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Dublin Area Theoretical Physics Colloquium
The colloquium is organized by the School of Mathematics and held
each Monday at 4.00 PM in the
AbstractsDate: Oct. 1st Speakeri: Joost Slingerland Title: Exotic exchanges: from braiding anyons to sliding rings Abstract: Perhaps the most useful bit of information one may have about a particle is whether it is a boson or a fermion. Systems of fermions have wave functions which are antisymmetric under exchange while bosonic wave functions are symmetric. This difference ensures that many particle systems have immensely different properties depending on whether they consist of bosons or fermions or a mixture. Obviously it would be interesting to have more classes of particles with different exchange behaviours. In 2+1 dimensional systems, this is possible and there are new types of particles, called anyons, which have exchange behavior described by braid groups. I will say a bit about anyons and the current quest for experimental detection of their exchange properties. Then I will return to 3+1 dimensions and give an introduction to the "loop braid group". This group governs the exchange properties of ring or loop shaped excitations. Loops can perform a number of nontrivial exchange motions including simple exchanges like those of point particles and "leapfrogging" like smoke rings. I will introduce the concept of a local representation of the loop braid group, analyze a number of examples of such representations and make a conjecture on their general structure.
Date: Oct. 15th Speaker: Thomas Klose Title: Worldsheet Form Factors in the AdS/CFT correspondence Abstract: Form factors are matrix elements of local operators in the basis of scattering states. Said in experimentalist's terms, particles are scattered off a target and thereby probe its "form". From these matrix elements, correlation functions and various observables can be constructed. In this talk, we discuss worldsheet form factors in the context of the AdS/CFT correspondence, thati is form factors in (a) the worldsheet theory of strings in Anti-de Sitter space and (b) the spin-chain description of planar Super-Yang-Mills theory. As these theories are integrable, we can formulate functional equations for the form factors. These equations are generalisations of the known form factor axioms for general two-dimensional, integrable, relativistic theories. We check our proposed equations in worldsheet perturbation theory and demonstrate that they naturally also hold for the Heisenberg spin-chain.
Date: Oct. 22nd Speaker: Agustin Sabio-Vera Title: A Modern look at the Regge limit in QCD, SUSY and gravity Abstract: A pedagogical introduction to the Regge limit of scattering amplitudes will be given. Its role in collider physics will be explained, together with its connections to integrability in QCD and SUSY theories. We will finish with a discussion on graviton scattering at high energies.
Date: Nov. 12th Speaker: Matthias Ihl Title: Backreaction of flavors in the Kuperstein-Sonnenschein model Abstract: In the framework of gauge/gravity duality, I will review some top-down approaches to large Nc holographic QCD, including the somewhat lesser known models by Dymarsky, Kuperstein and Sonnenschein whose most prominent feature is a geometric realisation of chiral symmetry breaking. Then I will briefly discuss the so-called smearing technique used to introduce a large number of flavour branes Nf ~ Nc into the background in order to account for their backreaction, and (in principle) allowing to study the Veneziano limit of the dual gauge. Finally, I report on some recent progress in applying this procedure to the Kuperstein-Sonnenschein background and discuss some of the novel physical features that emerge compared to the original, undeformed model.
Date: Nov. 26th Speaker: David Ireland Title: Quantifying Information in Baryon Spectroscopy Abstract: Experimental measurements always carry uncertainties, both from finite detector resolution and a limited sample of counts. Quantifying the information gained in experiments with information theoretic measures should allow one to determine what new physical insights can be inferred from a measurement. As an example, the N* programme with the CLAS detector at Jefferson Lab will be presented, including a survey of the latest results. The application of information theory to amplitude extraction in baryon spectroscopy will then be discussed.
Date: Dec. 3rd Speaker: Charles Patterson Title: Dielectric properties of surfaces Abstract: Surfaces are the window through which the interior of solid matter is viewed. Dielectric properties of matter arise from polarisation induced by external fields via excitation of optical transitions, phonons, etc. Surfaces have a dielectric response which is distinct from the bulk response because atomic structures and local fields are different at the surface. In this talk I will describe recent theoretical work on optical and phonon excitations at semiconductor surfaces induced by light or electron beams.
Date: Dec. 10th Speaker: Brian Wecht Title: New Superconformal Field Theories From Wra pped Branes Abstract: In string/M-theory, it is possible to engineer a wide variety of interesting supersymmetric conformal field theories (SCFTs) by wrapping branes on nontrivial manifolds. In this talk, I will describe a new infinite set of theories which come from M5-branes on Riemann surfaces. The corresponding supergravity solutions interpolate between and extend beyond a famous pair of solutions by Maldacena and Nuñez. Additionally, the dual SCFTs are "non-Lagrangian" theories, which have no weakly coupled UV descriptions, yet can (and will) be described explicitly.
Date: Jan. 21st Speaker: Karl Jansen Title: Spontaneous symmetry breaking and topology: what lattice QCD can tell us Abstract: After a short review of the progress made in lattice QCD simulations, I will introduce the so-called spectral projector method which allows for an accurate determination of the chiral condensate and the topological susceptibility. I will discuss the quark mass dependence of both quantities and give a value of the chiral condensate for a simulation of the first two fermion generations with quarks at (almost) their physical values. In addition, I will present first results for the Witten-Veneziano relation.
Date: Jan. 28th Speaker: Andreas Kümper Title: The Spin-1/2 Heisenberg Chain: Solved and Open Problems Abstract: The theory of integrable quantum chains is highly developed. Integrable systems satisfy the Yang-Baxter equation and allow for more explicit calculations than non-integrable systems. In my talk I intend to review results relevant for understanding experimental systems that were obtained by exact calculations. There are different techniques for calculating thermodynamical potentials and derived quantities, especially the logarithmic low-temperature singularities of the susceptibility. Thanks to recent progress in the field of integrable lattice systems there are exact data for the temperature dependent static correlation functions yielding for instance the frequency moments of electron spin resonance (ESR) lines. Despite the integrability, many properties are unknown. A notorious problem is posed by questions on transport. A very specific, but still difficult problem is posed by the spin Drude weight at finite temperature. I will sketch the different attempts of the literature to this problem by means of quantum Monte Carlo, exact numerical diagonalisation of finite chains, Bethe ansatz, and Mazur bounds applied to matrix product operators.
Date: Feb. 4th Speaker: Leonardo Giusti Title: Implications of Poincare' symmetry for thermal field theories Abstract: The analytic continuation to an imaginary velocity of the canonical partition function of a thermal system expressed in a moving frame has a natural implementation in the Euclidean path-integral formulation in terms of shifted boundary conditions. The Poincare' invariance underlying a relativistic theory implies a dependence of the free-energy on the compact length L0 and the shift xi only through the combination beta=L_0(1+xi^2)^(1/2). This in turn implies a set of Ward identities among the correlators of the energy-momentum tensor which have also interesting applications in lattice field theory. In particular, they offer identities to renormalize non-perturbatively the energy-momentum tensor and novel ways to compute thermodynamic potentials. At fixed bare parameters they also provide a simple method to vary the temperature in much smaller steps than with the standard procedure.
Date: Feb. 11th Speaker: Axel Kleinschmidt, AEI Potsdam Title: String theory scattering, automorphic forms and Kac-Moody symmetries Abstract: At low energies, string theory can be well described by supergravity field theory. Calculating scattering amplitudes at low energies in lowest order therefore is a problem in field theory, but string theory predicts very precise corrections to the supergravity answer. These corrections can be calculated either explicitly from string scattering calculations or indirectly by using constraints from discrete duality symmetries. I will review how this leads to automorphic forms and predictions for non-perturbative corrections to the scattering amplitude. Terra incognita in mathematics is explored by considering cases when the duality symmetry is of Kac-Moody type. Date: Feb. 18th Speaker: Jan Plefka, Humboldt University, Berlin Title: A spectral parameter for scattering amplitudes in N=4 super Yang-Mills theory Abstract: Planar N=4 super Yang-Mills appears to be integrable. While this allows to find this theory's exact spectrum, integrability has hitherto been of no direct use for scattering amplitudes. To remedy this, we deform all scattering amplitudes by a spectral parameter. The deformed tree-level four-point function turns out to be essentially the one-loop R-matrix of the integrable N=4 spin chain satisfying the Yang-Baxter equation. Deformed on-shell three-point functions yield novel three-leg R-matrices satisfying bootstrap equations. Finally, we supply initial evidence that the spectral parameter might find its use as a novel symmetry-respecting regulator replacing dimensional regularization. Its physical meaning is a local deformation of particle helicity, a fact which might be useful for a much larger class of non-integrable four-dimensional field theories. Date: March 4th Speaker: Toby Wiseman, Imperial College London Title: Black holes beyond astrophysics Abstract: In the context of modifications of gravity, large extra dimensions, and the AdS-CFT correspondence, there has been much interest in black holes solutions in theories of gravity and matter that are exotic - they might live in spacetime dimension other than 4, or have exotic matter and boundary conditions. One common theme is that traditional analytic methods to find solutions tend not to work when confronted with these more exotic contexts and instead we are increasingly forced to use numerical techniques. I will discuss a numerical approach to finding static and stationary black hole solutions, and give some example applications. Date: March 11th Speaker: Kareljan Schoutens Title: Strange Metals in One Spatial Dimension Abstract: We consider 1+1 dimensional SU(N) gauge theory coupled to a multiplet of massive Dirac fermions transforming in the adjoint representation of the gauge group. The high density limit is characterized by a deconfined Fermi surface state with Fermi wavevector equal to that of free gauge-charged fermions. Its low energy fluctuations are described by a coset conformal field theory with central charge c=(N^2-1)/3. It displays an emergent N=(2,2) supersymmetry as well as extensive higher spin W-symmetries. We determine the exact scaling dimensions of the operators associated with Friedel oscillations and pairing correlations. Date: March 25th Speaker: Matthias Staudacher Title: Integrability and Scattering in N=4 SYM Abstract: We discuss how a spectral parameter, crucial for the quantum inverse scattering method (QISM) in 1+1 dimensions, makes its appearance in the calculation of on-shell scattering amplitudes in N=4 Super Yang-Mills theory. This opens the door for applying QISM techniques to spacetime scattering in 1+3 dimensional quantum field theory. Excitingly, the regulator seems to allow for the replacement of dimensional regularization by what one might term spectral regularization.
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| Current organizers: Dr. Sergey Frolov Dr. Tristan McLoughlin Dr. Stefan Sint |
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