University of Crete HEP Seminars Fall 2011

**The Viscosity of Quark-Gluon Plasma at RHIC and the LHC**

Speaker: | Ulrich Heinz |

Department: | Ohio State University |

Slides: | [PDF] |

Time: | SATURDAY 3 September 2011 at 15:15 |

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

Abstract: | The specific shear viscosity (eta/s)_QGP of quark-gluon plasma (QGP) is of topical interest since its value appears to be very small, close to a conjectured lower bound imposed by quantum mechanics. It can be extracted from experimental elliptic flow data in heavy-ion collisions by comparing them with the dynamical model VISHNU which couples a viscous fluid dynamic description of the QGP with a microscopic kinetic description of the late hadronic rescattering and freeze-out stage. A robust method for fixing (eta/s)_QGP from the collision centrality dependence of the eccentricity-scaled charged hadron elliptic flow is presented. The systematic uncertainties associated with this extraction method are discussed, with specific attention to our presently restricted knowledge of initial conditions. With the (eta/s)_QGP extracted in this way, VISHNU yields an excellent description of all soft-hadron data from Au+Au collisions at top RHIC energy. Extrapolations to Pb+Pb collisions at the LHC, using both a purely hydrodynamic approach and VISHNU, are presented and compared with recent experimental results from the ALICE Collaboration. The LHC data are again well described by VISHNU, with the same (eta/s)_QGP value as at RHIC energies. This value ranges between one and two times the KSS bound of 1/(4pi), depending on the so far poorly constrained initial fireball deformation. I will show how this uncertainty can be eliminated by analyzing elliptic and triangular flow together. |

**Interface depinning with applications and generalizations**

Speaker: | Stefanos Papanikolaou |

Department: | Yale University |

Time: | Monday 26 September 2011 at 14:15 |

Venue: | The 1st floor seminar room of the physics department |

Abstract: | I will discuss the general properties of elastic manifolds driven in random media and their general behavior near to the critical "depinning" force needed to have a persistent motion. After reviewing the basic properties of the mean-field theory for such dynamical systems, I will consider its generalization in cases where there are coexisting relaxation processes. Applications to earthquakes and plastically deformed metals will be discussed. |

**Inhomogeneous holographic superconductors**

Speaker: | Stefanos Papanikolaou |

Department: | Yale University |

Slides: | [PDF] |

Time: | Tuesday 27 September 2011 at 14:15 |

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

Abstract: | The AdS/CFT correspondence provides a way to analytically study condensed matter phases, such as superfluids and superconductors, near quantum critical points. I will describe models of five-dimensional AdS gravity coupled to a gauge field and a charged inhomogeneous scalar field. Such models are believed to be dual to 2 + 1 dimensional superconductors. I will show that such systems display inhomogeneous phases, charge and pair density waves, that have consistent properties with the traditional BCS theory of superconductivity, even though with differences that I will discuss. |

**Dimensional Reduction **

Speaker: | Daiske Yamada |

Department: | Crete University |

Time: | Friday 7 October 2011 at 14:15 |

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

Abstract: | I plan to discuss generality of dimensional reduction and say something that I've been doing along the way. |

**On holographic nuclear interaction, nuclear matter and
multi-instanton configurations**

Speaker: | Jacob Sonnenschein |

Department: | Tev Aviv University |

Time: | Tuesday 11 October 2011 at 15:15 |

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

Abstract: | TBA |

**Towards brane-antibrane inflation in type IIA: The holographic MQCD model**

Speaker: | Jacob Sonnenschein |

Department: | Tev Aviv University |

Time: | Wednesday 12 October 2011 at 11:15 |

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

Abstract: | We describe type IIA cosmological brane inflation scenarios based on the holographic MQCD model of Aharony et al [1]. The scenarios can be related via T-duality to the type IIB KKLMMT model [2]. They describe a probe brane configuration of p D4 branes stretching between an NS5 and NS5′ branes in the holographic background of large N D4 branes. The resulting cosmological models have a Wick-rotated D4-brane metric, with transverse dimensions compactified, and a spiralling brane with flux p. In one model, the background has a small nonextremality, and the inflaton is provided by the position of a “sliding” D4-brane, and in the other, the background is supersymmetric, but with a sliding anti-D4-brane. We obtain good and generic inflationary models, though several unknowns remain, in particular about subleading corrections. The usual caveat of volume stabilization generically spoiling slow-roll still applies. |

**Holographic inflation: an update**

Speaker: | Kostas Skenderis |

Department: | Amsterdam University |

Slides: | [PDF] |

Time: | Friday 14 October 2011 at 14:15 |

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

Abstract: | I will discuss holographic models of inflation and the predictions for the power spectra and the bispectra of primordial perturbations. I will then show that these models are competitive to LambdaCDM in fitting the WMAP data. |

**The holographic fluid dual to vacuum Einstein gravity**

Speaker: | Marika Taylor |

Department: | Amsterdam University |

Slides: | [PDF] |

Time: | Tuesday 18 October 2011 at 15:15 |

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

Abstract: | We present an algorithm for systematically reconstructing a solution of the (d+2)-dimensional vacuum Einstein equations from a (d+1)-dimensional fluid, extending the non-relativistic hydrodynamic expansion of Bredberg et al in arXiv:1101.2451 to arbitrary order. The fluid satisfies equations of motion which are the incompressible Navier-Stokes equations, corrected by specific higher derivative terms. We establish the validity of a relativistic hydrodynamic description for the dual fluid, which has the unusual property of having a vanishing equilibrium energy density. The gravitational results are used to identify transport coefficients of the dual fluid, which also obeys an interesting and exact constraint on its stress tensor. We propose novel Lagrangian models which realise key properties of the holographic fluid. The talk will be based on 1103.3022 along with work in progress [to appear soon]. |

**Holographic models for QCD in the Veneziano limit**

Speaker: | Matti Järvinen |

Department: | Crete University |

Slides: | [PDF] |

Time: | Friday 21 October 2011 at 14:15 |

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

Abstract: | I will discuss ongoing work which explores holographic bottom-up models of QCD in the Veneziano limit of high N_f and N_c with x=N_f/N_c fixed. The models based on "the improved holographic QCD", and the actions are obtained from dilaton gravity by adding a generalized TDBI term. A class of models is identified where the behavior of the background solutions meets the qualitative expectations from QCD as x and the quark mass are varied (at zero temperature). In particular, for zero quark mass there is a conformal phase transition from the phase having chiral symmetry breaking to the one with an IR fixed point as x increases. |

**The physics of a new gauge boson in a Stueckelberg extension of
the two-Higgs-doublet model**

Speaker: | Grigoris Panotopoulos |

Department: | Valencia University |

Slides: | [PDF] |

Time: | Tuesday 25 October 2011 at 15:15 |

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

Abstract: | String theory constructions using D-brane physics offer a framework where ingredients like extra abelian factors in the gauge group, more than one Higgs doublet and a generalized Green-Schwarz mechanism appear at the same time. Motivated by works towards the direction of obtaining the Standard Model in orientifold constructions, we study in the present work a Stueckelberg extension of the two-Higgs-doublet model. The distinctive features of our model are i) a sharp decay width for the heavy gauge boson, and ii) a charged Higgs boson having two main decay channels at tree level with equal branching ratios. |

**Holographic fluids and vorticity**

Speaker: | Marios Petropoulos |

Department: | Ecole Polytechnique |

Slides: | [PDF] |

Time: | Wednesday 26 October 2011 at 11:30 |

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

Abstract: | In vue of the recent interest in reproducing holographically various properties of conformal fluids, I will analyze the emergence of rotation and vortices in the framework of AdS/CFT. The gravitational backgrounds involved in this study turn out to exhibit interesting relationships with motion of charged particles in magnetic monopoles, sailing in drift currents or sound wave propagation in moving media. The latter opens the way to the holographic description of analogue-gravity models. |

**Matrix models, duality and spontaneous SUSY breaking in 3D Quantum Field Theory**

Speaker: | Vasilis Niarchos |

Department: | Crete University |

Slides: | [PDF] |

Time: | Tuesday 1 November 2011 at 15:15 |

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

Abstract: | I will discuss recent developments in three-dimensional superconformal field theories based on the use of localization techniques. Special emphasis will be given to a particular example of three-dimensional Seiberg-like duality and the proposal of a new non-perturbative constraint on spontaneous supersymmetry breaking based on matrix models. |

**Holographic non-relativistic fermion liquids with the bulk dipole coupling**

Speaker: | Hongbao Zhang |

Department: | Crete University |

Slides: | [PDF] |

Time: | Friday 5 November 2011 at 14:15 |

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

Abstract: | With some success, applying AdS/CFT has gone beyong high energy physics and cosmology, to condensed matter physics, where in particular holographic implementation of fermi and non-fermi liquids has been achieved. After a review of the holographic framework to extract the retarded Green function at the non-relativistic fermionic fixed point, I will present our numerical results in the presence of bulk dipole coupling, where some exotic features show up and may wait to be identified in the realistic condensed matter systems. |

**Another talk about the Holographic Quantum Hall Effect**

Speaker: | Matt Lippert |

Department: | Crete University |

Slides: | [PDF] |

Time: | Friday 11 November 2011 at 14:15 |

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

Abstract: | I will present the D2-D8' model, which is a holographic model of 2+1-dimensional strongly-coupled fermions and which exhibits an integer quantum Hall effect. I'll review the construction of the system and the phase structure, and then I'll talk about the fluctuations of both the quantum Hall and the metallic states. |

**The characteristics of thermalization of boost-invariant plasma from holography**

Speaker: | Michal Heller |

Department: | Amsterdam University |

Slides: | [PDF] |

Time: | Tuesday 22 November 2011 at 15:15 |

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

Abstract: | We report on the evolution of boost-invariant N=4 super Yang-Mills plasma covering a large range of proper times starting from various nonequilibrium states at tau=0, through a transition to a hydrodynamic regime and following subsequent hydrodynamic expansion. The results are obtained through numerical solution of Einstein's equations for the dual geometries. Despite the very rich far from equilibrium evolution, we find surprising regularities in the form of simple phenomenological relations between initial entropy and total produced entropy, as well as between initial entropy and the temperature at thermalization. For 20 different initial conditions that we consider, hydrodynamics turns out to be applicable for proper times larger than at most 0.67 in units of inverse temperature at thermalization. |

**Multiplicities from black-hole formation in heavy-ion collisions**

Speaker: | Tassos Taliotis |

Department: | Crete University |

Slides: | [PPT] |

Time: | Wednesday 23 November 2011 at 11:30 |

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

Abstract: | The formation of trapped surfaces in the head-on collision of shock waves in conformal and non-conformal backgrounds is investigated. The backgrounds include all interesting confining and non-confining backgrounds that may be relevant for QCD. Several transverse profiles of the shocks are investigated including distributions that fall-off as powers or exponentials. Different ways of cutting-off the UV contributions (that are expected to be perturbative in QCD) are explored. Under some plausible simplifying assumptions our estimates are converted into predictions for multiplicities for heavy-ion collisions at RHIC and LHC. |

**Scattering in 3d Chern-Simons theories**

Speaker: | Andrea Mauri |

Department: | Milan University |

Time: | Friday 25 November 2011 at 14:15 |

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

Abstract: | I review the recently found dualities between MHV gluon scattering amplitudes, light-like polygonal Wilson loops and correlators of BPS operators in N=4 SYM. I then discuss the attempts to extend these correspondences to the case of three-dimensional Chern-Simons theories. |

**Heavy quarks in a magnetic field**

Speaker: | George Pavlopoulos |

Department: | Crete University |

Slides: | [PDF] |

Time: | Tuesday 29 November 2011 at 15:15 |

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

Abstract: | The motion of a heavy charged quark in a magnetic field is analyzed in the vacuum of strongly coupled CFT. The motion is damped and time dependent. The quark moves in spiral until it eventually comes to rest. The world-sheet geometry is locally AdS_2 but has a time dependent horizon. The string profile in the static gauge extends from the boundary till a point hidden by the horizon. Connections with other circular string motions are established. |

**Transport in Anisotropic Superfluids: A Holographic Description**

Speaker: | Hansjörg Zeller |

Department: | Munich, MPI |

Slides: | [PDF] |

Time: | Tuesday 6 December 2011 at 15:15 |

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

Abstract: | We study transport phenomena in p-wave superfluids in the context of gauge/gravity duality. Due to the spacetime anisotropy of this system, the tensorial structure of the transport coefficients is non-trivial in contrast to the isotropic case. In particular, there is an additional shear mode which leads to a non-universal value of the shear viscosity even in an Einstein gravity setup. In this talk, I present a study of the helicity two and helicity one fluctuation modes. In addition to the non-universal shear viscosity, I show the thermoelectric effect, i.e. the mixing of electric and heat current, we find in our system. Moreover, I present an additional effect due to the anisotropy, the so-called flexoelectric effect. |

**Scalar Bremsstrahlung in Gravity-Mediated
Ultrarelativistic Collisions**

Speaker: | Yiannis Constantinou |

Department: | Crete University |

Slides: | [PDF] |

Time: | Friday 9 December 2011 at 14:15 |

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

Abstract: | Classical bremsstrahlung of a massless scalar field $\Phi$ is studied in gravity mediated ultra-relativistic collisions with impact parameter $b$ of two massive point particles in the presence of $d$ non-compact or toroidal extra dimensions. The spectral and angular distribution of the scalar radiation are analyzed, while the total emitted $\Phi-$energy is found to be enhanced by a $d-$dependent power of the Lorentz factor $\ga$. The direct radiation amplitude from the accelerated particles is shown to interfere destructively (in the first two leading ultra-relativistic orders) with the one due to the $\Phi-\Phi-graviton$ interaction in the frequency regime $\ga/b\lesssim \om \lesssim \ga^2/b$ in all dimensions. |