HEP-GR Colloquium Series

**Place:** Meeting Room 2

**Time:** Wednesdays, 2:15pm (unless otherwise stated.)

**Organisers:** Dr Eric Perlmutter and Dr Joan Camps

Please stay for tea, coffee and biscuits in the Central Core!

HEP-GR Colloquium Series

**Place:** Meeting Room 2

**Time:** Wednesdays, 2:15pm (unless otherwise stated.)

**Organisers:** Dr Eric Perlmutter and Dr Joan Camps

Please stay for tea, coffee and biscuits in the Central Core!

### Easter Term 2014

#### 7th-May-2014: The generalized bootstrap program for N=4 SCFT

Fernando Alday (Mathematical Institute, Oxford)

*Abstract: *We describe recent progress in applying the bootstrap program to N=4 SCFT. In particular, we will show how to obtain non-trivial non-perturbative results for the spectrum of non-planar N=4 SYM.

#### 14th-May-2014: Anomalies and transport: from the quark gluon plasma to Weyl semi-metals on a superstring

Karl Landsteiner (Madrid)

#### 21st-May-2014: The Entropic Origin of Gravity, Dark Energy and Dark Matter

Erik Verlinde (Amsterdam)

#### 28th-May-2014: Black Hole Hair

Ruth Gregory (Durham)

*Abstract:* Black holes are perhaps the most fascinating objects in General Relativity – they have a very simple mathematical description, characterised by mass, charge and angular momentum – the lack of larger numbers of parameters being typically summed up by “no hair” theorems. Yet the no hair theorems ignore nonperturbative aspects of the field theories used to describe matter. I will discuss how black holes can acquire very spectacular hair when symmetries of nature are spontaneously broken – in the form of topological defects emanating from the black hole horizon. I will discuss how these vacuum defects interact with the black hole, causing it to buckle and change in shape. Although the black hole can be bland, it can have rather literal, and very long hair!

#### 4th-June-2014: The cosmological constant problem (and its sequester)

Tony Padilla (Nottingham)

*Abstract: *I will review the notorious cosmological constant problem, sometimes described as the worst fine tuning problem in Physics. I will explain the true nature of the problem, which is one of radiative instability against any change in the effective description. I will recall Weinberg’s venerable no-go theorem that prohibits certain attempts to “solve” this problem before going on to explain a new mechanism that circumvents Weinberg. This is the vacuum energy sequester, a global modification of GR that results in the cancellation of large vacuum energy contributions from a protected matter sector (taken to include the Standard Model) at each and every order in the perturbative loop expansion. Cosmological consequences are a Universe which has finite space-time volume, will ultimately crunch, and for which dark energy can only be a transient.

#### 11th-June-2014: Yangian Symmetry in the AdS/CFT Correspondence

Niklas Beisert (ETH, Zurich)

### Lent Term 2014

### 22nd-Jan-2014: Quantum simulations of high energy physics models

Ignacio Cirac (Max Planck Institute, Munich)

*Abstract:* Many-body quantum systems are very hard to describe and simulate in general, since the dimension of the state space grows exponentially with the number of particles, volume, etc. Cold atomic systems may help us in that task, as one can in principle engineer the interactions among the atoms to emulate many-body quantum problems. So far, this possibility has been mainly addressed in the context of condensed matter systems, and a significant experimental effort is nowadays trying to pursue this goal. In this talk I will present some recent work where we have analysed how to use cold atomic systems to simulate simple high-energy models. In particular, I will explain how lattice (compact) QED and QCD in different dimension may be simulated. Time permitting, I will also mention other efforts to describe and simulate (classically) some of those problems using tensor-network techniques, as developed in the context of quantum information theory.

#### 29-Jan-2014: Hidden Spacetime and M-branes

Neil Lambert (Kings College, London)

*Abstract:* In this talk we will discuss mechanisms by which the extra eleventh-dimension arises in the field theories that live on the worldvolumes of branes in M-theory. These include three-dimensional monopole operators as well as higher-dimensional analogues.

#### 5th-Feb-2014: Cancelled.

#### 12th-Feb-2014: Effective Field Theory in Cosmology

Leonardo Senatore (Stanford)

*Abstract*:The recent tremendous progress in Observational Cosmology makes it now sensitive to non-linear corrections in the evolution of the density perturbations during the early epochs of the universe. In this context, the effective field theory paradigm represents the ideal setup to explore and systematically study the signatures that come from interactions, and additionally to directly map what we are learning from data into theory. I will describe two recent applications of this paradigm to Cosmology: the Effective Field Theory of Inflation and the Effective Field Theory of Cosmological Large Scale Structures (EFTofLSS). The first example represents the general parametrization of adiabatic fluctuations around an inflationary solutions, and it allows us to study the non-Gaussian signatures of Inflation, that I will describe. The second example is in the context of the gravitational clustering of dark matter. In our universe matter perturbations are large on short distances and small on large distances: strongly coupled in the UV and weakly coupled in the IR. We formulate an effective description based on an IR fluid-like system that allows us to formulate a manifestly convergent perturbative expansion to describe weak dark matter clustering. I will present the predictions of the EFTofLSS up to 2-loops. We find that it matches to percent accuracy the non-linear matter power spectrum up to k\sim 0.6 h/Mpc, requiring just one unknown coupling constant that needs to be fit to observations. Given that former perturbative techniques stop converging at k\sim 0.1 h/Mpc, our results demonstrate the possibility of accessing a factor of order 200 more dark matter quasi-linear modes than naively expected. If the remaining observational challenges to accessing these modes can be addressed with similar success, our results show that there is tremendous potential for large scale structure surveys to explore the primordial universe.

#### 19th-Feb-2014: What happens at the horizon on an extreme black hole?

Harvey Reall (DAMTP)

*Abstract: *An extreme black hole is one with the lowest mass consistent with a fixed charge or angular momentum. Supersymmetric black holes are necessarily extreme. Such black holes are generally believed to be classically stable. However, recently it was discovered that any extreme black hole exhibits an instability at the event horizon. I will review this discovery and discuss the nonlinear evolution of the instability.

#### 26th-Feb-2014: Higher Spin Holography: a new perspective on black holes and locality

Alejandra Castro (Amsterdam)

*Abstract: *In recent years there has been a renewed interest in higher spin gravity. These theories have the potential to be the simplest models of AdS/CFT, and hence can provide insight to the fundamental mechanics behind holography. In this talk I’ll overview recent progress on non-perturbative aspects of these theories with particular emphasis on black holes and its associated thermodynamics. We will also discuss issues related to holographic entanglement entropy and the Hawking-Page phase transitions as interesting applications of these new developments.

#### 5th-March-2014: LUX on dark matter

Henrique Araujo (Imperial)

*Abstract*: The Large Underground Xenon experiment is searching for interactions of galactic dark matter particles in a 250 kg liquid xenon target. LUX features a double-phase xenon Time Projection Chamber at its core, and operates in a low background environment at the 4850-ft level of the Sanford Underground Research Facility, South Dakota, USA. Results from its first science run completed in 2013 put new constraints on the elastic scattering of Weakly Interacting Massive Particles (WIMPs) off nucleons. Very low energy threshold for nuclear recoil detection and novel calibration techniques have allowed a 20-fold improvement in sensitivity for low mass WIMPs relative to the previous leading experiment, changing the landscape of direct searches for light WIMPs. The null result from LUX can only be reconciled with controversial signal claims by other experiments under the most contrived set of astrophysical, particle physics and experimental assumptions. I will describe the experiment and this significant result, and discuss future prospects as LUX enters a longer science run in 2014.

#### 12th-March-2014: TBC

Joan Simon (Edinburgh)

*Abstract* tbc

### Michaelmas term 2013

#### 16th-Oct-2013: Hidden Structures in the Gravitational S-Matrix.

*Please note room change to MR3*

David Skinner (DAMTP)

*Abstract: *Scattering amplitudes in General Relativity contain a rich geometric structure that is completely obscured by their traditional representation as sums of Feynman diagrams. I will give an overview of various recent developments that weave together aspects of string theory, twistor theory and quantum field theory.

#### 23rd-Oct-2013: *Precision physics: the new LHC frontier.*

Alexander Mitov (Cavendish/DAMTP)

*Abstract: *The Large Hadron Collider was conceived as a discovery machine. So far it has performed brilliantly: a Higgs boson was discovered (Nobel Prize) and very many powerful limits on physics beyond the Standard Model were placed.

As a result of the physics observed so far at the LHC, however, a new demand on the LHC physics program is starting to take shape: to achieve unprecedented for a hadron collider experimental and theoretical precision. I will demonstrate how this new requirement emerged, why it is important, and what its implications for a range of important physics problems are. I will also discuss the two-way relationship between collider physics applications and more formal theoretical developments in, for example, gauge theory amplitudes.

#### 6th-Nov-2013: Holographic Conductivity

David Tong (DAMTP)

*Abstract:* I'll describe attempts to compute Ohm's law in strongly interacting field theories using holography.

#### 13th-Nov-2013: Exploring Moonshine in String Theory

Miranda Cheng (Paris 7)

*Abstract:* The term “moonshine” was introduced in the 70’s to describe a mysterious relation between automorphic functions and representations of finite groups, two distinct and fundamental concepts in mathematics. Recently, a novel type of moonshine phenomenon, named “umbral moonshine”, was discovered. Evidence suggests that umbral moonshine enjoys a close connection to string theory compactified on K3 surfaces. In this talk I will report on the progress of this moonshine exploration from a string theoretic perspective.

#### 20th-Nov-2013: Gravity & Hydrodynamics

Mukund Rangamani (Durham)

*Abstract: *I will describe recent developments in relativistic hydrodynamics inspired in part by the fluid/gravity correspondence. I

will focus on general lessons for the hydrodynamic effective field theory: signature of quantum anomalies, domain of validity of

hydrodynamics, etc.. Time permitting I will also attempt to extract some lessons about gravity: in particular, clarify the connections between the black hole membrane paradigm and the fluid/gravity correspondence.

#### 27th-Nov-2013: Holographic Baryons

Paul Sutcliffe (Durham)

*Abstract:* Baryons in holographic QCD correspond to topological solitons in the bulk. I shall describe some recent results on the study of bulk solitons within the Sakai-Sugimoto model, which is a holographic QCD model with a string theory embedding. A low-dimensional analogue will also be described and results presented that are relevant for holographic investigations of QCD at finite density.

#### 4th-Dec-2013: Entanglement in Quantum Field Theory

John Cardy (Oxford)

*Abstract: *Entanglement is a characteristic property of quantum systems. In a relativistic quantum field theory, even the vacuum state is entangled. Quantifying the amount of entanglement between different regions of space gives fresh insight into the nature of quantum field theories, and of the many-body condensed matter systems whose long-distance physics they capture. In this talk I show how various measures of entanglement can be computed using the Feynman path integral approach, and describe various analytic results that have been obtained both in two, and higher dimensions of space-time.

### Easter Term 2013

***CANCELLED***17th-April-2013: Entanglement entropy and negativity in quantum field theories.

Pasquale Calabrese (Pisa)

*Abstract: *The study of the entanglement content of many body quantum systems has prompted an intense research activity at the crossroad of different disciplines such as statistical mechanics, quantum information, condensed matter, and quantum field theory. The quantification of the entanglement allowed a novel and finer characterization of many extended quantum systems. In this talk I will present systematic methods to calculate the entanglement entropy and negativity in the ground state of 1+1 dimensional quantum field theories, with particular emphasis on conformal invariant ones. I will show how to apply these powerful tools to the calculation of the entanglement of a single interval, and the generalization to different situations such as the case of several disjoint intervals.

#### 24th-April-2013: From Quantum Black Holes to Number Theory.

Don Zagier (College de France)

*Abstract:* tbc

#### 1-May-2013: The Planck satellite, inflation and non-Gausianity

Paul Shellard (DAMTP, Cambridge)

*Abstract:* The Planck satellite crosses a new quantitative threshold, like COBE and WMAP before it, with results which appear to be consistent with the standard cosmology and the inflationary paradigm. However, it also crosses new qualitative thresholds by opening up new windows on the Universe, for example, through the exploration of non-Gaussian statistics and gravitational lensing. As well as a brief overview of the Planck results, I will discuss the search for non-Gaussian fingerprints of new physics in both the late and early universe.

#### 8-May-2013: The large D limit of General Relativity

Roberto Emparan (ICREA & Universitat de Barcelona)

Abstract: Although at first sight it may seem an odd idea, I will argue thatit is actually quite natural to investigate the properties of General Relativity and its black holes in the limit in which the number of spacetime dimensions grows to infinity. The theory simplifies dramatically: it reduces to a theory of non-interacting particles, of finite radius but vanishingly small cross sections, which do not emit nor absorb radiation of any finite frequency. This leads to efficient calculational approaches in an expansion around this limit, as well as to intriguing connections to low-dimensional string-theory black holes.

#### 15-May-2013: Heavy Flavour Physics

Alexander Lenz (Durham)

*Abstract:* We review the current status of Flavour Physics in the light of recent experimental data in particular from the LHC. We also investigate some implications for beyond standard model physics.

#### 22-May-2013: (In) Stabilities and complementarity in AdS/CFT

Eliezer Rabinovici (Racah Institute of Physics, The Hebrew University of Jerusalem)

Abstract: I will discuss the holographic description of crunching AdS cosmologies. The relations between various types of Boundary QFTs and the corresponding bulk properties will be reviewed. Some of the systems are stable, some unstable and some can be narrated using a complementarity as either stable or unstable. I will describe systems in which bulk singularities are healed and others which need not be resolved. The talk is based on a series of works with J.L. Barbon and work with R. Auzzi, S. Elitzur and B. Gudnason.

#### 29-May-2013: Quantum black holes from gauge/string duality

Toby Wiseman (Imperial)

*Abstract: *Gauge/string duality arguably provides our best framework for computing quantum properties of gravity and black holes from first principles. In the simplest instances it reformulates the problem of understanding a quantum gravity black hole in terms of understanding certain (rather special) gauge theories at finite temperature. I will review this surprising and powerful duality, and then discuss progress over the last 5 years in both analytic and numerical attempts to directly solve such gauge theories with the aim of performing direct quantum gravity calculations.

#### 5-June-2013: M5-branes

Seok Kim (Seoul National University)

Abstract: M5-branes are 5+1 dimensional objects in M-theory. Putting N M5-branes together, a 6d CFT was predicted with N^{3} degrees of freedom, very different from N D-branes hosting N^{2} degrees of Yang-Mills theories. Their novel properties are observed mostly by indirect methods rather than 6d QFT calculations, due to the lack of its microscopic formulations. Curiously, the mere existence of this brane and CFT leads to nontrivial predictions on lower dimensional QFT’s, like the compactification of string/M-theory leading to a variety of interesting quantum systems. In this talk, after a review, I will briefly explain recent studies on a class of interesting observables in this 6d QFT, from exact calculations in 5 dimensional super Yang-Mills theory.

#### 12-June-2013: Entanglement entropy and negativity in quantum field theories.

Pasquale Calabrese (Pisa)

*Abstract: *The study of the entanglement content of many body quantum systems has prompted an intense research activity at the crossroad of different disciplines such as statistical mechanics, quantum information, condensed matter, and quantum field theory. The quantification of the entanglement allowed a novel and finer characterization of many extended quantum systems. In this talk I will present systematic methods to calculate the entanglement entropy and negativity in the ground state of 1+1 dimensional quantum field theories, with particular emphasis on conformal invariant ones. I will show how to apply these powerful tools to the calculation of the entanglement of a single interval, and the generalization to different situations such as the case of several disjoint intervals.

### Lent Term 2013

#### 16th-Jan-2013: Amplitudes and number theory

Francis Brown (CNRS, Paris)

*Abstract:* I will give an overview of some recent results on amplitudes in massless phi^4 theory, and explain their connection to algebraic geometry, and multiple zeta values and modular forms in number theory

#### 23rd-Jan-2013: Unoriented D-brane instantons

Massimo Bianchi (Rome)

*Abstract:* We give a pedagogical introduction to D-brane instanton effects in vacuum configurations with open and unoriented strings. We focus on quiver gauge theories for unoriented D-branes at orbifold singularities and describe in some detail the Z_3 case, where both "gauge" and "exotic" instantons can generate non-perturbative super potentials, and the Z_5 case, where supersymmetry breaking may arise from the combined effect of `gauge' instantons and a FI D-term.

#### 30th-Jan-2013 The Wilkinson Microwave Anisotropy Probe (WWAMP) Observations: The Final Results

Eiichiro Komatsu (Max Planck Institute for Astrophysics)

*Abstract:* The Cosmic Microwave Background (CMB), the fossil light of the Big Bang, is the oldest light that one can ever hope to observe in our Universe. The CMB provides us with a direct image of the Universe when it was still an "infant" - 380,000 years old. The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped the microwave sky in five frequency bands for nine years since 2001, creating a full-sky CMB map with the unprecedented precision. The WMAP data have enabled us to obtain a wealth of cosmological information, such as the composition, age, geometry, and history of the Universe. Yet, can we go further and learn about the primordial universe, when it was much younger than 380,000 years old, perhaps as young as a tiny fraction of a second? If so, this gives us a hope to test competing theories about the origin of the Universe at ultra high energies. In this talk, we will review the physics of CMB and the WMAP mission, present the basic results from nine years of observations, and discuss their cosmological implications.

#### 6th-Feb-2013: Old and New in Constructive Field Theory

Arthur Jaffe (Harvard)

*Abstract:* We describe the motivation and background for the theoretical understanding of relativistic field theory (cqft), and summarize major solved and unsolved problems. We highlight reflection positivity and explain some recent discoveries. These apply to symmetries in field theory and also give insight into certain spin chains.

#### 13-Feb-2013: New Observations about Quantum Field Theory

Zohar Komargodski (Weizmann Institute, Israel)

*Abstract:* We review recent progress in understanding Quantum Field Theory. The new results connect three thorny and fundamental questions: The symmetries of second order phase transitions, monotonicity of the renormalization group flow, and the entanglement entropy of the vacuum. We give examples of various applications for theories ranging from boiling water to Quantum Chromodynamics.

#### 20th-Feb-2013: A holographic view of the very early universe

Kostas Skenderis (Southampton)

*Abstract:* In this talk I will give an overview of holographic cosmology. I will first discuss standard inflation, show that it is holographic and discuss the new insights that come from this. I will then present new holographic models that describe a universe that that was non-geometric at early times and describe the phenomenology and the observational signatures of these models.

#### 27-Feb-2013 Title TBC

Speaker TBC

#### 6th-March-2013: Cancelled

#### 13th-March-2013: The LHC update

Ben Gripaios (Cavendish Lab, Cambridge)

*Abstract:* I will describe some recent results from the LHC and the implications for physics Beyond the Standard Model

#### 20th-March-2013: Exploring the infrared properties of gluons and ghosts

John Gracey (Liverpool)

*Abstract: *We discuss several different field theoretic approaches to explain recent lattice data of the infrared structure of the gluon and ghost propagators. These include effective massive gluons as well as the Gribov approach. Loop calculations using the Gribov-Zwanziger Lagrangian are presented

### Michaelmas 2012

#### 24-Oct-2012: Complementarity vs. Firewalls: Are there large quantum effects near black hole horizons?

Don Marolf (UC Santa Barbara)

*Abstract:* We argue that the following three statements cannot all be true: (i) Hawking radiation is in a pure state, (ii) the information carried by the radiation is emitted from the region near the horizon, with low energy effective field theory valid beyond some microscopic distance from the horizon, and (iii) the infalling observer encounters nothing unusual at the horizon. Perhaps the most conservative resolution is that the infalling observer burns up at the horizon. Alternatives would seem to require novel dynamics that nevertheless cause notable violations of semiclassical physics at macroscopic distances from the horizon.

#### 26-Oct-2012: New Horizons in Black Hole Physics and Holography.

***NB: Friday 26th Oct in 2:30 pm in MR3***

Shamit Kachru (Stanford)

*Abstract:* tbc.

#### 7-Nov-2012: Simulating black-hole spacetimes on supercomputers.

Ulrich Sperhake (DAMTP, Cambridge)

*Abstract:* This talk presents a review of numerical relativity, the generation of solutions to the Einstein field equations using numerical methods. After an overview of the numerical techniques, we will summarize results about black-hole systems in the context of astrophysics, gravitational wave physics and high-energy physics. We conclude with a summary of the main future directions of numerical relativity.

#### 14-Nov-2012: Some Properties of String Theory Scattering Amplitudes

Michael Green (DAMTP, Cambridge)

*Abstract:* tbc

#### 21-Nov-2012: New Superconformal Field Theories From Wrapped Branes.

Brian Wecht (Queen Mary, London)

*Abstract:* In string/M-theory, it is possible to engineer a wide variety of interesting supersymmetric conformal field theories (SCFTs) by wrapping branes on nontrivial manifolds. In this talk, I will describe a new infinite set of theories which come from M5-branes on Riemann surfaces. The corresponding supergravity solutions interpolate between and extend beyond a famous pair of solutions by Maldacena and Nuñez. Additionally, the dual SCFTs are "non-Lagrangian" theories, which have no weakly coupled UV descriptions, yet can (and will) be described explicitly.

#### 28-Nov-2012: Quark flavour and lattice QCD: investigations into electroweak symmetry

breaking

Matt Wingate (DAMTP, Cambridge)

*Abstract:* This colloquium will review how theoretical calculations and experimental measurements of weak decays of hadrons are testing the Standard Model and constraining models which would supplant it. The role played by lattice QCD will be a focus of the talk, with an emphasis on the bigger picture rather than specific details.