University of Crete HEP Seminars: Spring 2017


A universal theory for fluid dynamics with explicitly broken boost symmetries

Speaker: Jelle Hartong
Institution: The Niels Bohr Institute, Copenhagen University
Time: Tuesday 7 February 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Theoretical investigations into the properties of strange metals suggest that these are described by effective field theories that are strongly coupled and that have non-relativistic scaling exponents. Such systems do not have boost invariance. In this talk I will discuss how to describe such theories in the hydrodynamic limit. More explicitly, I will first discuss an ideal gas of particles with dispersion relation w ~ k^z for any z. I will then describe the general thermodynamic properties of systems with no boost symmetries by introducing velocity as a chemical potential whose conjugate variable is momentum. I will then define perfect fluids and finally discuss first order corrections in the case of broken Galilean boost invariance.

Mixed Correlator 4D N=1 Superconformal Bootstrap

Speaker: Andreas Stergiou
Institution: CERN
Time: Tuesday 21 February 2017, 14:15
Venue: 2nd floor seminar room
Abstract: The numerical conformal bootstrap program has produced a wide range of impressive results concerning conformal field theories in various spacetime dimensions. In this talk I will focus on an application of this program to four-dimensional conformal theories with N=1 supersymmetry. In such theories a kink was found some time ago in the upper bound on the dimension of the first real scalar operator that appears in the operator product expansion of a chiral operator with its conjugate. Kinks signify isolated solutions to crossing equations, and are typically associated with specific conformal field theories. The nature of the putative theory at the N=1 kink remains mysterious. In this talk I will present numerical results concerning 4D N=1 theories using mixed correlation functions, and provide ample evidence that a special conformal field theory lives at the kink.

MOND cosmology

Speaker: Richard Woodard
Institution: University of Florida
Time: Tuesday 14 March 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Milgrom's MOdified Newtonian Dynamics (MOND) provides a wonderful explanation for galactic structure, as shown by the striking recent work of McGaugh and collaborators (arXiv:1609.05917). However, MOND must be regarded as the weak field, static limit of some larger, relativistic field theory. It is this larger theory which controls MOND cosmology, and also the model's predictions for recently disturbed systems such as the Bullet Cluster. I report on a nonlocal, pure metric realization of MOND which was developed by Deffayet, Esposito-Farese and myself in arXiv:1405.0393. The model is based upon an algebraic function of a nonlocal scalar which is typically positive for gravitationally bound systems, but is typically negative for cosmological settings. Correctly reproducing MOND phenomenology for galaxies therefore determines how the algebraic function depends on positive arguments but does not constrain its dependence on negative arguments. I report on a recent determination (arXiv:1608.07858) of the negative branch which succeeds in enforcing the Lambda CDM expansion history, without dark matter, until very late redshifts (z < 0.088). This means that the model agrees with the usual predictions of Big Bang Nucleosynthesis and at least has the same cosmological background geometry as usual during Recombination. Interestingly, the expansion history deviates at very late times in such a way as to reduce the growing tension between inferences of the Hubble parameter based on data from large redshifts and inferences based on data from small redshifts. Because no more freedom exists in the model, its response to perturbations can now be studied to see if it agrees with the observed pattern of Doppler peaks in the Cosmic Microwave Radiation, and also if it is in rough agreement with structure formation.

Theta dependence in holographic QCD

Speaker: Francesco Bigazzi
Institution: INFN Florence and Pisa University
Time: Tuesday 4 April 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Using the holographic gauge/gravity duality as a tool, I will consider the effects of the CP-breaking topological theta term in the large N QCD model by Witten, Sakai and Sugimoto. I will first focus on the ground state energy density, the topological susceptibility and the masses of the lowest lying mesons, showing agreement with expectations from the chiral effective action. Then I will consider the baryonic sector in the case with two degenerate light quarks. I will show that while the baryon spectrum does not receive corrections to first order in theta, this is not the case for observables like the electromagnetic form factor of the nucleons. In particular, it exhibits a dipole term, which turns out to be vector-meson dominated. The resulting neutron electric dipole moment, which is exactly the opposite as that of the proton, is of the same order of magnitude of previous estimates in the literature. Finally, I will show that the CP-violating pion-nucleon coupling constant is zero to leading order in the large N limit.

Black hole formation from pointlike particles and emerging thin shell spacetimes

Speaker: Jonathan Lindgren
Institution: Vrije Universiteit Brussel
Time: Wednesday 5 April 2017, 14:15
Venue: 2nd floor seminar room
Abstract: I will discuss some recent work on collisions of pointlike particles in three-dimensional anti-de Sitter space. I will explain how to construct exact solutions corresponding to an arbitrary number of particles colliding in the center of AdS to form either a new particle or a black hole, which is in general a non-trivial generalization of the well known case with two particles. I will also discuss the limit of an infinite number of particles, resulting in novel solutions consisting of a thin shell of matter collapsing to form a black hole. These solutions are new since they are the first examples of thin shell spacetimes that break rotational symmetry, generalizing the well known rotationally symmetric thin shell AdS3-Vaidya spacetimes. Some implications for the AdS/CFT correspondence will also be discussed.

Theta angle in V-QCD

Speaker: Matti Jarvinen
Institution: Ecole Normale Superieure de Paris
Time: Thursday 6 April 2017, 14:15
Venue: 2nd floor seminar room
Abstract: I discuss the results of [arXiv:1609.08922] on CP-odd physics in holographic models for QCD in the Veneziano limit (V-QCD). Observables such as the topological susceptibility and the meson masses are analyzed over the whole parameter space, i.e., as a function of x=Nf/Nc, the quark mass, and the theta angle. At small quark mass, where effective chiral Lagrangians lead to reliable results, an extensive agreement with V-QCD is found.

Random matrices and the Sachdev-Ye-Kitaev model

Speaker: Antonio Garcia Garcia
Institution: Cambridge University
Time: Tuesday 18 April 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Fermions with k-body infinite-range interactions, originally introduced in the context of quantum chaos and recently relabelled Sachdev-Ye-Kitaev models, are attracting a great deal of attention as toy models of strongly coupled metals with a gravity dual. After a pedagogical introduction. I study analytically and numerically the spectral and thermodynamic properties of the model. The main finding is that for sufficiently long times, the Sachdev-Ye-Kitaev model evolves to a fully ergodic state whose dynamics only depends on the global symmetry of the system. This strongly suggests that random matrix correlations are a universal feature of quantum black holes and that fermions with k-body infinite-range interactions may still be relevant to model certain aspects of the dynamics of realistic strongly coupled fermions.

Mazur-Suzuki bounds in holography

Speaker: Aurelio Bermudez Romero
Institution: Leiden University
Time: Wednesday 19 April 2017, 14:15
Venue: 2nd floor seminar room
Abstract: The Mazur-Suzuki (MS) bounds allow to decompose 2-point correlation functions of a single trace operator A in terms of the conserved quantities which overlap with A. After an introduction of the MS bounds, I will consider the current-current 2-point function and the related MS bound in a broad variety of strongly coupled field theories with a gravity dual. In particular, I will show that for Einstein-Maxwell-dilaton (EMd) theories, including R-charged backgrounds, the MS bound involving the conserved momentum operator is saturated. I will also consider EMd theories with U(1) spontaneous symmetry breaking and gravity duals of nonrelativistic field theories in which this MS bound is not saturated. Finally, I will study the effect of a weak translational symmetry-breaking and show that, in this limit, part of the conductivity may be recovered from the product of the MS bound and the scattering time obtained from the leading quasinormal mode.

CME in asymmetric Weyl semimetals

Speaker: Rene Meyer
Institution: Wurzburg University
Time: Thursday 20 April 2017, 14:15
Venue: 2nd floor seminar room
Abstract: After a general introduction to the chiral magnetic effect in Weyl semimetals, I discuss a new kind of chiral magnetic current which is present even in the absence of an externally applied electric field. It is instead driven by non equilibrium pumping of non-chiral charges (electrons) into the band structure of an asymmetric Weyl semimetal, which is a Weyl semimetal with an asymmetry in the dispersion relations of the left and right chiralities. I will present the derivation of the current from chiral kinetic theory, as well as a proposed experimental setup to measure the nonequilibrium chiral magnetic current. I will conclude by arguing that the effect should be present in generic band structures with asymmetric dispersion relations, and discuss a candidate material.

Flavor Physics

Speaker: Yossi Nir
Institution: Weizmann Institute of Science
Time: Wednesday 26 April 2017, 14:15
Venue: 1st floor seminar room
Abstract: A general overview of the field, and more details about two of the more interesting measurements of recent years: the search for h to tau mu and the measurement of B to D tau nu.

A Vicious Circle: Spatially Modulated Deformations and Intermediate Scaling from M-Theory

Speaker: Christopher Rosen
Institution: Imperial College London
Time: Tuesday 9 May 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Renormalization group flows between fixed points provide a powerful framework for both exploring and cataloging the myriad ways in which field theory degrees of freedom can behave at low energies. Under favorable conditions, the physics of such flows can be reformulated geometrically, and examined in the context of gauge/gravity duality. I will present one such flow, which is driven by a careful tuning of spatially modulated mass terms for fermions and scalars of strongly coupled ABJM theory in 2+1 dimensions. Interestingly, this deformation results in a "boomerang" RG flow, in which the theory returns to itself at low energies (with a renormalization of length scales). For sufficiently large deformations, the flow approaches two distinct intermediate energy scaling regimes along its journey to the IR fixed point. The holographic description of this trajectory is encoded in a new solution to D=11 supergravity which deforms the AdS_4 x S^7 vacuum in a peculiar way.

The S-matrix bootstrap

Speaker: Balt van Rees
Institution: Durham University
Time: Wednesday 10 May 2017, 14:15
Venue: 1st floor seminar room
Abstract: We take a fresh look at the S-matrix bootstrap program using modern tools inspired by the revived conformal bootstrap. There exist various approaches to taming the landscape of S-matrices. We will discuss two different methods and show that they both lead to the same universal bound for the interaction strength in 2-dimensional QFTs. We propose a specific numerical algorithm for higher-dimensional QFTs and present some initial results.

Transport peak and other effects of finite coupling in holographic plasmas

Speaker: Andrei Starinets
Institution: Oxford University
Time: Tuesday 16 May 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Transport properties of liquids and gases in the regime of weak coupling (or effective weak coupling) are determined by the solutions of relevant kinetic equations for particles or quasiparticles, with transport coefficients being proportional to the minimal eigenvalue of the linearized kinetic operator. At strong coupling, the same physical quantities can sometimes be determined from dual gravity, where quasinormal spectra enter as the eigenvalues of the linearized Einstein's equations. We discuss the problem of interpolating between the two regimes using results from higher derivative gravity.

Yukawas of light stringy states

Speaker: Pascal Anastasopoulos
Institution: Vienna University
Time: Thursday 18 May 2017, 14:15
Venue: 2nd floor seminar room
Abstract: We investigate light massive string states that appear at brane intersections. They replicate the massless spectrum in a richer fashion and may be parametrically lighter than standard Regge excitations.We compute tri-linear Yukawa couplings of such open-string states to massless ones and to one another. Due to ambiguities in the normalisation of the vertex operators, that involve twist fields, we proceed via factorization of appropriate scattering amplitudes. Some peculiar features are observed that may lead to interesting signatures at colliders in the future.

Lessons and surprises in compactified gauge theories

Speaker: Mohamed Anber
Institution: Lewis & Clark College
Time: Tuesday 20 June 2017, 14:15
Venue: 2nd floor seminar room
Abstract: During the last decade there has been a tremendous amount of effort to understand QCD-like theories with one compact spatial dimension. This class of theories is weakly coupled and under complete analytic control. In this talk, I will report on a few surprises and lessons we have learned from this line of research. First, I will discuss the general vacuum structure of QCD on R^3XS^1. Then, I will show that strings in QCD with adjoint fermions have qualitatively new features not previously known. Finally, I will use a supersymmetric version on R^3XS^1 to argue that the quantum phase transition in this theory is continuously connected to the thermal deconfinement transition in pure Yang-Mills on R^4. This continuity enables us to predict the qualitative behavior of a few quantities in pure Yang-Mills including the order of the transition, string tension, its N-laity, and string melting at high temperature.

Matrix Quantum Mechanics and the S^1/Z_2 orbifold

Speaker: Panagiotis Betzios
Institution: Utrecht University
Time: Tuesday 4 July 2017, 14:15
Venue: 2nd floor seminar room
Abstract: We revisit c=1 non-critical string theory and its formulation via Matrix Quantum Mechanics (MQM). In particular we study the theory on an S^1/Z_2 orbifold of Euclidean time and try to compute its partition function in the grand canonical ensemble that allows one to study the double scaling limit of the matrix model and connect the result to string theory (Liouville theory). We will also compare the partition function with the cases of the thermal circle and Euclidean 2d black hole and compute the contribution of the twisted states using a formula by Dyson. Finally, we will make some comments regarding the possibility of using this model as a toy model of a two dimensional big-bang big-crunch universe.

A magnetically induced quantum critical point in holography

Speaker: Papadoulaki Olga
Institution: Utrecht University
Time: Thursday 6 July 2017, 14:15
Venue: 2nd floor seminar room
Abstract: We are using holography to study quantum phase transitions in a 2+1 dimensional strongly coupled system at finite chemical potential and constant external magnetic field. Our gravity dual is in the context of 3+1 dimensional N = 2 Fayet-Iliopoulos gauged supergravity. We find a line of quantum critical points at a critical value of the magnetic field between extremal dyonic black branes and extremal magnetically charged "thermal gas" solutions. We will introduce the two types of solutions, their thermodynamics and their phase diagram. Then we will discuss the physics of the critical point and we will point out qualitative similarities between our field theory and Nambu- Jona- Lasinio model in 2+1 dimensions in the presence of an external magnetic field. We will conclude with some open questions both on the gravity and field theory side and we will discuss future directions.