CCTP 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.

### Proton Structure and Tensor Gluons

 Speaker: George Savvidy Department: Demokritos National Research Center Time: Tuesday 4 February 2014 at 13:00 ***Note special time*** Venue: The 2nd floor seminar room of the physics department Abstract: We consider a possibility that inside the proton and, more generally, inside the hadrons there are additional partons - tensor-gluons, which can carry a part of the proton momentum. The tensor-gluons have zero electric charge, like gluons, but have a larger spin. Inside the proton a nonzero density of the tensor-gluons can be generated by the emission of tensor-gluons by gluons. The last mechanism is typical for non-Abelian tensor gauge theories, in which there exists a gluon-tensor-tensor vertex of order g. Therefore the number of gluons changes not only because a quark may radiate a gluon or because a gluon may split into a quark-antiquark pair or into two gluons, but also because a gluon can split into two tensor-gluons. The process of gluon splitting suggests that part of the proton momentum which was carried by neutral partons is shared between vector and tensor gluons. We derive evolution equations for the parton distribution functions which take into account these new processes. The momentum sum rule allows to find the tensor-gluons contribution to the Callan-Simanzik beta function and to calculate the corresponding anomalous dimensions. This contribution changes the behavior of the structure functions, and the logarithmic correction to the Bjorken scaling becomes more mild. This also influences the unification scale at which the coupling constants of the Standard Model merge, shifting its value to lower energies of order of 40 TeV.

### Entanglement entropy and higher derivative Gravity

 Speaker: Joan Camps Department: DAMTP, University of Cambridge Slides: [PDF] Time: Tuesday 11 February 2014 at 13:30 ***Note special time*** Venue: The 2nd floor seminar room of the physics department Abstract: I will discuss the Maldacena-Lewkowycz derivation of the Ryu-Takayangi prescrition for holographic entanglement entropy, and extend it to a more general class of theories of gravity in the bulk. This analysis results in a new euclidean entropy functional, that generalises Wald's entropy. In the lorentzian, this functional is a natural extension of Wald's black hole entropy to time dependent situations.

### 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.

### AdS/CFT and Landau Fermi liquids

 Speaker: Mikhail Goykhman Department: Leiden University Slides: [PDF] Time: Thursday 6 March 2014 at 15:15 Venue: The 2nd floor seminar room of the physics department Abstract: In this talk I am going to talk about the field theory dual to a charged gravitational background in which the low temperature entropy scales linearly with the temperature. This systems exhibits the existence of a sound mode which is described by hydrodynamics, even at energies much larger than the temperature. This and other properties of the field theory are consistent with those of a 3+1-dimensional Landau Fermi liquid, finely tuned to Pomeranchuk critical point ($F_2=-5$). I will also talk about how one could engineer a higher-derivative gravitational Lagrangian which reproduces the correct low temperature behavior of shear viscosity in a generic Landau Fermi liquid.

### Smaller is different and more: surprises in low dimensional superconductivity (department colloquium)

 Speaker: Antonio M. Garcia-Garcia Department: University of Cambridge Time: Thursday 13 March 2014 at 17:00 Venue: The 3rd floor seminar room of the physics department Abstract: I review recent experimental and theoretical advances in the problem of superconductivity in nanograins, thin films, heterostructures and Josephson arrays of nanograins of conventional and high temperature superconductors. As a guiding principle the talk focuses on theoretical mechanisms and experimental settings that can exploit quantum size and non equilibrium effects to boost superconductivity in these systems.

### The out of equilibrium birth of a holographic superfluid

 Speaker: Antonio M. Garcia-Garcia Department: University of Cambridge Time: Friday 14 March 2014 at 14:15 ***Note special day*** Venue: The 2nd floor seminar room of the physics department Abstract: We study the time evolution of a holographic superconductor after a linear thermal quench from a disordered to an ordered phase. We identify the time scale for which the condensate starts to form. For later times, and sufficiently slow quenches, the scaling of the number of vortices with the quench speed follows the Kibble-Zurek prediction. We put forward a theoretical prediction for the total number of vortices that improves substantially the Kibble-Zurek result. We also identify the maximum quench speed for which KZ scaling is valid and propose a theoretical prediction for the number of vortices for faster quenches. Finally we identify a time scale for which a pseudogap phase is observed, namely, the condensate is formed but there is no phase coherence.

### 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.

### Universal response in anomalous cold holographic superfluids

 Speaker: Irene Amado Department: Technion Slides: [PDF] Time: Thursday 27 March 2014 at 15:15 Venue: The 2nd floor seminar room of the physics department Abstract: Using the frame of the fluid/gravity correspondence, I will argue that at low temperature the response of generic U(1) anomalous s-wave superfluids is universal, i.e independent of the particular details of the action and of the emergent low energy geometry and that it is completely determined by the anomalies.

### 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.

### Momentum dissipation and charge transport in holography

 Speaker: Richard Davison Department: Leiden University Slides: [PDF] Time: Tuesday 15 April 2014 at 14:15 Venue: The 2nd floor seminar room of the physics department Abstract: Holography can be used to study the physics of strongly coupled quantum field theories at finite density, which may be at work in certain real materials. However, the conservation of momentum in these field theories means that they have unrealistic transport properties, such as an infinite electrical conductivity. I will give a summary of recent progress in constructing gravitational solutions which are dual to field theories which dissipate momentum. I will explain how these gravitational solutions are related to theories of massive gravity, and what the transport properties of the dual field theories are. Finally, I will describe a simple mechanism, which is natural from the holographic point of view, by which strongly interacting metals can acquire a linear in temperature resistivity, as is observed in the strange metal phase of the cuprates.

### Aspects of 3d N=2 dualities: RG flows, localization and contact terms

 Speaker: Antonio Amariti Department: École Normale Supérieure Time: Thursday 17 April 2014 at 15:15 Venue: The 2nd floor seminar room of the physics department Abstract: I discuss some aspects of dualities between three dimensional N=2 supersymmetric gauge theories. First I review basic aspects of these field theories: the action, the moduli space and the role of the global symmetries. After I introduce some known dualities and discuss how to move with RG flows between these dual pairs. I explain how to follow these flows on the three sphere partition function and discuss the role of the contact terms of the two point global current correlators.

 Speaker: Andrei Parnachev Department: Leiden University Time: Tuesday 22 April 2014 at 14:15 Venue: The 2nd floor seminar room of the physics department Abstract: We will discuss various measurements of quantum entanglement and their relations to the c-functions in quantum field theories.

### 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.