** Seminars: Spring 2014**

**Minding the Gap in N=4 Super-Yang-Mills**

Speaker: | Christopher Rosen |

Department: | University of Crete |

Time: | Tuesday 21 January 2014 at 15:00 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | Fermions exhibit interesting behaviors in the geometry holographically dual to zero-temperature N = 4 Super-Yang-Mills theory with two equal nonvanishing chemical potentials. This background is characterized by a singular horizon and zero ground state entropy. Curiously, fermionic fluctuations are completely stable within a gap in energy around a Fermi surface singularity, beyond which non- Fermi liquid behavior returns. I will discuss the novel features of this gap, and attempt to interpret it in the context of N=4 SYM gauge theory, its five dimensional gravitational dual, and a six dimensional theory which resolves the singular horizon. |

**Gravity duals of N = 2 superconformal field theories with no electrostatic description**

Speaker: | Konstantinos Siampos |

Department: | Université de Mons |

Slides: | [PDF] |

Time: | Thursday 13 February 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | We construct the first eleven-dimensional supergravity solutions, which are regular, have no smearing and possess only SO(2,4) x SO(3) x U(1)_R isometry. They are dual to four-dimensional field theories with N = 2 superconformal symmetry. We utilise the Toda frame of self-dual four-dimensional Euclidean metrics with SU(2) rotational symmetry. They are obtained by transforming the Atiyah--Hitchin instanton under SL(2,R) and are expressed in terms of theta functions. The absence of any extra U(1) symmetry, even asymptotically, renders inapplicable the electrostatic description of our solution. |

**Aspects of entanglement: entropies, negativity and causal holographic information**

Speaker: | Erik Tonni |

Department: | SISSA |

Time: | Tuesday 18 February 2014 at 14:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | Entanglement of quantum states and its measures play an important role in many areas of theoretical physics. The entanglement entropy is a good measure for pure states, while the negativity allows to measure entanglement for mixed states. A method to compute negativity in QFT through the replica trick will be described. Analytic results and their numerical checks will be presented for simple 2D CFTs like the compactified boson and the Ising model. Within the class of theories with a holographic dual, besides the holographic entanglement entropy, a natural quantity to introduce is the causal holographic information. Its definition and properties will be discussed. |

**Superfluid Hydrodynamics, Thermal Partition Function and Lifshitz Scaling**

Speaker: | Shira Chapman |

Department: | Tel-Aviv University |

Slides: | [PDF] |

Time: | Thursday 20 February 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | We will review the basics of superfluidity, and discuss how quantum anomalies are manifested in superfluid hydrodynamics. We will present an algebraic method to derive Kubo formulas for the anomalous transport coefficients using the equilibrium partition function. We will outline the special features revealed in superfluid hydrodynamics, when it exhibits a Lifshitz scaling symmetry. Such a scaling is expected to hold in the quantum critical regime, and is potentially the underlying reason for the exotic properties of heavy fermions compounds and high T_c superconductors. We will discuss possible experimental signatures. The Seminar will be based on works with Carlos Hoyos and Yaron Oz. |

**Null surface geometry, fluid vorticity, and turbulence**

Speaker: | Christopher Eling |

Department: | Max Planck Institute |

Slides: | [PDF] |

Time: | Tuesday 25 February 2014 at 14:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | I will review work over the past several years relating the dynamics of null horizons to hydrodynamics using the fluid/gravity correspondence. I will show how the fluid vorticity in 2+1 dimensions can be mapped into a geometric, gauge invariant quantity (a Newman-Penrose scalar) on the event horizon of a four dimensional black brane. I will use this result to characterize the horizon geometry dual to a turbulent flow and discuss the potential implications for the study of turbulent flows themselves. |

**Exact quantum entropy of black holes**

Speaker: | Sameer Murthy |

Department: | King’s College London |

Slides: | [PDF] |

Time: | Thursday 27 February 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | The pioneering work of Bekenstein and Hawking in the 70's produced a universal area law for black hole entropy valid in the infinite size limit. Quantum corrections to the gravitational action induce finite size corrections to the black hole entropy. I shall report on progress in the computation of the \emph{exact} quantum entropy of supersymmetric black holes in supergravity, using techniques of localization. In simple examples in string theory, one has a solvable dual microscopic description as an ensemble of microscopic excitations. I shall describe how the gravity functional integral leads to the microscopic \emph{integer} degeneracies of this black hole, and its associated number theoretic properties. |

**High-energy neutrino astronomy: a first glimpse to the promised land (department colloquium)**

Speaker: | Christian Spiering |

Department: | DEST |

Time: | Thursday 27 February 2014 at 17:00 |

Venue: | The 3rd floor seminar room of the physics department |

Abstract: | First ideas to build a large underwater neutrino detector started in 1973. After a fourty-year march we now are close to the promised land: IceCube, the cubic kilometer neutrino telescope at the South Pole reports high-energy neutrino events which hardly can be explained by interactions of neutrinos generated in the Earth's atmosphere. If confirmed, these observations would open a third window to the high-energy universe (after charged cosmic rays and gamma rays). The talk gives a short introduction into history, physics goals and functional principles of neutrino telescopes and then focuses to the results obtained during the last 2 years by IceCube and ANTARES. A discussion of future perspectives of the field will conclude the talk. |

**p-wave superconductors and spatial modulation**

Speaker: | Christiana Pantelidou |

Department: | Imperial College London |

Slides: | [PDF] |

Time: | Tuesday 4 March 2014 at 14:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | The AdS/CFT correspondence is a very powerful tool for studying strongly coupled CFTs at finite temperature and charge density and/or magnetic field, commonly found in condensed matter physics. Two of the main focus in this direction is to get a better understanding of superconductivity and spatially modulated phases in the holographic setup. In this talk, I will discuss how the two merge to give rise to spatially modulated superconducting p-wave states. |

**Holographic interaction effects on transport in Dirac semimetals**

Speaker: | Stefan Vandoren |

Department: | Utrecht University |

Slides: | [PDF] |

Time: | Thursday 20 March 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | Strongly interacting Dirac semimetals are investigated using a holographic model especially geared to compute the single-particle correlation function for this case, including both interaction effects and a non-zero temperature. We calculate the (homogeneous) optical conductivity at zero chemical potential, and show that it scales as a power law either in frequency or in temperature for low frequency. The precise power is related to a critical exponent of the dual holographic theory, which is a parameter in the model. The behavior for Coulomb interactions is obtained as a special limiting case. |

**Resurgent Transseries: Beyond (Large N) Perturbative Expansions
**

Speaker: | Ricardo Schiappa |

Department: | Instituto Superior Técnico |

Time: | Thursday 10 April 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | In this talk I will review recent progress in generically defining perturbation theory even in non-Borel summable situations, by making use of resurgent analysis and transseries. As an example, I will address the large N perturbative expansion and discuss new nonperturbative contributions found deep in the large-order expansion of the genus expansion. If time allows, I will also briefly comment on the large N dual picture (via matrix models), where resurgence and transseries may also be used to obtain nonperturbative solutions of closed (topological) string theory. |

**CFT constraints from three point functions**

Speaker: | Manuela Kulaxizi |

Department: | Université Libre de Bruxelles |

Slides: | [PDF] |

Time: | Thursday 24 April 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | We discuss energy flux (Hofman-Maldacena) constraints on non-conserved operators and an attempt to reformulate these constraints in the language of deep inelastic scattering. |

**Black Hole and fundamental fields in Numerical Relativity **

Speaker: | Hirotada Okawa |

Department: | University of Lisbon |

Slides: | [PDF] |

Time: | Tuesday 29 April 2014 at 14:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | Black Hole(BH) is a key player in the phenomina of our universe. Especially, we are interested in the dynamical interaction between BH and matter, which would affect the final states in such systems. Numerical Relativity allows us to investigate dynamics of BHs by directly solving Einstein's equations. In this talk, we would like to explain how to apply it to our problems and show you our results, for example, role of massive fields and "gravitational Magnus effect". |

**100 years of gravitation (department colloquium)**

Speaker: | Vitor Cardoso |

Department: | University of Lisbon |

Slides: | [PDF] |

Time: | Wednesday 30 April 2014 at 17:00 ***Note special day*** |

Venue: | The 3rd floor seminar room of the physics department |

Abstract: | General Relativity remains one of the most fascinating constructions of the human mind, explaining gravitation as curved-spacetime effects. Among the strangest predictions, are time-warps, gravitational waves and black holes, topics which are now playing a crucial role in fundamental physics, astrophysics, high energy physics and particle physics. In the last 10 years, our ability to understand strongly nonlinear phenomena in General Relativity has opened up a new Golden Age in the field. From Cosmic Censorship tests to superkicks and black hole bombs, the possibilities are endless. I will describe some of the current activity in the field along with prospects for the future. |

**High energy black hole collisions**

Speaker: | Vitor Cardoso |

Department: | University of Lisbon |

Slides: | [PDF] |

Time: | Friday 2 April 2014 at 14:15 ***Note special day*** |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | Gravity-dominated high energy collisions are important for a number of reasons, ranging from conceptual issues (cosmic censorship, maximum luminosity in the universe, hoop conjecture), to practical questions such as understanding of black hole production, gravitational-wave emission etc. I will summarize the state-of-the-art and some open problems in asymptotically flat spacetimes. |

**Thermodynamics of the BMN matrix model at strong coupling**

Speaker: | Miguel Sousa Costa |

Department: | University of Porto |

Slides: | [PDF] |

Time: | Tuesday 6 May 2014 at 14:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | We construct the black hole geometry dual to the deconfined phase of the BMN plane wave matrix model, therefore bring light into the phase diagram of this theory. In particular, we make a prediction for the critical temperature of this system at strong coupling that is amenable to being tested with Monte Carlo simulations of the matrix model. |

**The MOND paradigm of modified dynamics as an alternative to a dark universe (department colloquium)**

Speaker: | Mordehai Milgrom |

Department: | Weizmann Institute |

Time: | Thursday 8 May 2014 at 17:00 |

Venue: | The 3rd floor seminar room of the physics department |

Abstract: | MOND is and alternative paradigm of dynamics proposed to supersede Newtonian dynamics and General Relativity in the realm of the galaxies and the Universe at large. It strives to account for the various mass discrepancies observed in the Universe--which are generally attributed to "dark matter" and "dark energy"--without such hypothetical entities. Emphasis will be put on the general principles underlying MOND, and on its phenomenology: the various predictions that MOND makes, and how they fare observationally. |

**A Holographic Model of the Kondo Effect**

Speaker: | Andrew O'Bannon |

Department: | Oxford University |

Slides: | [PDF] |

Time: | Tuesday 20 May 2014 at 14:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | The Kondo effect occurs in metals doped with magnetic impurities: in the ground state the electrons form a screening cloud around each impurity, leading to dramatic changes in the thermodynamic and transport properties of the metal. Although the single-impurity Kondo effect is considered a solved problem, many questions remain, especially about the fate of the Kondo effect in the presence of multiple impurities. In particular, for a sufficiently dense concentration of impurities, a competition between the Kondo effect and inter-impurity interactions can lead to quantum criticality and non-Fermi liquid behavior, which remains poorly understood. In this talk I will present a model of the single-impurity Kondo effect based on holography, also known as gauge-gravity duality or the AdS/CFT correspondence, which may serve as a foundation for a new approach to the multiple-impurity system. |

**Deformations of special geometry and the holomorphic anomaly equation**

Speaker: | Gabriel Cardoso |

Department: | University of Lisbon |

Time: | Thursday 22 May 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | Special geometry is based on a holomorphic function F(X), with an associated vector (X^I, \partial F / \partial X^I) that undergoes symplectic transformations. The Hesse potential of special geometry is related to F(X) by a Legendre transformation, and it is expressed in terms of real variables that transform as canonical variables under symplectic transformations. Using the formulation based on the Hesse potential, we show that special geometry can be consistently deformed by a class of non-holomorphic terms that satisfy the so-called holomorphic anomaly equation of topological string theory. |

**Entanglement in General Holographic 2d CFTs**

Speaker: | Eric Perlmutter |

Department: | DAMTP, University of Cambridge |

Time: | Thursday 29 May 2014 at 15:15 |

Venue: | The 2nd floor seminar room of the physics department |

Abstract: | There has been phenomenal recent progress in computing entanglement and Renyi entropy in conformal field theories with pure gravity duals. As a step toward further understanding the intersection of holography, entanglement and the fundamental structure of CFTs, we extend these methods to more general 2d CFTs and their gravity duals. In particular, we work in the realm of higher spin holography, by computing ground state entanglement and Renyi entropy in certain classes of holographic 2d CFTs with higher spin symmetry. This involves a precise match between CFT and higher spin gravity calculations, performed at both leading and next-to-leading order in large central charge (small Newton's constant). We also present results on entanglement and Renyi entropy in 2d CFTs with gravitational anomalies, and a holographic modification of the Ryu-Takayanagi formula that encodes the anomalous contributions to entanglement. |