### University of Crete HEP Seminars: Spring 2015

**Transport and integrability. Lecture I**

Speaker: | Xenophon Zotos |

Department: | University of Crete |

Time: | Wednesday 11 February 2015 at 11:15 |

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

Abstract: | Linear response theory and the Drude weight. Integrable systems and conservation laws. The "heavy" particle paradigm. A classical problem — the Toda chain. The puzzle of the one-dimensional spin-1/2 Heisenberg model. The Hubbard, t-J and other integrable spin models. Integrable effective theories. Impurities and perturbations in integrable models. Quasi-local conservation laws. Results from novel numerical simulation methods. Integrability, experiments in quasi-1D magnets and applications. |

**Demystifying the numerical tools in holography**

Speaker: | Daniel Fernández |

Department: | Max Planck Institute for Physics, Munich |

Time: | Thursday 19 February 2015 at 15:30 |

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

Abstract: | The holographic duality is now understood to be a powerful tool for studying strongly interacting systems, and to a great extent this is motivated from experimental data. In order to bring this paradigm forward with more accurate results, the focus should go into finding efficient ways to solve the classical partial differential equations in the gravity side. I will comment on several open problems for which this challenge becomes unavoidable, while reviewing a few key ingredients which are necessary for the numerical techniques. |

** Signatures of natural SUSY**

Speaker: | Antonio Delgado |

Department: | Univeristy of Notre Dame |

Time: | Tuesday 24 February 2015 at 14:15 |

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

Abstract: | In this seminar I will introduce the concept of natural SUSY and analyze quite thoroughly two scenarios, one with heavy stops and another one with compressed spectrum of neutralinos. |

** Reconstructing holography as an RG flow**

Speaker: | Ayan Mukhopadhyay |

Department: | University of Crete |

Time: | Thursday 5 March 2015 at 14:15 |

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

Abstract: | We will discuss how the holographic correspondence can be reconstructed as a special RG flow in a strongly interacting large N field theory. An explicit construction will be given in the hydrodynamic limit. |

** The information paradox in AdS/CFT**

Speaker: | Kyriakos Papadodimas |

Department: | CERN and Centre for Theoretical Physics, University of Groningen |

Time: | Tuesday 10 March 2015 at 14:15 |

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

Abstract: | By refining recent versions of the information paradox in AdS/CFT, we show how the CFT successfully describe local physics behind the horizon, not only for single-sided black holes but even in the eternal black hole. The solution requires operators to be state-dependence. In this setup the infalling observer does not observe any violations of quantum mechanics or firewall. We comment on how to resolve a large class of potential ambiguities in the state-dependence construction. |

** An effective microscopic description of black brane thermodynamics**

Speaker: | Benjamin Withers |

Department: | STAG group, University of Southampton |

Time: | Tuesday 17 March 2015 at 14:15 |

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

Abstract: | I will construct effective large-N microscopic field theories which provide a description of a general class of black branes near extremality. In the regime where the gravity description is valid, the field theory is strongly coupled and exhibits a scaling regime. With a simple application of the virial theorem, the microscopic theory can be used to estimate the parametric form of the black brane thermodynamics, including the correct strong-coupling N-dependence. The class of theories is general enough to encompass black branes in string theory, where they can be derived from SYM. In these cases it is possible to estimate phase boundaries and make predictions for a large class of operators at finite temperature. |

**Anomaly induced negative magnetoresistivity in Anti-de Sitter space
and Dirac metals**

Speaker: | Karl Landsteiner |

Department: | University of Madrid, IFT, CSIC |

Time: | Thursday 19 March 2015 at 14:15 |

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

Abstract: | In recent years it has been realized that chiral anomalies play a distihished role in transport theory. One particular phenomenon is the dramatic increase in electric conductivity induced by the axial anamaly via the chiral magnetic effect (CME). I will review the theory of anomalous transport and negative magnetoresistivity and present its treatment in hydrodynamics and holography. I will also review recent expermiental results with Dirac metals. |

**Transport and integrability. Lecture II**

Speaker: | Xenophon Zotos |

Department: | University of Crete |

Time: | Monday 23 March 2015 at 11:15 |

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

Abstract: | Linear response theory and the Drude weight. Integrable systems and conservation laws. The "heavy" particle paradigm. A classical problem — the Toda chain. The puzzle of the one-dimensional spin-1/2 Heisenberg model. The Hubbard, t-J and other integrable spin models. Integrable effective theories. Impurities and perturbations in integrable models. Quasi-local conservation laws. Results from novel numerical simulation methods. Integrability, experiments in quasi-1D magnets and applications. |

** Monopole correlations in holographically flavored liquids **

Speaker: | Nabil Iqbal |

Department: | Institute for Theoretical Physics, University of Amsterdam |

Time: | Tuesday 24 March 2015 at 14:15 |

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

Abstract: | Given a many-body system in (2+1) dimensions with a conserved U(1) current, an interesting probe of the IR dynamics is provided by weakly gauging the current and then studying the correlation functions of magnetic monopole operators in the resulting dynamical gauge theory. This correlation function is a non-perturbative object and so is somewhat difficult to calculate using conventional techniques. We review the expected behavior of such monopole correlations in different field-theoretical states. We then turn to holography and study monopole correlations in holographic liquids with fundamental flavor, where the monopole operator is dual to a bulk particle that carries magnetic charge, permitting a simple geometric understanding. We display consistency with expected field-theoretical results. For a charge-gapped phase this requires a discussion of monopole condensation in the bulk, which turns out to be equivalent to the capping off of the bulk flavor brane. In a compressible finite-density phase we present a novel calculation of the monopole correlation in space and time. |

**Real Time Dynamics and Screening in Finite Temperature QCD **

Speaker: | Harvey Meyer |

Department: | Institute of Nuclear Physics, Johannes Gutenberg University Mainz |

Time: | Thursday 26 March 2015 at 14:15 |

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

Abstract: | We investigate the low-temperature phase of QCD and the crossover region with two light flavors of quarks. The chiral expansion around the point (T,mq=0) in the temperature vs. quark-mass plane indicates that a sharp real-time excitation exists with the quantum numbers of the pion. We determine its dispersion relation and test the applicability of the chiral expansion. Finally, we test the predictions of ordinary chiral perturbation theory against lattice results. |

** Quantum entanglement of locally perturbed thermal states **

Speaker: | Joan Simón |

Department: | School of Mathematics and Maxwell Institute, University of Edinburgh |

Time: | Tuesday 31 March 2015 at 14:15 |

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

Abstract: | We will review the main arguments leading to the connection between quantum entanglement and geometry in the context of the AdS/CFT correspondence. Building on this, we will perturb a thermal state and ask what the time scale is for the mutual information between two subsystems to vanish. We will motivate this question from black hole physics and relate it to the notion of scrambling time. We will explicitly compute the mutual information for a thermofield double state perturbed by a localised primary operator in 2d CFTs in the large c limit. We will briefly comment on the holographic calculation involving a BTZ black hole and the relation with previous work in the literature. |

**Large scale structure of the Universe: The relativistic angular power spectrum and bispectrum **

Speaker: | Ruth Durrer |

Department: | Department of Theoretical Physics, University of Geneva |

Time: | Thursday 2 April 2015 at 14:15 |

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

Abstract: | I shall discuss observations of large scale structure taking fully into account relativistic effects coming from the fact that we can only observe on our background light cone and that we observe the direction of incoming photons and their true (perturbed redshift). In addition to the well known redshift space distortion this leads to effects from lensing convergence, integrated Sachs Wolfe terms and other relativistic effects. I shall consistently define a power spectrum and Bispectrum in angular and redshift space which can take into account all these effects. I shall also discuss some numerical results and some forecasts for future observations e.g. with Euclid. |

**Fermion spectral functions in zero temperature states of ABJM theory**

Speaker: | Oscar Henriksson |

Department: | University of Colorado Boulder |

Time: | Tuesday 7 April 2015 at 14:15 |

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

Abstract: | Top down realizations of holographic matter at zero temperature provide an excellent framework for uncovering interesting properties of strongly interacting theories. A useful diagnostic of these properties is the linear response of the matter to various field theory probes. In this talk I will highlight the lessons to be learned from fermionic probes, which can help reveal the fermionic nature of candidate holographic ground states. From the dual supergravity perspective, this involves identifying and solving the fermionic sector of various supergravity theories. Recent progress towards computing fermion spectral functions in novel states of ABJM theory will frame the discussion. |

** Adiabatic hydrodynamics and the eightfold way to dissipation**

Speaker: | Felix Haehl |

Department: | Centre for Particle Theory and Department of Mathematical Sciences, Durham University |

Time: | Tuesday 21 April 2015 at 14:15 |

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

Abstract: | I will describe a complete solution to hydrodynamic transport at all orders in the gradient expansion compatible with the second law constraint. A key ingredient is the notion of adiabaticity, which allows to take hydrodynamics off-shell. I will explain a sevenfold classification scheme of adiabatic fluids which, together with the dissipative sector, establishes the "eightfold way of hydrodynamic transport". While this completes the classification, I will furthermore argue for a new symmetry principle, an Abelian gauge invariance which guarantees adiabaticity in hydrodynamics. This new symmetry can be seen as the macroscopic manifestation of the microscopic KMS condition. In a Schwinger-Keldysh setup it enables us to keep influence functionals under control and to formulate an off-shell effective action that encompasses the entirety of adiabatic fluids in a consistent way, while elucidating the origin of the second law constraint. I will explain why this new symmetry promises to be of importance to many applications in non-equilibrium physics and holography. |

** Permutation orbifolds and holography **

Speaker: | Felix Haehl |

Department: | Centre for Particle Theory and Department of Mathematical Sciences, Durham University |

Time: | Thursday 23 April 2015 at 14:15 |

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

Abstract: | Two-dimensional CFTs with large central charge and a sparse low-lying spectrum are expected to admit a holographic description in terms of classical string theory. This manifests itself in a certain universality of the free energy which mimics a Hawking–Page phase transition. Using orbifold techniques, I will describe how to construct a large class of such CFTs. I will describe group theoretic constraints on permutation groups which ensure the existence of a holographic description of the corresponding orbifold CFT. Along the way I will also elaborate on how to construct partition functions for general 2d orbifold theories in the language of covering spaces. This gives insight into the question of how to construct higher genus partition functions and thus probe orbifold CFTs beyond their spectral information. |

**Viscous leptons in the quark gluon plasma**

Speaker: | Di-Lun Yang |

Department: | University of Crete |

Time: | Tuesday 28 April 2015 at 13:15 |

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

Abstract: | We investigate the shear viscosity of leptons in a strongly coupled quark gluon plasma (QGP). We find that the shear viscosity due to the lepton–quark scattering is inversely proportional to the ratio of electric conductivity of the QGP to temperature up to the leading logarithmic order of the electromagnetic coupling. The finding implies that the thermal leptons form a more viscous fluid than the quarks by a large ratio. Using the known result for the electrical conductivity of strongly coupled plasmas obtained from gauge/gravity duality, we find that the lepton shear viscosity is suppressed compared with the one from lepton-lepton scattering. Consistently, we find an enhancement of the energy loss of hard leptons in a strongly coupled scenario compared with that in a weakly coupled plasma. |

**Non perturbative effects of primordial curvature perturbations on the Hubble parameter **

Speaker: | Antonio Enea Romano |

Department: | University of Crete |

Time: | Thursday 30 April 2015 at 14:30 |

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

Abstract: | Recent measurements of the cosmic microwave background (CMB) radiation have shown an apparent tension with the present value of the Hubble parameter inferred from local observations of supernovae, which look closer, i.e. brighter, than what is expected in a homogeneous model with a value of H0 equal to the one estimated from CMB observations. We examine the possibility that such a discrepancy is the consequence of the presence of a local inhomogeneity seeded by primordial curvature perturbations, finding that a negative peak of the order of less than two standard deviations could allow to fit low red-shift supernovae observations without the need of using a value of the Hubble parameter different from HCMB0. The type of inhomogeneity we consider does not modify the distance to the last scattering, making it compatible with the constraints of the PLANCK mission data. The effect on the luminosity distance is in fact localized around the region in space where the transition between different values of the curvature perturbations occurs, producing a local decrease, while the distance outside the inhomogeneity is not affected. Comparison with studies of local structure based on galaxy surveys analysis reveals that the density profile we find could in fact be compatible with the one obtained for the same region of sky where most of the supernovae used for the local H0 estimation are located, suggesting a possible directional dependence which could be partially attributed to the presence of the Sloan Great Wall and hinting to the need of a more careful investigation, including a wider set of low redshift supernovae in different regions of the sky. |

** An exact model for non-equilibrium phenomena**

Speaker: | Ioannis Bakas |

Department: | Department of Physics, National Technical University, Athens |

Time: | Tuesday 5 May 2015 at 14:15 |

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

Abstract: | I consider the class of AdS_4 Robinson–Trautman metrics and use them to explore aspects of non-equilibrium dynamics in the bulk as well as on the boundary in terms of geometric flows. This class of metrics illustrates non-trivially the validity of Penrore's inequality as gravitational radiation is emitted from bounded sources and space-time equilibrates to a black-hole state. The holographic energy-momentum tensor is also obtained and comparison with black-hole hydrodynamics is made at late times, showing that there is entropy production. [Based on joint work with K. Skenderis, arXiv:1404.4824 [hep-th]] |

** A series of lectures on the topic of "magnetic fields and transport in holography"**

Speaker: | Umut Gürsoy |

Department: | University of Utrecht |

Time: | Monday 4 May, 16:00-17:00, Magnetohydrodynamics, charged currents and directed flow in heavy ion collisions - Tuesday 5 May, 16:00-17:00 Magnetohydrodynamics, charged currents and directed flow in heavy ion collisions, Part II - Wednesday 6 May 16:00-17:00 (Non)renormalization of Anomalous Conductivities and Holography. - Thursday 7 may 16:00-17:00 Thermodynamics and CP-odd transport in Holographic QCD with Finite Magnetic Field - Friday 8 may, 10:00-11:00 Thermodynamics and CP-odd transport in Holographic QCD with Finite Magnetic Field, Part II |

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

Abstract: | The physics of magnetic fields in holographic systems at finite density will be described. The effect of anomalies on transport in the presence of magnetic field will be detailed. |

** Semi-holography, the bulk vertex and Weyl semimetals**

Speaker: | Umut Gürsoy |

Department: | University of Utrecht |

Time: | Wednesday 6 May 2015 at 14:15 |

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

Abstract: | I will discuss how semi-holography should be set up in accordance with the Ward identities of the boundary theory. As an example, we consider a situation where both the free fermions and fermionic operators of the CFT are electrically charged and compute the total electrical conductivity of the combined system. We express the resulting conductivity in terms of vacuum CFT correlators which we compute in the dual holographic space-time. This involves computation of a 3-point vertex in the gravitational background. We also find an anomalous magnetic moment of fermions in this system. |

** Hairy black holes in scalar tensor theories**

Speaker: | Christos Charmousis |

Department: | Laboratoire de Physique Théorique d'Orsay, Université Paris-Sud |

Time: | Thursday 7 May 2015 at 14:15 |

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

Abstract: | We will review scalar tensor theories of gravity, where we have an additional scalar field coupling non minimally to the metric tensor. By means of a theorem given by Horndeski back in 1974 we will briefly discuss the most general of these theories acquiring second order field equations. We will examine a particular sub-class of Horndeski theory which has interesting properties with respect to the cosmological constant problem. We will then find black hole solutions of this subclass which in some cases will be identical to GR solutions. The novel ingredient will be the presence of a time and space dependent scalar field. As we will see time dependence and higher order Galileon terms will bifurcate no hair theorems and provide scalar tensor black holes with a non trivial scalar field. |

** Holographic RG flow and the Quantum Effective Action**

Speaker: | Francesco Nitti |

Department: | APC, Universite Paris 7, |

Time: | Monday 11 May 2015 at 11:30 |

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

Abstract: | The calculation of the full (renormalized) holographic action is undertaken in general Einstein-scalar theories. The appropriate formalism is developed and the renormalized effective action is calculated up to two derivatives in the metric and scalar operators. The holographic RG equations involve Ricci flow for the space-time metric as well as the standard â-function flow for scalar operators. Several examples are analyzed and the effective action is calculated. A set of conserved quantities of the holographic flow is found, whose interpretation is not yet understood |

** A series of lectures on Improved Holographic QCD**

Speaker: | Francesco Nitti |

Department: | APC, Universite Paris 7, |

Time: | 11 May, 16:00-17:00 Improved Holographic QCD, the model, Part I 12 May, 16:00-17:00 Improved Holographic QCD, the model, Part II 13 May, 16:00-17:00 Improved Holographic QCD, finite temperature aspects, Part I 14 May 16:00-17:00 Improved Holographic QCD, finite temperature aspects, Part II 15 May 16:00-17:00 Improved Holographic QCD, The phase diagram. |

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

Abstract: | A review is provided to holographic models based on Einstein-dilaton gravity with a potential in 5 dimensions. Such theories, for a judicious choice of potential are very close to the physics of large-N YM theory both at zero and finite temperature. The zero temperature glueball spectra as well as their finite temperature thermodynamic functions compare well with lattice data. The model can be used to calculate transport coefficients, like bulk viscosity, the drag force and jet quenching parameters, relevant for the physics of the Quark-Gluon Plasma and condensed matter physics applications. |

** A series of lectures on "Techniques for solving Einstein's equations"**

Speaker: | Christos Charmousis |

Department: | Laboratoire de Physique Théorique d'Orsay, Université Paris-Sud |

Time: | May 15, 15:00-16:00. General techniques for Einstein equations - May 18 16:00-17:00 Techniques for charged Galileon black holes - May 19 16:00-17:00 Techniques for biscalar extensions of Hornedki's theories - May 20 16:00-17:00 Techniques for conformally coupled gravity - May 21 16:00-17:00 Techniques for non-linear charge black holes |

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

Abstract: | We will review scalar tensor theories of gravity, where we have an additional scalar field coupling non minimally to the metric tensor. By means of a theorem given by Horndeski back in 1974 we will briefly discuss the most general of these theories acquiring second order field equations. We will examine a particular sub-class of Horndeski theory which has interesting properties with respect to the cosmological constant problem. We will then find black hole solutions of this subclass which in some cases will be identical to GR solutions. The novel ingredient will be the presence of a time and space dependent scalar field. As we will see time dependence and higher order Galileon terms will bifurcate no hair theorems and provide scalar tensor black holes with a non trivial scalar field. |

** Exact results in non-conformal holography from localization **

Speaker: | Konstantin Zarembo |

Department: | Nordita and Department of Physics and Astronomy, Uppsala University |

Time: | Tuesday 12 May 2015 at 14:15 |

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

Abstract: | The partition function on S^4 can be calculated exactly for any N=2 supersymmetric gauge theory. We exploit this result to solve massive N=2* Super-Yang-Mills at large-N. Its holographic dual is explicitly known, and taking the strong-coupling limit of the exact result proves a direct test of holography in a non-conformal setting. Quite unexpectedly, the model displays very intricate phase structure with infinitely many quantum phase transition at finite 't Hooft coupling. |

** Trace anomaly on the conformal manifold **

Speaker: | Andreas Stergiou |

Department: | Department of Physics, Yale University |

Time: | Thursday 14 May 2015 at 14:15 |

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

Abstract: | Conformal field theories (CFTs) defined at fixed points of the renormalization group flow can be lifted to curved space where they develop the well-known trace anomaly. In this talk we will consider the trace anomaly of CFTs with exactly marginal operators. The couplings that source the marginal operators are promoted to spacetime-dependent functions, and new terms, proportional to derivatives of the couplings, appear in the trace anomaly. Using both field-theoretic as well as holographic methods we will analyze these contributions in four and six spacetime dimensions, and illustrate their relevance to the a-theorem and the relation between scale and conformal invariance. The six-dimensional case displays novel features not seen in lower dimensions. |

** Consistent massive graviton on arbitrary backgrounds **

Speaker: | Cédric Deffayet |

Department: | Institut d'Astrophysique de Paris |

Time: | Monday 18 May 2015 at 11:30 |

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

Abstract: | I will discuss how to derive from the massive gravity theory of de Rham–Gabadadze–Tolley (which involves both a dynamical and a non dynamical flat metric, and which will be introduced) a theory for a massive graviton moving on an arbitrary background metric which always has 5 (or less) physical polarizations. The obtained theory features a non trivial mass term with non trivial coupling to curvature. It will also be shown how to count there the number of degrees of freedom in a covariant way, as can be done for the flat background space-time theory of Fierz and Pauli. The talk is based on the two papers arXiv:1504.04382 and arXiv:1410.8302, and our results can also been compared to the old results of Buchbinder et al. (hep-th/991018 where the first non trivial coupling to curvature was computed and compared to a stringy computation. |

**Holography, probe branes and isoperimetric inequalities **

Speaker: | Frank Ferrari |

Department: | Université Libre de Bruxelles |

Time: | Tuesday 19 May 2015 at 14:15 |

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

Abstract: | I will first briefly review recent investigations on the gauge-theoretic construction of probe brane actions, and show that they yield a rather surprising consistency condition in holography: the minimal value of Euclidean probe-brane actions in holographic background should be directly related to the on-shell value of the gravitational action. I will then investigate the validity of the consistency condition directly on the gravity side and show that, very remarkably, it is always precisely satisfied, at least in a wide variety of cases, including when alpha corrections are taken into account. Moreover, the consistency condition can be naturally generalized to asymptotically flat backgrounds, and seems to be always valid in this case too. Possible consequences of these unexpected results will be discussed. |

** Towards unity of GUT families: anti-SU(7)
from Z(12-I) orbifold compactification **

Speaker: | Jihn Kim |

Department: | Seoul National University and Kyung Hee University |

Time: | Thursday 21 May 2015 at 11:30 |

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

Abstract: | I discuss how a family unified GUT model is constructed from orbifold compactification: It simplifies the study of flavor problems in the low energy effective theory. |

**Simulations of black hole collisions and relations to QCD physics**

Speaker: | Paul Romatschke |

Department: | University of Colorado Boulder |

Time: | Tuesday 2 June 2015 at 14:15 |

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

Abstract: | In the context of gauge/gravity duality, it has been suggested that the far-from equilibrium strongly coupled dynamics encountered in ultrarelativistic heavy-ion collisions may be modeled as the collisions of black holes in asymptotic anti-de Sitter spacetimes. I will present results from the evolution of spacetimes that describe the merger of asymptotically global AdS black holes in 5D with an SO(3) symmetry. The initial trapped regions are sourced by scalar field collapse and we are able to evolve through the ensuing black hole merger as well as subsequent ring-down. The boundary stress tensor corresponding to this evolution is found to correspond to hydrodynamics at late times, but not at early times. Implications and generalizations of this work and signatures that could be relevant to experimental observations at RHIC and the LHC will be discussed. |

**On holographic insulators and supersolids**

Speaker: | Jie Ren |

Department: | University of Crete |

Time: | Thursday 4 June 2015 at 14:15 |

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

Abstract: | We obtain holographic realisations for systems that have strong similarities to Mott insulators and supersolids, after examining the ground states of Einstein–Maxwell–scalar systems. The real part of the AC conductivity has a hard gap and a discrete spectrum only. We add momentum dissipation to resolve the delta function in the conductivity due to translational invariance. We develop tools to directly calculate the Drude weight for a large class of solutions and to support our claims. Numerical RG flows are also constructed to verify that such saddle points are IR fixed points of asymptotically AdS geometries. |

**Instanton dynamics in finite temperature QCD via holography**

Speaker: | Yoshinori Matsuo |

Department: | University of Crete |

Time: | Tuesday 9 June 2015 at 14:15 |

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

Abstract: | We investigate instantons in finite temperature QCD via Witten's holographic QCD. To study the deconfinement phase, we use the setup proposed in arXiv:1107.4048. We find that the sizes of the instantons are stabilized at certain values both in the confinement and deconfinement phases. This agrees with the numerical result in the lattice gauge theory. Besides we find that the gravity duals of the large and small instantons in the deconfinement phase have different topologies. We also argue that the fluctuation of the topological charges is large in confinement phase while it is exponentially suppressed in deconfinement phase. |

** Turbulent strings in AdS/CFT**

Speaker: | Takaaki Ishii |

Department: | University of Crete |

Time: | Thursday 11 June 2015 at 14:15 |

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

Abstract: | I will talk on nonlinear dynamics of the flux tube between an external quark-antiquark pair in N=4 SYM using the gravity dual given by a string hanging in AdS. I will perturb the endpoints of the string and compute the string's nonlinear time evolution numerically. As a result, cusp formation on the string is observed, and this will be examined in detail. In particular, I will discuss the turbulent behavior in the energy spectrum of the string. Reference: arXiv:1504.02190 |

** Massive holographic QCD in the Veneziano limit, Part II **

Speaker: | Matti Jarvinen |

Department: | Ecole Normale Superieure, Paris |

Time: | Monday 15 June 2015 at 14:15 |

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

Abstract: | QCD at finite, flavor independent quark mass is analyzed by using bottom-up holography in the Veneziano limit, where the backreaction of quarks to the gluon dynamics is fully included. The dependence on the quark mass of observables such as the bound state masses, the chiral condensate, the S-parameter, and the critical temperatures is studied. Many of the results are argued to be universal, i.e., independent of the details of the holographic model, and compared to explicit computations in the V-QCD models. The effect of adding four-fermion operators in QCD is also discussed. |

**Can an extremal black hole provide an effective description of the many-body localized phase?**

Speaker: | Wei-Jia Li |

Department: | University of Crete |

Time: | Tuesday 16 June 2015 at 14:15 |

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

Abstract: | N/A |

**Towards high temperature superconductor phase diagram from holography**

Speaker: | Li Li |

Department: | University of Crete |

Time: | Thursday 18 June 2015 at 14:15 |

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

Abstract: | I will introduce our ongoing work about a general holographic model aiming at realising the phase diagram of high temperature superconductors in terms of temperature and doping. We show the possibility of building a phase diagram of a system with the anti-ferromagnetism at small doping, the superconductivity occupying a dome-shaped region, as well as spatially modulated phases between them, which is resembling more of a real-world doped high-Tc superconductor. |

** Electric response of a holographic strange metal **

Speaker: | Francisco Peña-Benitez |

Department: | University of Crete |

Time: | Thursday 25 June 2015 at 14:15 |

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

Abstract: | In this talk I will talk about the frequency dependence of the optical conductivity for a holographic non relativistic model. The system shows in the UV a power law. Also some features related with the existence of a Drude peak and its relation to the so called pair creation and drag conductivities will be discussed. |

**Fine tuning may not be enough**

Speaker: | Richard Woodard |

Department: | University of Florida |

Time: | Tuesday 30 June 2015 at 14:15 |

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

Abstract: | This talk is based on arXiv:1506.07306, with Shun-Pei Miao. We argue that the fine-tuning problems of scalar-driven inflation may be worse than is commonly believed. The reason is that reheating requires the inflaton to be coupled to other matter fields whose vacuum fluctuations alter the inflaton potential. The usual response has been that even more fine-tuning of the classical potential V(phi) can repair any damage done in this way. We point out that the effective potential in de Sitter background actually depends in a complicated way upon the dimensionless combination of phi/H. We also show that the factors of H which occur in de Sitter do not even correspond to local functionals of the metric for general geometries, nor are they Planck-suppressed. |

**Emergent large N matrices from a nonlocal spin system**

Speaker: | Victoria Martin |

Department: | Stanford University |

Time: | Tuesday 18 August 2015 at 14:15 |

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

Abstract: | Large N matrices underpin the best understood models of emergent spacetime. We suggest that large N matrices can themselves be emergent from simple quantum mechanical spin models with finite dimensional Hilbert spaces. Specifically, we start with a nonlocal "Ising" model and transform this into a matrix integral. As a result, we find an example of a nontrivial emergence of a continuous symmetry group from a discrete one, in the large N limit. Furthermore, we can use the well-known mechanics of solving matrix integrals to study the properties of the original spin system. We verify our analytic results by Monte Carlo simulation. |