CCTP Seminars: Fall 2014

Aspects of RG flow with intermediate scalings

Speaker: Sera Cremonini
Department: DAMTP at Cambridge University and Mitchell Institute at Texas A&M University
Time: Tuesday 9 September 2014 at 11:00
Venue: The 2nd floor seminar room of the physics department
Abstract: Scaling geometries have recently played an interesting role in holography as a laboratory for probing strongly coupled condensed matter systems with unusual transport properties. In this talk I will discuss solutions which realize holographically RG flows with intermediate scaling regimes. I will comment on various aspects of their possible ground states, and on applications to transport.

Some Applications in AdS/CMT

Speaker: Li Li
Department: University of Crete, CCTP
Time: Thursday 11 September 2014 at 15:00
Venue: The 2nd floor seminar room of the physics department
Abstract: In this seminar, I will introduce my past and current work with my collaborators. For the first part, I will focus on a holographic multi-component superconductor and a new holographic p-wave model. Then I will show two current attempts. One is to construct the Josephson Junction with Lifshitz scaling. The other is to study the time evolution of entanglement entropy in a far from equilibrium response in the s-wave holographic superfluid model.

Non-equilibrium dynamics of strongly coupled systems by holography

Speaker: Wei-Jia Li
Department: University of Crete, CCTP
Time: Thursday 11 September 2014 at 15:30
Venue: The 2nd floor seminar room of the physics department
Abstract: This talk includes two sections. In the first section, I would like to talk about the previous work on real time dynamics of periodically driven holographic superconductors. By performing numerical analysis, as a result, the superconductivity of the strongly coupled system is suppressed with the periodically driving electrical field no matter how large the frequency is, which is not the case of weakly coupled limit. Furthermore, in the high frequency limit, three distinct channels towards the final steady state are found, which can be captured essentially by the spectrum of the low lying quasi-normal modes in the time averaged approximation. In the last section, I turn to talk about an ongoing project on evolution of the entanglement entropy in semi-local liquids. Using some analytical methods, we find a slow growth of the entanglement entropy and capture novels scaling behaviors during the quenching process of the systems with spatial localization.

Large dimensionality in Holographic Superconductors and Designer (multi)gravity in Condensed Matter

Speaker: Aurelio Romero
Department: Cavendish Laboratory at Cambridge University
Time: Thursday 11 September 2014 at 16:00
Venue: The 2nd floor seminar room of the physics department
Abstract: We study the usual model for holographic superconductors in the limit in which the number of dimensions in the dual theory, d, is large. We study analytically the low, intermediate and large frequency-dependence of the conductivity, in the large-d limit at $T=0$. Similarly, we provide simple expressions for the entanglement entropy between a rectangular strip and its complement in the boundary. We study the entanglement in the large-d limit at $T=0$ and at $T\sim T_c$. We also compute numerically, in the probe limit, the order parameter and the electrical conductivity for up to 10 dimensions. Both the numerical and analytical results provide evidence that, as the number of dimensions increases, the dual field theory becomes more and more weakly coupled. As a second topic, we use the so-called designer (multi)-gravity technique to study multitrace deformations on different systems. The aim is to study the holographic dual on Josephson Junctions and the Proximity effect using different backgrounds like boost invariant planar solutions and the AdS soliton. This is an ongoing research at this stage.

Cosmology with Mimetic Matter

Speaker: Alexander Vikman
Department: CERN and MPI for Physics at Munich University
Time: Tuesday 23 September 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: I will discuss the recently introduced "mimetic" modification of general relativity. This modification is manifestly Weyl-invariant and can mimic practically any cosmological epoque. In particular, this "mimetic" modification can naturally account for a part of dark matter. It is also possible to unify such dark matter with dark energy. Finally one can construct an interesting single-field model for the early universe inflation. This model has parametrically suppressed gravity waves but does not seem to produce any non-Gaussianity, moreover the usual consistency relation for the single-field inflationary models gets modified.

Electric Field Quench in AdS/CFT

Speaker: Keiju Murata
Department: Keio University and DAMTP at Cambridge University
Time: Thursday 25 September 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: An electric field quench, a suddenly applied electric field, can induce nontrivial dynamics in confining systems which may lead to thermalization as well as a deconfinement transition. In order to analyze this nonequilibrium transitions,we use the AdS/CFT correspondence for N=2 supersymmetric QCD that has a confining meson sector. We find that the electric field quench causes the deconfinement transition even when the magnitude of the applied electric field is smaller than the critical value for the static case (which is the QCD Schwinger limit for quark-antiquark pair creation). The time dependence is crucial for this phenomenon, and the gravity dual explains it as the weakly turbulent instability of a D-brane in the bulk AdS spacetime. Interestingly, the deconfinement time takes only discrete values as a function of the magnitude of the electric field. We advocate that the new deconfinement phenomenon is analogous to the exciton Mott transition.

From integrands to integrated amplitudes: on contours and regularization for on-shell diagrams

Speaker: Paolo Benincasa
Department: IFT at Madrid University
Time: Tuesday 30 September 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: In recent years a big deal of progress has been made towards a deeper understanding of particle theory. In this talk, we discuss the on-going program of formulating perturbative physics just in terms of on-shell processes. Through the analysis of the analytic structure of the amplitude, this approach can generate both tree-level amplitudes and *on-shell integrands* at loop level. We discuss the integration contours which are used to obtain the loop amplitudes from these on-shell integrands. The integration procedure by itself needs a regularisation scheme that deals both with IR and UV divergencies. The four-dimensional nature of this on-shell approach makes the standard dimensional regularization inadequate. We propose a general four-dimensional regularization scheme which is in principle theory-independent and shows how the IR-devergences emerge. Finally, we discuss what insights this point of view provides, and the issues of its current status.

Warped Conformal Field Theory and Non-AdS holography

Speaker: Diego Hofman
Department: University of Amsterdam
Time: Thursday 2 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: I'll discuss two-dimensional field theories that, while translationally invariant, are not Lorentz invariant. They have left (but not right) scaling symmetry. These theories have appeared in the context of AdS/CFT as holographic duals of warped AdS backgrounds in Topologically Massive Gravity, TsT deformations of exact string theory solutions and Kerr/CFT duality. I'll show that, as in the usual CFT case, these global symmetries extended to an infinite dimensional algebra. Furthermore, I'll explain that these symmetries are enough to show the existence of a Cardy-like formula for the asymptotic density of states. This purely field theoretic result matches calculations of the Bekenstein-Hawking entropy of black holes in the bulk dual, including Kerr Black Holes. Lastly I'll discuss the background fields that couple to these theories and I'll propose a framework for building a holographic dual systematically.

Non-thermal fixed points: From turbulent Bose gases to high-energy physics

Speaker: Thomas Gasenzer
Department: Ruprecht-Karls University of Heidelberg
Time: Thursday 2 October 2014 at 17:00
Venue: The 3rd floor seminar room of the physics department
Abstract: Many-body systems far away from thermal equilibrium can show a much wider range of characteristics than equilibrium systems. Among the wealth of possible non-equilibrium many-body configurations most interesting candidates are those which show universal behavior. A selection of such phenomena in ultracold Bose gases, characterized by specific power-laws in space and time, will be presented. Our results open a path to explore a new class of universal far-from-equilibrium dynamics well accessible in ultracold gas experiments. These phenomena are of importance far beyond the realm of cold gases, via solid-state systems to high-energy heavy-ion collisions and cosmology.

Cardy formulae in SUSY theories

Speaker: Lorenzo di Pietro
Department: Weizmann Institute of Science
Time: Tuesday 7 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: We consider 4d N=1 SUSY theories on spaces with compact space-like slices. One can define a generalized Witten index by a weighted sum over states. This object is computed by the path integral on the background with a compactified time direction and periodic conditions for the fermions. We will show a universal asymptotic formula for the index in the limit of small radius of the Euclidean time circle. The asymptotic behavior is fixed by the anomalies of the theory. If time permits, we will discuss some applications and the generalization to 6d (1,0) theories.

Vortices in holographic superfluids and superconductors

Speaker: Oscar Dias
Department: University of Southampton
Time: Tuesday 14 October 2014 at 15:30
Venue: The 2nd floor seminar room of the physics department
Abstract: I will present a nonlinear gravitational solution that describes a single vortex in a holographic symmetry breaking phase. At low energies the system flows to an nontrivial conformal fixed point. Novel vortex physics arises from the interaction of these gapless degrees of freedom with the vortex: at low energies the vortex may be understood as a conformal defect in this low energy theory. Defect conformal symmetry allows the construction of a simple infrared geometry describing a new kind of extremal horizon: a Poincare horizon with a small bubble of magnetic Reissner-Nordstrom horizon inside it that carries a single unit of magnetic flux and a finite amount of entropy even at zero temperature. I will also present the full geometry describing the vortex at finite temperature in a UV complete theory. I will discuss both superfluid and superconducting boundary conditions and calculate thermodynamic properties of the vortex. A study of vortex stability reveals that the dual superconductor can be Type I or Type II, depending on the charge of the condensed scalar.

Gravity as (gauge theory)^2: from amplitudes to black holes

Speaker: Ricardo Monteiro
Department: Oxford University
Time: Thursday 16 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: We will discuss the relation between perturbative gauge theory and perturbative gravity, and look at how this relation extends to some exact classical solutions. First, we will review the double copy prescription that takes gauge theory amplitudes into gravity amplitudes, which has been crucial to progress in perturbative studies of supergravity. Then, we will see how the relation between the two theories can be made manifest when we restrict to the self-dual sector, in four dimensions. A key role is played by a kinematic algebraic structure mirroring the colour structure, which can be extended from the self-dual sector to the full theory, in any number of dimensions. Finally, we will see how these ideas can be applied also to some exact classical solutions, namely black holes and plane waves. This leads to a relation of the type Schwarzschild as (Coulomb charge)^2.

Holographic baryons and the Sakai-Sugimoto soliton

Speaker: Stefano Bolognesi
Department: IFN, University of Pisa
Time: Tuesday 21 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: The Sakai-Sugimoto model is the preeminent example of a string theory description of holographic QCD, in which baryons correspond to topological solitons in the bulk. In the first part of the talk we investigate the validity of various approximations of the Sakai-Sugimoto soliton that are used widely to study the properties of holographic baryons. These different approaches have produced contradictory results in the literature regarding properties of the baryon, such as relations for the electromagnetic form factors. Our analysis clarifies the source of the contradictory results in the literature and resolves some outstanding issues. In the second part we discuss physics at finite baryon density. In holographic QCD models this gets translated into a multi-instanton problem in the bulk, and a state with a high density baryonic charge consists of a non-diluted multi-instanton solution. The instanton bag is a good candidate for this high-density state. We compute its parameters via moduli stabilization. Chiral symmetry restoration is exhibited by this state, and it is a direct consequence of its non-diluted features.

GUTs without guts

Speaker: Bert Schellekens
Department: NIKHEF Amsterdam
Time: Wednesday 22 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: The discrete structure of the Standard Model (gauge group, representations and in particular charge quantization) is derived in a class of intersecting brane models from a mild anthropic constraint, namely the existence of electrodynamics. Although Grand Unification is realized in a subclass, this offers no advantages whatsoever. In all other cases the Standard Model is realized as a GUT with its intestines removed, to keep only the good parts: a GUT without guts.

Simple models of holographic superconducting lattices

Speaker: Tomas Andrade
Department: Durham University
Time: Thursday 23 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: We will discuss two mechanisms of momentum relaxation in holography which preserve homogeneity in the bulk geometry. Studying the condensation of a complex scalar field on these black hole solutions we will describe superconductivity on the dual theory. We will then compute the optical conductivity of the so constructed black holes finding good matching with the field theory expectations.

Teleparallel equivalent of Gauss-Bonnet gravity

Speaker: Giorgos Kofinas
Department: University of the Aegean
Time: Monday 27 October 2014 at 13.30
Venue: The 2nd floor seminar room of the physics department
Abstract: A discussion of the teleparallel equivalent of Einstein gravity and other theories is given. The teleparallel equivalent term of Gauss-Bonnet gravity is presented in arbitrary dimensions, as well as a new class of modified gravities based on this term. A cosmological analysis of this theory is performed together with a stability analysis. For a non-minimal coupling of a scalar field with the torsion scalar a solution is presented.

Black holes in N=8 supergravity

Speaker: David Chow
Department: CCTP, Crete Center for Theoretical Physics
Time: Wednesday 29 October 2014 at 11.00
Venue: The 2nd floor seminar room of the physics department
Abstract: I will discuss the construction of general rotating, non-extremal black holes of maximal N=8 supergravity in 4 dimensions. These are asymptotically flat, multi-charge, dyonic exact solutions that generalize the Kerr-Newman solution of Einstein-Maxwell theory. I will also discuss some properties and generalizations to gauged supergravity.

What is inside a black hole?

Speaker: Marika Taylor
Department: University of Southampton
Time: Thursday 30 October 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: There has been renewed interest in the black hole information paradox following the claims by AMPS that unitary evolution is possible only if black holes have firewalls located just inside their event horizons. In this talk we will argue that holography implies an alternative, and rather more natural, resolution of the information loss paradox: significant modifications to the naive black hole geometry at sub-horizon scales. AdS/CFT calculations will be used to infer how the region "inside" a black hole should be described: we will use holography to explain why certain families of microstates of BPS black holes can be well-described by horizonless supergravity geometries (often called fuzzballs or microstate geometries in the literature) but we will also that generic black hole microstates cannot be captured within supergravity. Throughout the talk we will relate our calculations to the recent claims of Hawking, Martinec and others.

Dirty Holographic Superconductors

Speaker: Daniel Arean
Department: Max Planck Institute, Munich
Time: Tuesday 4 November 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: The study of disorder, and in particular the phenomenon of Anderson localization, in interacting many-body systems is a challenging problem in condensed matter physics. By introducing disorder in holographic realizations of superconductivity we can get predictions from the gauge/gravity duality on how strongly coupled superconductors behave in the presence of impurities. In the framework of holographic s- and p-wave superconductors we will study the effect of disorder on the critical temperature and the conductivity of the system.We will also characterize the spectral properties of both the condensate and the charge density, finding a fairly universal response consisting of linear functions of the input (disordered) power spectrum.

New results in backreacted holographic QCD

Speaker: Matti Jarvinen
Department: ENS, Paris
Time: Thursday 6 November 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: I discuss some of the latest developments in holographic bottom-up models for QCD in the Veneziano limit (V-QCD) where the flavor fully backreacts to the glue. Finite quark mass and the axial anomaly can be included in the model in a natural way. I will show how the quark mass affects the phase diagram and the bound state masses. The eta' meson mass satisfies the Witten-Veneziano relation at small quark mass and at small N_f/N_c. If I have time I will also discuss double trace deformations in the model.

FQH/CFT and its deformation

Speaker: Taro Kimura
Department: Institut de Physique Theorique, CEA Saclay
Time: Tuesday 11 November 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: Fractional quantum Hall (FQH) system is the most fundamental example of topological states of matter. The wave function of the FQH system is given as a conformal block of a certain CFT, and the anyonic statistics of its quasi-particle/hole can be described by the fusion rule in the corresponding CFT. In this talk, after reviewing some basic aspects of the FQH/CFT relation, we would like to present the deformation and its application to a system involving spin degrees of freedom. This talk is partly based on [arXiv:1201.1903].

Electromagnetic-induced Effects in Strongly Coupled Plasmas

Speaker: Di-Lun Yang
Department: CCTP, Crete Center for Theoretical Physics
Time: Thursday 13 November 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: The interplay between the electromagnetic fields and chiral fermions in strongly coupled plasmas could play an important role in heavy ion collisions. In this talk, we will discuss about the anomalous transport induced by the electric fields and thermal photon production engendered by a strong magnetic field in strongly coupled plasmas in the framework of the AdS/CFT correspondence. It has been found that the presence of both a vector and an axial chemical potential could induce an axial current parallel to the applied electric field known as the chiral electric separation effect (CESE). We find that an axial Hall current can also be generated when introducing an electric field and a magnetic field perpendicular to each other with an axial chemical potential, which could be dubbed as the chiral Hall effect (CHE). The fluctuations of chemical potentials will further result in chiral electric waves (CEW) as propagating density waves led by the applied electromagnetic fields. We argue that the CHE could lead to nontrivial charge distributions at different rapidity in asymmetric heavy ion collisions. In addition, the strong magnetic field could enhance the thermal photon production and result in large Fourier coefficient v_2 associated with the elliptic flow. We evaluate thermal-photon v_2 caused by a constant magnetic field in a strongly coupled supersymmetric plasma. It turns out that the magnetic field can generate large v_2 when it reaches the magnitude around the maximum magnetic field produced in heavy ion collisions in early times.

Non-equilibrium steady states in many-body quantum systems

Speaker: Benjamin Doyon
Department: King's College
Time: Tuesday 18 November 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: Let an infinite, homogeneous, many-body quantum system be unitarily evolved for a long time from a state where two halves are independently thermalized. A non-equilibrium steady state emerges if there are nonzero steady currents in the central region, and their presence is a signature of ballistic transport. I will describe recent results for such non-equilibrium steady states, in any dimensionality. I will explain how, using the Lieb-Robinson bound, there are, under certain conditions, bounds on the average current and on the noise. These suggest definitions for nonlinear sound velocities in non-integrable models, and for generalized sound velocities, encoding generalized Gibbs thermalization, in integrable models. Depending on time, I will then describe results for the exact steady state, the energy current and its fluctuations in various models (one- and higher-dimensional CFT, free-particle models), involving various techniques including aspect of the AdS/CFT duality. I will discuss the bounds within these various models, and I will explain how large-time fluctuations are described by independent Poisson processes. This is based in part on works done in collaboration with Denis Bernard and Marianne Hoogeveen, and with Joe Bhaseen, Andrew Lucas and Koenraad Schalm.

Near horizon geometry of M2/M5-branes

Speaker: John Estes
Department: Imperial College
Time: Tuesday 25 November 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: I will discuss the construction of half-BPS solutions of 11-dimensional supergravity, with isometry superalgebra D(2,1;\gamma) x D(2,1;\gamma). Their space-time has the form AdS_3 x S^3 x S^3 warped over a 2d Riemann surface. The solutions include the near-horizon geometries of M2-branes ending on, or intersecting with, M5-branes along a common string.

Supersymmetric partition functions and Twisted Holomorphic Theories

Speaker: Guido Festuccia
Department: NBI at University of Copenhagen
Time: Tuesday 2 December 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: I will consider supersymmetric field theories on compact manifolds M and obtain constraints on the dependence of their partition functions Z_M on the geometry of M. For N=1 theories with a U(1) R symmetry in four dimensions, M must be a complex manifold with a Hermitian metric. I will show how describe the theory in terms of twisted variables that make easy to analyze the dependence of Z_M on the parameters entering the Lagrangian. I will also show that Z_M is "almost" topological: Z_M is independent of the Hermitian metric and depends holomorphically on the complex structure moduli.

AdS/CFT and non-Fermi liquids

Speaker: Giuseppe Policastro
Department: ENS, Paris
Time: Tuesday 9 December 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: In this talk I will review the recent efforts to understand some aspects of strongly correlated fermionic systems using the AdS/CFT correspondence. The motivation is partly phenomenological, coming from the desire to have models capable of describing the behavior of systems like the high-temperature cuprate superconductors, and partly purely theoretical, namely understanding what phases of matter at finite density are possible. Particular emphasis will be given to the deviations from Fermi liquid behavior exhibited by several holographic constructions.

Magnetohydrodynamics and charged currents in heavy ion collisions

Speaker: Umut Gursoy
Department: Utrecht University
Time: Wednesday 10 December 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: The hot QCD matter produced in any heavy ion collision with a nonzero impact parameter is produced within a strong magnetic field. We study the imprint the magnetic fields produced in non-central heavy ion collisions leave on the azimuthal distributions and correlations of the produced charged hadrons. The magnetic field is time-dependent and the medium is expanding, which leads to the induction of charged currents due to the combination of Faraday and Hall effects. We find that these currents result in a charge-dependent directed flow v1 that is odd in rapidity and odd under charge exchange. It can be detected by measuring correlations between the directed flow of charged hadrons at different rapidities.

Fluid Dynamics from Lattice QCD

Speaker: Simon Mages
Department: University of Regensburg
Time: Thursday 11 December 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: In the strongly interacting medium produced in heavy ion collisions non-perturbative effects of QCD play an important role. Lattice QCD is a well established framework to answer questions about non-perturbative properties of QCD. In this talk I will give an introduction on the principles and methods that constitute Lattice QCD and report about our latest efforts on reconstructing transport properties of the medium like the shear viscosity. I will also introduce the concept of the so called "Wilson Flow" as a recently developed tool for accessing the infrared properties of a gauge theory.

Gravity waves from Kerr/CFT

Speaker: Achilleas Porfyriadis
Department: Harvard University
Time: Tuesday 16 December 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: Astronomical observation suggests the existence of near-extreme Kerr black holes whose horizons spin at nearly the speed of light. Properties of diffeomorphisms imply that the dynamics of the high-redshift near-horizon region of near-extreme Kerr, which includes the innermost-stable-circular-orbit (ISCO), is governed by an infinite-dimensional emergent conformal symmetry. This symmetry may be exploited to analytically, rather than numerically, compute a variety of potentially observable processes. In this talk I will show how we compute and study the conformal transformation properties of the gravitational radiation emitted by an orbiting massive object in the large-redshift near-horizon region. I will also use conformal symmetry of the near-horizon region to compute the gravitational radiation produced during the plunge phase following the object's crossing of the ISCO.

Holographic RG flows and quantum effective actions

Speaker: Francesco Nitti
Department: APC, Universite Paris 7,-
Time: Thursday 18 December 2014 at 14:00
Venue: The 2nd floor seminar room of the physics department
Abstract: I will show how one can write explicit covariant expressions for the renormalized holographic generating functional, in a derivative expansion with respect to the space-time coordinates. Independence of the radial coordinate of the generating functional gives rise to holographic RG equations. I will describe the connection between QFT local RG transformations and radial diffeomorphisms, and show how beta-functions for space-time-dependent couplings can be computed from the bulk Einstein equations. I will discuss the application to the calculation of the quantum effective potential for gauge-invariant opertors in holographic models for QCD.