constrain the set of all possible theories that can describe the nature.These symmetry principles are much more powerful in the quantum version of a theory compared to their classical counterpart. For example, the consequence of Poincare symmetry which, by our present understanding, is an exact symmetry of nature, tells us that a massless relativistic quantum particle in 4 dimensions can have only 2 degrees of freedom. The purpose of this talk is to demonstrate how a quantum theory of massless particles is completely constrained by the symmetries of nature. More specifically, one can show that the theory of massless spin 1 particle has to be a yang-Mills theory and the theory of massless spin 2 particle has to be a theory of gravity.

#### 14-Nov-2012: Energy and Stability in GR

**note start time is 4pm**

Joe Keir, DAMTP

*Abstract:* It is often stated that various positive energy theorems tell us something about the stability of certain spacetimes in general relativity. In this talk I will first explain why a naive interpretation of such a statement fails. I will then go on to show how energy can correctly be related to (linear) stability properties. I will construct the appropriate "energy" functional for use with linear perturbations of initial data, and briefly discuss some of its properties and the restrictions that must be made on the space of initial data perturbations. Along the way I will indicate how to prove the first law of black hole mechanics, for arbitrary variations, in an arbitrary diffeomorphism invariant theory. Finally, I will argue that this energy is related to stability and instability.

#### 21-Nov-2012: Generalised geometry and 11-dimensional supergravity

*note start time is 4pm and the location is MR 4*

Emanuel Malek, DAMTP

*Abstract:* I will discuss how U-dualities arise in 11-dimensional supergravity and how generalised geometry allows a reformulation of the action so that it is manifestly invariant under dualities. If time permits, I will introduce the concepts of ''non-geometric'' backgrounds (U-folds).

#### 28-Nov-2012: Symmetries of Inflation

Valentin Assassi, DAMTP

*Abstract:* First introduced to solve several shortcomings of the standard model of cosmology, inflation provides an elegant explanation for the origin of the cosmic microwave background anisotropies and the large scale structure. It is indeed believed that quantum fluctuations produced during inflation seeded the primordial inhomogeneity of our universe. In this talk, I will give a short review of inflation and explain how quantum fluctuations produced during inflation relate to the observed inhomogeneity in the universe. Then, after reviewing the symmetries of adiabatic fluctuations, I will show how these symmetries constrain the low-momentum physics of these perturbations. Finally, I will discuss the possibility of probing the spectrum of particles present during inflation through the statistics of the primordial fluctuations.