Dublin Theoretical Physics Colloquium

2015/2016

Speaker

Title

September

30

Karl Landsteiner

(IFT-UAM Madrid)

Chiral Anomalous Transport: Theory and Applications

The concept of symmetry is a cornerstone of modern physics. Quantum theory is another. They are not always compatible. When a symmetry is incompatible with  quantum theory we speak of an anomaly. Chiral symmetries are typically affected by quantum  anomalies and traditionally were invoked to explain the decay of the neutral pion into  two photons. In the recent years it has turned out that anomalies are also  responsible for new and dissipationless transport phenomena, the chiral magnetic and vortical  effects (CME,CVE). I will review the theory of anomalous transport and some of its  applications in high energy and condensed matter physics.

October

7

Marcus Marino (Universite de Geneve)

Topological strings from quantum mechanics

Abstract:

     The gauge/string correspondence postulates an equivalence

between certain string theories and gauge theories at large N. Topological strings are simplified versions of string theory, and one could suspect that they have even simpler dual descriptions.

   In this talk I present a conjectural correspondence between topological string theory on toric Calabi-Yau manifolds, and the spectral properties of quantum-mechanical operators on the real line, in the spirit of large N dualities. The operators turn our to be quantum versions of the algebraic curves which define the mirror manifolds to the Calabi-Yau manifolds. This conjecture provides a rigorous, non-perturbative definition of topological string theory on these backgrounds. In addition, it gives a precise and explicit prediction for the Fredholm determinants and spectra of the corresponding operators, providing in this way a new class of exactly solvable problems in spectral theory.

14

Juan Jottar (ETH, Zurich)

Aspects of the Chern-Simons/CFT_2 correspondence

Abstract:  In the context of the AdS_3/CFT_2 correspondence, the holographic principle asserts that two-dimensional Conformal Field Theories are dual to three-dimensional gravity with anti-de Sitter boundary conditions. The latter may be formulated as a Chern-Simons theory based on two copies of the sl(2) algebra, and various questions about the universal behavior of 2d CFTs at large central charge can be then addressed in classical Chern-Simons theory. We discuss generalizations of this correspondence in which the bulk Chern-Simons theory is based on a larger gauge algebra: via holography, such theories are dual to CFTs with additional conserved currents beyond the stress tensor, which generate chiral algebras that extend the Virasoro algebra. In particular, we describe how to exploit the topological formulation of the 3d bulk theory in order to efficiently compute relevant CFT quantities in the presence of these extended symmetries. Such quantities include partition functions, BPS bounds (in supersymmetric setups), and even non-local observables such as entanglement and Rényi entropies, and we discuss a few concrete examples.

21

Lionel Mason (Oxford)

Ambitwistor strings and the scattering equations at one loop

Abstract:

Ambitwistor strings are holomorphic string theories whose target space is the space of complex null geoedesics in a complexified space-times. I will explain how these theories explain the origin of the scattering equations in twistor strings and the CHY formulae in arbitrary dimensions and provide a reformulation of standard gauge, gravity and other theories in a holomorphic infinite tension analogue of conventional string theories. I will show how these results extend to 1-loop both on a torus and on a nodal Riemann sphere.

28

Mariapaola Lombardo (INFN Frascati)

Adding flavors to strong interactions

Abstract:

Quantum-Chromodynamics - the field theory of strong interactions - is extremely successful in describing the physics of quarks and gluons.  In a lattice simulation of QCD we can artificially increase the number of quarks' species - quark flavours - and study how fundamental issues like mass generation and symmetry breaking are related to the interplay of flavor symmetry and gauge dynamics.It turns out that when the number of flavors grows large, chiral symmetry is no longer spontaneously broken,  the theory is infrared conformal and strongly interacting.  After reviewing the theoretical predictions and possible phenomenological implications, the talk will focus on recent lattice results at zero and non-zero temperature, Evidence for the conformal phase and its anomalous dimension will be presented. It will be shown that this cold exotic phase is continuously connected to the physical strongly interacting quark gluon plasma, while its  precursor effects  might be used to study beyond-the-standard-model mechanisms of electroweak symmetry breaking.

November

4

Michael Kraemer (Aachen)

Simplify your life: towards model-independent searches for new physics at the LHC

Abstract:

The main goal of the LHC physics program is to explore the mechanism of electroweak symmetry breaking and to search for physics beyond the Standard Model. I briefly summarize the results obtained in the first period of the LHC and discuss the prospects for new physics searches at the upcoming high-energy run. I introduce the concept of simplified models for generic new physics searches and analyse the complementarity of LHC, low-energy and astroparticle physics in the context of dark matter searches.

18

Alberto Ramos (CERN)

Charged hadrons in a finite volume

Isospin breaking represents a small correction to most hadronic processes. Nevertheless, these small corrections have drastic consequences, among them the stability of the hydrogen atom. On the other hand, flavour physics is reaching a level of precision such that isospin breaking corrections cannot be ignored. The first principles computation of many of these quantities requires a lattice formulation of QCD and QED in a finite volume. In this talk I will describe the existing approaches in the literature and focus on a recent proposal to use charge conjugate boundary conditions in order to describe electrically charged states in a finite volume.  

25

Aidan Robson (Glasgow)

Physics with heavy boson pairs at the ATLAS Experiment

The Large Hadron Collider beam energies are high enough to produce large numbers of pairs of electroweak heavy bosons.  These allow a sensitive probing of the gauge structure of the Standard Model, are crucial in our ongoing Higgs physics programme, and provide a channel for searching for further new physics at high masses.  I will introduce the relevant LHC physics, detectors, and methods, and show diboson results from the ATLAS Experiment from the LHC's 8TeV running - including a search for high-mass resonances that uses new techniques to tag hadronic decays of highly boosted W and Z bosons and shows a small excess.  I will also show first results and prospects for Run 2 at 13TeV.

December

2

Alessandro Vichi (CERN)

Conformal bootstrap and the O(N) vector model

The past few years have seen great progresses in the understanding of Conformal Field Theories (CFTs). I  will review the 'conformal bootstrap program', namely the use of unitarity and crossing symmetry to extract quantitative informations about CFTs.

I will then apply the conformal bootstrap machinery to extract precise estimates of the critical exponents of the three dimensional O(N) vector model.

9

Gabriele Travaglini (Queen Mary)

Harmony of scattering amplitudes

I will describe some of the  hidden structures recently discovered in the scattering amplitudes of elementary particles, such as those measured at the Large Hadron Collider. These structures are responsible for the mysterious simplicity of these quantities, which is completely obscured by a calculation based on textbook techniques such as Feynman diagrams. I will then move on to discuss form factors. Form factors are slightly off-shell quantities and, similarly to amplitudes, are also much simpler than what expected based on conventional approaches. In particular I will focus on form factors of particular (half-BPS) operators in a special theory, known as N=4 super Yang-Mills, and briefly discuss some unexpected connections to scattering amplitudes in phenomenologically relevant theories. Finally I will discuss some recent connections between amplitudes, form factors and the dilatation operator in N=4 super Yang-Mills.

January

20

27

Luis Fernando Alday (Oxford)

The analytic bootstrap program

We will use basic properties of conformal field theories, such as symmetries and the structure of the operator product expansion, to get analytic results for the scaling dimensions of higher spin operators.

February

3

Daniel Persson (Chalmers)

Postponed to March 30th.

 10

Luigi Del Debbio (Edinburgh)

Energy momentum tensor on the lattice and the

gradient flow

The gradient flow has recently been formulated in great detail, and has turned out to be a very successful tool

for studying several aspects of quantum field theory.

In this talk we discuss the use of the gradient flow for the nonperturbative renormalization of the energy momentum tensor on the lattice,  and its numerical implementation for the case of scalar and gauge field theories.

We present some preliminary numerical results.

17

Pierpaolo Mastrolia (Universidad Autónoma de Madrid)

Scattering Amplitudes: the Frontier of Feynman Calculus

The connection between theoretical models and experimental data in Particle Physics lies in the evaluation of “scattering amplitudes”, numbers that represent the likelihood that a certain set of particles will turn into certain other particles upon colliding. Particle collisions are conveniently described by Feynman diagrams, drawing

pictures of the various ways the particles can morph or shuffle during an interaction. Basic reactions are described by tree-shaped diagrams, while quantum corrections receive contributions from diagrams containing either a larger number emitted particles (legs) or closed loops. Advances in High Energy Particle Physics necessarily depend on our ability to describe the scattering processes involving many light and heavy particles at very high accuracy. The complexity of Feynman calculus grows with the number of loops, the number of legs, and the masses of the involved particles, because of the severe computational

limitations due to the large amount of algebra required, as well as due to the analytic structure dictated by the singularities.

I discuss about the fundamental role played by scattering amplitudes at the core of Quantum Field Theory. I analyze the generic patterns of amplitudes in two domains - algebraic and analytic - turning the qualitative property of the "unitarity" of the Scattering-Matrix, related to probability conservation, into quantitative methods for

amplitudes determination. I propose a novel algebraic-geometrical approach for unitarity, showing a unique and simple framework underlying scattering amplitudes to all orders in perturbation theory, based on Cauchy's Residue Theorem implemented via Multivariate

Polynomial Division, which yields the decomposition of Feynman integrals in terms of elementary integrals. Also, I establish a correspondence between the systems of differential equations obeyed by elementary Feynman integrals and Schroedinger equation in Quantum

Mechanics, so that they can be determined at once by means of purely algebraic methods, such as the Magnus series.

I present the dramatic impact of these new methods on the study of more theoretical aspects of Quantum Field Theory, as well as, very importantly, on collider phenomenology. Relevance is given to Higgs boson associated production at the LHC, recently computed at an unprecedented level of accuracy: one of the most beautiful calculation ever performed with Feynman diagrams. I conclude with the possible investigation paths which can be accessed by our novel understanding of Feynman calculus, which complements the original work of Landau and Cutkosky

March

9

Ben Hoare (ETH, Zurich)

The eta- and lambda-deformations of the AdS_5 x S^5 superstring and supergravity.

In this talk we will discuss recent progress understanding two proposals for q-deformations (eta and lambda) of the AdS_5 x S^5 superstring and their interplay with supergravity. In the first part of the talk we introduce and motivate the models and, restricting to simple bosonic examples, explore the connections between the two theories. In the second part we will use the results to try to understand the extent to which the eta-deformed AdS_5 x S^5 background is a solution of Type IIB supergravity. We will conclude with a discussion of the meaning of the results.

23

Valentina Forini (Humboldt University)

String worldsheet, AdS/CFT and lattice

String sigma-models relevant in AdS/CFT are highly non-trivial two-dimensional field theories whose perturbative analysis is being crucially used to verify various conjectures, among which their integrability. I will discuss how to address the extraction of information at finite values of the string effective tension via the use of lattice-based methods.

30

Daniel Persson (Chalmers)

Moonshine: from finite groups to black holes and beyond

In mathematics and physics the word 'moonshine'

represents surprising and deep connections between a priori

unrelated fields, such as number theory, representation theory

and quantum field theory. The most famous example

is Monstrous Moonshine which relates Fourier coefficients

of modular forms with representations of the largest finite sporadic

group, known as the Monster group. This connection can be

partially explained through a certain bosonic string theory. Recently, a new moonshine

phenomenon was uncovered which connects the largest Mathieu group

M24 with modular forms, via superstring theory on K3-surfaces.

In this talk I will give a general introduction to moonshine

and describe some recent exciting developments.

April

6

13

Boris Pioline (CERN)

Unfolding methods for string amplitudes