The Arnold Sommerfeld Center for Theoretical Physics organizes regular colloquia about topics of current interest in the field of theoretical physics.

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Sommerfeld Theory Colloquium (ASC)

The Arnold Sommerfeld Center for Theoretical Physics organizes regular colloquia about topics of current interest in the field of theoretical physics.

Sommerfeld Theory Colloquium (ASC) was last modified: February 19th, 2020 by Kurt Laufer

Feb 14, 2020

With Run II, the LHC experiment already reached an unprecedented level of precision compared to previous hadron colliders. The amount of data collected in Run III and the High-Luminosity run will increase quickly and dramatically. I will discuss specific challenges to theorists that must be overcome to provide predictions that have the precision necessary to match the accuracy of data. A few novel ideas to exploit this precision directly or indirectly will also be discussed.

Jan 24, 2020

Condensed matter is found in a variety of phases, the vast majority of which are characterized in terms of symmetry breaking. For example, magnets spontaneously break time-reversal and spin rotation symmetries. A notable exception was provided by the discovery of the quantum Hall effects which exhibit new kinds of topological orders not associated with any symmetry breaking. One of the characterizing features of topological order is the existence of excitations with exotic properties. These so-called anyonic excitations might make topologically ordered systems ideal building blocks of fault-tolerant quantum computers. In this colloquium, I will start by giving a general introduction to the concept of topological order and then address some of the recent developments. In particular, I will introduce theoretical frameworks that allow us to classify topological phases and discuss dynamical signatures that are useful to experimentally detect them in experiments.

Jan 16, 2020

String theory seems to offer an enormous number of possibilities for low energy physics. The huge set of solutions is often known as the String Theory Landscape. In recent years, however, it has become clear that not all quantum field theories can be consistently coupled to gravity. Theories that cannot be ultraviolet completed in quantum gravity are said to be in the Swampland. In this talk, I'll discuss some conjectured properties of quantum gravity, evidences for them, and their applications to cosmology and particle physics.

Jan 16, 2020

The functional renormalization group (RG) is an ideal tool for dealing with the diversity of energy scales and competition of instabilities in interacting fermion systems. Starting point is an exact flow equation which yields the gradual evolution from a microscopic model action to the effective low-energy action as a function of a continuously decreasing energy scale. Expanding in powers of the fields yields an exact hierarchy of flow equations for vertex functions. Truncations of this hierarchy have led to powerful new approximation schemes [1]. Applications reviewed in the colloqium include: (i) d-wave superconductivity and other instabilities in the two-dimensional Hubbard model, and (ii) transport through a barrier and resonant tunneling in a one-dimensional Luttinger liquid metal. Recently, the functional RG has been upgraded from a weak-coupling method to a computational tool for strongly interacting fermion systems [2,3]. [1] W. Metzner et al., Rev. Mod. Phys. 84, 299 (2012). [2] C. Taranto et al., Phys. Rev. Lett. 112, 196402 (2014). [3] D. Vilardi et al., Phys. Rev. B 99, 104501 (2019).

Jan 16, 2020

Although the neutrino oscillations are well established phenomenon, new and unusual oscillation effects in matter are still emerging. I will describe three such effects which have applications to the solar, supernova and low energy atmospheric neutrinos: (i) Parametric resonance for neutrinos propagating in a flux of background neutrinos. (ii) Oscillation waves emitted from borders between layers in a multilayer medium, like in the Earth. (iii) The attenuation effect related to the energy uncertainty in oscillation setup.

Jan 16, 2020

From the softest of interactions of a magnetic field with an electron, to the most violent collisions at the Large Hadron Collider, precision quantum field theory produces numbers and functions with interesting number-theoretic properties. In many examples a co-action principle holds, an invariance under a ”cosmic” Galois group. I will provide several arenas in which this principle can be seen at work, including perhaps the richest set of theoretical data, scattering amplitudes in planar N = 4 super-Yang-Mills theory.

Jan 16, 2020

Quantum field theory (QFT) is the universal language of theoretical physics, underlying the Standard Model of elementary particles, the physics of the early Universe and a host of condensed matter phenomena such as phase transitions and superconductivity. A great achievement of 20th-century physics was the understanding of weakly coupled quantum field theories where interactions can be treated as small perturbations of otherwise freely moving particles. Critical challenges for the 21st century include solving the problem of strong coupling and mapping the whole space of consistent QFTs. In this lecture, I will overview the bootstrap approach, the idea that theory space can be determined from the general principles of symmetry and quantum mechanics. This strategy provides a new unifying language for QFT and has allowed researchers to make predictions for physical observables even in strongly coupled theories. I will illustrate the general framework in a few examples, ranging from the concrete (boiling water) to the abstract (supersymmetric theories in various spacetime dimensions).

Jan 16, 2020

A unified theory for space-time and matter might be based on finite projective geometries instead of differentiable manifolds and gauge groups. Each point is equipped with a quadratic form over a finite Galois field which define neighbors in the finite set of points. Due to the projective equivalence of all quadratic forms this world is necessarily a 4-dimensional Lorentz-invariant space-time with a gauge symmetry G(3)xG(2)xG(1) for internal points which represent elementary particle degrees of freedom. Matter appears as a geometric distortion by an inhomogeneous field of quadrics and all physical properties (spins, charges) of the standard model seem to follow from its geometric structure in a continuum limit. The finiteness inevitably induces a fermionic quantization of all matter fields and a bosonic for gauge fields. This unity of space-time and matter was already sought 1918 by Hermann Weyl in a gauge theory as an extension of Einstein’s general theory of relativity, but not found - probably because of the assumption of a continuous geometry.

Jan 16, 2020

I describe an algebraic scheme for quantizing the Ruijsenaars-Schneider models in the R-matrix formalism. It is based on a special parametrization of the cotangent bundle over GL(n,C). In new variables the standard symplectic structure is described by a classical (Frobenius) r-matrix and by a new dynamical r¯-matrix. Quantizing these r-matrices, I will exhibit the quantum L-operator algebra and construct its particular representation corresponding to the RuijsenaarsSchneider system. I will also indicate a couple of open problems.

Jan 16, 2020

The apparent crisis of black holes inconsistency with foundational physical principles provides a sharp focus for the conflict between quantum mechanics and classical spacetime. Various resolutions have been proposed; a very plausible one is that small interactions can transfer sufficient information between the black hole and outgoing radiation, with a quantum enhancement from the enormous number of black hole states. Such interactions must however violate conventional notions of locality, perhaps as a symptom of the more basic subtlety of information localization in quantum gravity, and hinting at aspects of the fundamental structure of quantum gravity. An intriguing question is whether further clues can be found from new observational windows on black holes.

Jan 16, 2020

The study of black holes has revealed a deep connection between quantum information and spacetime geometry. Its origin must lie in the quantum theory of gravity, which offers a valuable hint in our search for a unified theory. Precise formulations of this relation recently led to new insights in Quantum Field Theory, some of which have been rigorously proven. An important example is our discovery of the first universal lower bound on the local energy density. The energy near a point can be negative but it is bounded below by a quantity related to the information flowing past the point.

Jan 16, 2020

Sommerfeld Theory Colloquium, The knowledge of all correlation functions of a system is equivalent to solving the corresponding quantum many-body problem. If one can identify the relevant degrees of freedom, the knowledge of a finite set of correlation functions is in many cases sufficient to determine a sufficiently accurate solution of the corresponding field theory. Complete factorization is equivalent to identifying the relevant degrees of freedom where the Hamiltonian becomes diagonal. I will give examples how one can apply this powerful theoretical concept in experiment. A detailed study of non-translation invariant correlation functions reveals that the pre-thermalized state a system of two 1-dimensional quantum gas relaxes to after a splitting quench [1], is described by a generalized Gibbs ensemble [2]. This is verified through phase correlations up to 10th order. Interference in a pair of tunnel-coupled one-dimensional atomic super-fluids, which realize the quantum Sine-Gordon / massive Thirring models, allows us to study if, and under which conditions the higher correlation functions factorize [3]. This allowed us to characterize the essential features of the model solely from our experimental measurements: detecting the relevant quasi-particles, their interactions and the different topologically distinct vacuum-states the quasi-particles live in. The experiment thus provides a comprehensive insight into the components needed to solve a non-trivial quantum field theory. Our examples establish a general method to analyse quantum systems through experiments. It thus represents a crucial ingredient towards the implementation and verification of quantum simulators. Work performed in collaboration with E.Demler (Harvard), Th. Gasenzer und J. Berges (Heidelberg). Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF): SFB FoQuS: F40-P10 and the EU: ERC-AdG QuantumRelax [1] M. Gring et al., Science, 337, 1318 (2012); [2] T. Langen et al., Science 348 207-211 (2015). [3] T. Schweigler et al., Nature 545, 323 (2017), arXiv:1505.03126

Jan 16, 2020

Sommerfeld Theory Colloquium, We shall review the attempts to extend the quantum mechanical property of non-commutativity from phase space to ordinary space. These attempts took a more precise form in the case of gauge theories for which some concrete results have been obtained. In flat space they amount to a reformulation of the theory which looks interesting but they have not given so far any novel physical results. However, the introduction of gravity gives a richer structure and may offer some new insights.

Jan 16, 2020

Sommerfeld Theory Colloquium, The construction of a quantum theory of gravity remains an open problem despite decades of efforts. In time, the very perspective on this problem evolved. From quantising General Relativity, the goal is now mostly understood to be unraveling a more fundamental microstructure of spacetime, based on non-geometric building blocks, and to show how spacetime and matter emerge as effective, approximate notions. Given some candidate building blocks, the task becomes analogous to that of extracting the macroscopic, collective behaviour of the atoms of a condensed matter system, but even more challenging since we cannot use the usual spacetime intuition and no direct observational input is available to guide theory construction. Lacking a fundamental theory of quantum gravity, existing cosmological models which have proven extremely successful in accounting for the observed features of the very early universe (via CMB data) remain without a solid foundation, having to make a number of assumptions about a physical regime (close to the big bang), where the quantum nature of gravity and spacetime is expected to be relevant. This is all the LUDWIG-MAXIMILIANS-UNIVERSITÄT MÜNCHEN SEITE 2 VON 2 more unfortunate, since the very early universe is also where any proposed quantum theory of gravity has the highest chance of finding its observational test-bed. The gap needs to be bridged. In this talk I will first of all review the basic aspects of the problem of quantum gravity, and of some current approaches. I will then focus on one specific formalism for quantum gravity, so-called group field theories (strictly related to a number of other modern approaches). I will introduce its main features, trying to clarify the nature of the suggested building blocks of spacetime and their mathematical description. Next, I will outline a general strategy to extract an effective cosmological dynamics from quantum gravity, within this formalism. In this setting, the universe emerges as a quantum condensate of the fundamental “atoms of spacetime”, and cosmology is its corresponding hydrodynamics. Finally, I will summarize the recent results obtained along this research direction.

Jan 16, 2020

Sommerfeld Theory Colloquium, Metagenomics has revealed hundreds to thousands of microbial species coexisting in almost all microbiota. It is increasingly appreciated that microbial communities condition their own environments. To better understand the role of this environmental conditioning in promoting diversity, we physically model the population dynamics of microbes that compete for steadily supplied resources. In a model where cells require multiple nutrients, we find that population dynamics generally leads to the coexistence of different metabolic types, which satisfy an extended competitive exclusion principle. Moreover, we establish that these consortia of metabolic types act as cartels, whereby population dynamics pins down resource concentrations at values for which no other strategy can invade. Strikingly, these cartels also yield maximum biomass, constituting a microbial example of Adam Smith’s “invisible hand” leading to collective optimal usage of resources. Curiously, in a model where only total resource acquisition is considered, diversity can arbitrarily exceed that predicted by the competitive exclusion principle.

Jan 16, 2020

Sommerfeld Theory Colloquium, The observed deviations from the laws of gravity of Newton and Einstein in galaxies and clusters can logically speaking be either due to the presence of unseen dark matter particles or due to a change in the way gravity works in these situations. Until recently there was little reason to doubt that general relativity correctly describes gravity in all circumstances. In the past few year insights from black hole physics and string theory have lead to a new theoretical framework in which the gravitational laws are derived from the quantum entanglement of the microscopic information that is underlying space-time. An essential ingredient in the derivation is of the Einstein equations is that the vacuum entanglement obeys an area law, a condition that is known to hold in Anti-de Sitter space due to the work of Ryu and Takayanagi. We will argue that in de Sitter space due to the positive dark energy, that the microscopic entanglement entropy also contains also a volume law contribution in addition to the area law. This volume law contribution is related to the thermal properties of de Sitter space and leads to a total entropy that precisely matches the Bekenstein-Hawking formula for the cosmological horizon. We study the effect of this extra contribution on the emergent laws of gravity, and argue that it leads to a modification compared to Einstein gravity. We provide evidence for the fact this modification explains the observed phenomena in galaxies and clusters currently attributed to dark matter.

Jan 16, 2020

Sommerfeld Theory Colloquium, In February 2016 the LIGO team announced the detection of gravitational waves (GW) created by the merger of two black holes. In addition to confirming a major prediction of general relativity, successful GW detection would provide a powerful new tool for astrophysics. Given their evident importance, the LIGO results and the methods which led to them deserve independent critical analysis. This talk will present the results of one such study in a manner suitable for non-specialists.

Jan 16, 2020

Sommerfeld Theory Colloquium, The upshot of extensive studies of �uctuations in condensed matter systems is that their qualitative importance is typically con#ned to isolated critical points of continuous transitions between phases of matter. This conventional wisdom also predicts the number of low energy Goldstone modes based on the so-called “G/H” pattern of symmetry breaking. I will discuss a class of systems, some quite wellknown, that violate this standard paradigm. Namely, they exhibit a fewer than “G/H” number of low-energy modes due to an emergent Higgs mechanism. Even more spectacularly, such systems exhibit “critical” ordered phases, with universal power-law properties reminiscent of a critical point, but requiring no #ne-tuning and extending throughout the ordered phase. One exciting recently discovered state is the heliconical nematic that in addition to above phenomena also exhibits spontaneous chiral symmetry breaking.

Jan 16, 2020

Sommerfeld Theory Colloquium, Over the past ten years, there has been a growing interest among physicists for ‘active matter’, a codename that encompasses systems in which energy is taken from the environment to generate self-propulsion at the single particle level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic colloids or actin filaments in motility assays, are strongly out-of-equilibrium and exhibit much richer behaviours than their passive counterpart. In this talk I will review recent progresses regarding the physics of active particles. I will show how simple concepts like pressure, the force density exerted by assemblies of particles on their container, play a new role for active systems because of the lack of equation of state. I will also show how new collective phenomena emerge, from the transition to collective motion to the existence of cohesive matter without cohesive forces, that have no counterpart in thermal equilibrium.

Jan 16, 2020

Sommerfeld Theory Colloquium, I will review for a general audience some recent developments in our understanding of the mathematical structure of scattering amplitudes in quantum field theory. Many of these developments involve properties that have been discovered ”experimentally”: not in actual experiments, but by carrying out a tedious calculation and then observing that the result has some remarkable hidden simplicity. I will give examples of this phenomenon, and in particular I will discuss some aspects of the geometry of the ”amplituhedron”, a geometric object that is believed to completely encode certain scattering amplitudes.

Jan 16, 2020

Sommerfeld Theory Colloquium, I describe the physics of black holes and show how the traditional approach leads to the information paradox. I will then discuss some of the proposed resolutions and the difficulties they need to overcome. I then discuss soft black hole hair and describe how it may help to resolve the information paradox. Finally, I will review the problems that still need to be overcome.

Jan 16, 2020

Sommerfeld Theory Colloquium, In this talk I will discuss connections between the physics of complex systems such as spin glasses and attempts to solve optimization problems by ”Adiabatic Quantum Computing” (AQC), a version of ”Quantum Annealing” (QA). An optimization problem is one in which one has to minimize (or maximize) an energy function in which there is competition between different terms so no single configuration of the variables minimizes each term in the energy. In statistical physics this competition is called ”frustration”. It leads to a complex energy ”landscape” with many valleys separated by barriers, so simple algorithms easily get trapped in local minima which have a higher energy than the global minimum. Many problems in science, and engineering are formulated as optimization problems. In quantum annealing one tries to avoid being trapped in a local minimum by adding quantum fluctuations so the system can tunnel to regions of lower energy. The strength of the quantum fluctuations is gradually reduced to zero during the annealing schedule. This method applies to problems with binary variables, known as qubits in the quantum case. There is considerable interest in AQC at present, in large part because a company, D-Wave, has produced an actual device, the latest version of which has about one thousand qubits. In addition, there has been considerable theoretical work mainly using computer simulations to see if there is a ”quantum speedup” compared with analogous classical algorithms in which thermal, rather than quantum, fluctuations are used to escape from local minima. In the talk I will discuss difficulties in obtaining a quantum speedup due to (i) (quantum) phase transitions that the system can undergo during the annealing schedule, and (ii) the sensitivity of the state of the system to the precise values of the interactions, i.e. chaos. A related chaotic effect is that the state of the system can change dramatically with small changes in the temperature (temperature-chaos), for thermal annealing, and the strength of the quantum fluctuations, for quantum annealing.

Jan 16, 2020

Sommerfeld Theory Colloquium, Low-mass boson dark matter particles produced after the Big Bang form a classical field and/or topological defects. Effects produced by the interaction of ordinary matter with dark matter may be first power in the underlying interaction strength rather than the second power. This may give a big advantage, since the dark matter interaction constant is extremely small. Limits on certain types of dark matter have been improved up to 15 orders of magnitude. New experiments are proposed.

Jan 16, 2020

Sommerfeld Theory Colloquium, Metals are found frequently in nature and their properties are usually very well described within Landaus Fermi liquid theory. Various strongly correlated materials exhibit strange metallic phases which do not fit into the Fermi-liquid framework, however. The theoretical description of such non-Fermi liquids remains one of the main unsolved problems in condensed matter physics. In this talk I will give an introduction to the problem and show how interesting strongly coupled field theories arise in the low energy description of such states, which are still very poorly understood. I will focus on the paradigmatic problem of a metal coupled to fluctuations of a critical Ising order parameter and discuss unexpected scaling properties at finite temperature.

Jan 16, 2020

Sommerfeld Theory Colloquium, Protoplanetary discs are natural consequence of star formation. These discs hold the left-over material from star formation, which constitutes the reservoir from which new planetary systems may form. The fate of a new planetary is then intimately linked to the evolution and ﬁnal dispersal of the disk from which is born, which determines also the striking diversity observed in extra-solar planetary systems. I will brieﬂy review our understanding of disc dispersal via a photoevaporative wind in the context of planet formation, and show how both processes are ﬁnally dominated by the irradiation from their central star.

Jan 16, 2020

Sommerfeld Theory Colloquium, Condensed matter provides us deep insights into quantum physics. Giving just two examples, wave-corpuscle duality manifests itself in spectroscopy of strongly correlated systems as coexistence of itinerant and atomic-like features, and graphene and other Dirac materials provide a natural playground to study vacuum reconstruction, Klein tunneling and other fundamental quantum relativistic phenomena. Electron-photon interaction is the key tool to understand this rich and nontrivial physics.

Jan 16, 2020

Sommerfeld Theory Colloquium

Jan 16, 2020

Sommerfeld Theory Colloquium, New Physics searches at the LHC are mostly being performed with the aim of detecting new states. A complementary strategy is to look for new interactions, something that typically involves precise measurements. In this talk I argue how many of the current SM (and BSM) measurements in the top sector could be used to efficiently and consistently determine of the couplings of an effective field theory and in particular that of the SM at dimension 6.

Jan 16, 2020

Sommerfeld Theory Colloquium, Do quantum many-body systems necessarily come to thermal equilibrium after a long enough time evolution? The conventional wisdom has long been that they do and that, in the process, any quantum information encoded in the initial state is lost irretrievably. Thus the dynamics of many-interacting particles becomes effectively classical. But these ingrained notions of thermalization and ergodicity have recently been called into question. In this talk I will discuss how ergodicity can break down in disordered quantum systems through the phenomenon of many-body localization. In contrast to thermalizing fluids, quantum correlations can persist through time evolution of the localized state even at high energy densities. Thus, investigating the many-body localization transition offers a concrete route to address fundamental unsolved questions concerning the boundary between classical and quantum physics in the macroscopic world. I will emphasize the important role that quantum entanglement plays in current attempts to understand this fascinating dynamical phase transition. Finally I will present recent progress in confronting the emerging theoretical understanding of many-body localization with experimental tests using systems of ultra-cold atoms.

Jan 16, 2020

Sommerfeld Theory Colloquium, Quantum mechanics is important for determining the geometry of spacetime. We will review the role of quantum fluctuations that determine the large scale structure of the universe. In some model universes we can give an alternative description of the physics in terms of a theory of particles that lives on its boundary. This implies that the geometry is an emergent property. Furthermore, entanglement plays a crucial role in the emergence of geometry. Large amounts of entanglement are conjectured to give rise to geometric connections, or wormholes, between distant and non-interacting systems.

Jan 16, 2020

Sommerfeld Theory Colloquium, Highest resolution Event Horizon Telescope (EHT) observations will probably soon tell us more about the supermassive black hole at the Galactic Centre (Sgr A*) and the cores of active galactic nuclei (AGN). It might also help to clarify the long-standing question whether the central massive objects in AGN are instead close pairs of black holes. Mergers of supermassive black hole pairs would provide the strongest gravitational wave signals. I will present examples of how we identify potential close binary black hole candidates based on the combined analysis of high resolution radio interferometric (VLBI) observations and multi- wavelength data. I will also provide an outlook on the scientific prospects with regard to future EHT-observations.

Jan 16, 2020

Sommerfeld Theory Colloquium, 2015 is the International Year of Light, and of its purposes is “to raise awareness of optical technologies”. One such technology, high-power lasers of the petawatt class and beyond, provides the most intense light sources created by humankind so far. The intensities and field strengths in question are in excess of 1022 W/cm2 and 1014 V/m, respectively magnitudes that correspond to concentrating the total solar radiation on a pinhead. This talk will present an overview of the uses and consequences of such extreme environments within the realm of particle physics. The relevant theory is strong-field QED, with the laser beams providing a rather peculiar electromagnetic background field. The magnitudes above are such that a nonperturbative treatment of the background becomes a necessity. Using appropriate theoretical tools, a number of phenomena will be addressed, including radiation reaction, nonlinear Thomson/Compton scattering, laserstimulated pair production and photon-photon scattering. A particular incarnation of the latter, polarisation flip forward scattering, or vacuum birefringence, will be discussed in some detail, with an outlook on a planned experiment at the European XFEL at DESY.

Jan 16, 2020

Sommerfeld Theory Colloquium, All flows show a transition from a laminar phase to a turbulent one for sufficiently high flow speeds. In many cases turbulence develops in a succession of instabilities that create flows of increasing temporal and spatial complexity (Lord Rayleigh, Sommerfeld, Heisenberg, Taylor, Landau etc), For the classroom example of pipe flow and several other flows, the linear stability analysis of the laminar profile does not reveal any instability, so that the very first point in that cascade of instabilities is absent. Over the last decade much of the mystery of the transition in pipe flow has been resolved, primarily thanks due to suitable adaptations and extensions of ideas from nonlinear dynamics. Numerical and experimental data corroborate a scenario where the appearance of new classes of fully 3d solutions and their increasing entanglement provides the key ingredients for the transition. Further studies on the spatio-temporal dynamics in the transition region, where turbulence is not space-filling, reveal intriguing similarities to the directed percolation transition in statistical mechanics.

Jan 16, 2020

Sommerfeld Theory Colloquium, The hard-disk model has exerted outstanding influence on computational physics and statistical mechanics. Decades ago, hard disks were the first system to be studied by Markov-chain Monte Carlo methods and by molecular dynamics. It was in hard disks, through numerical simulations, that a two-dimensional melting transition was first seen to occur even though such systems cannot develop long-range crystalline order. Analysis of the system was made difficult by the absence of powerful simulation methods. In recent years, we have developed powerful Monte Carlo algorithms for hard disks and related systems. I will in particular show how the event-chain Monte Carlo algorithm has allowed us to prove that hard disks melt with a first-order transition from the liquid to the hexatic and a continuous transition from the hexatic to the solid. I will finally describe how a new factorized Metropolis filter transforms the event-chain algorithm into a paradigm for general Monte Carlo calculations. First results with the generalized algorithm have allowed us to establish the phase diagram for two-dimensional soft disks and Yukawa particles.

Jan 16, 2020

Sommerfeld Theory Colloquium, We review the current status of measurements of the spectrum, an isotropy and polarization of the CMB, and their impact on cos- mology. We show that there is still a large discovery potential in these measurements, complementing and synergic to other cosmology observations. We also describe a selection of new measurement eff�orts, including high precision measurements of CMB polarization with ground-based, stratospheric balloon and satellite experiments, focusing on the astrophysical and technological limits of these measurements.

Jan 16, 2020

Sommerfeld Theory Colloquium, I will review the basic defi�nitions and ideas of the twistor program for fundamental physics, as started by Roger Penrose around 1970. I will give particular attention to certain conformally invariant struc- tures involving deformed helicities, relevant to scattering amplitudes for massless �elds. These played a role in Penrose's earliest contour integral expressions, but have more recently been rediscovered and greatly developed as an approach to regularization.

Jan 16, 2020

Sommerfeld Theory Colloquium, To pinpoint the microscopic mechanism for superconductivity has proven to be one of the most outstanding challenges in the physics of correlated quantum matter. Thus far, the most direct evidence for an electronic pairing mechanism is the observation of a new symmetry of the order-parameter, as done in the cuprate high-temperature superconductors. Like distinctions based on the symmetry of a locally defi�ned order-parameter, global, topological invariants allow for a sharp discrimination between states of matter that cannot be transformed into each other adiabatically. Here we propose an unconventional pairing state for the electron uid in two-dimensional oxide interfaces and establish a direct link to the emergence of nontrivial topological invariants. Topological superconductivity and Majorana edge states can then be used to detect the microscopic origin for superconductivity. In addition, we show that also the density wave states that compete with superconductivity sensitively depend on the nature of the pairing interaction. Our conclusion is based on the special role played by the spin-orbit coupling and the shape of the Fermi surface in SrTiO3/LaAlO3-interfaces and closely related systems.

Jan 16, 2020

Sommerfeld Theory Colloquium

Jan 16, 2020

Sommerfeld Theory Colloquium, The role of Type Ia supernovae in observational cosmology has evolved from being ”avant-garde” in the early 1990’s until today’s mature status of precision cosmology. Several large transient surveys have been detecting supernovae routinely, near and far, with the aim of probing what is causing the accelerated expansion of the Universe. With time, the focus has changed towards addressing the intricacies of astrophysical effects that could bias the fits of cosmological parameters, most notably the nature of dark energy. In this talk, aimed at high-energy physics theorists(!), I will try to convey the status of the field, high-lighting some progress and set-backs, based on studies of the closest Type Ia SN in modern time that exploded in the beginning of 2014 in the near-by galaxy M82.

Jan 16, 2020

Sommerfeld Theory Colloquium, Various aspects of lattice gauge theory will be briefly discussed including, general principles, sources of systematic errors, dynamical fermions, QCD phenomenology, the FLAG project and, if time allows, some applications of lattice theory to other non-perturbative BSM phenomena.

Jan 16, 2020

Sommerfeld Theory Colloquium, The conjectured relation between higher spin theories on anti de-Sitter (AdS) spaces and weakly coupled conformal �field theories is reviewed. I shall then outline the evidence in favour of a concrete duality of this kind, relating a speci�c higher spin theory on AdS3 to a family of 2d minimal model CFTs. Finally, I shall explain how this relation �ts into the framework of the familiar stringy AdS/CFT correspondence.

Jan 16, 2020

Sommerfeld Theory Colloquium, Equivalence principle violation, deviations from the inverse-square law of gravity, and long-range interactions between particle spins o�er powerful tests of beyond-the-standard-model physics. Using a torsion pendulum, a technique that dates to the 18th century, is still the most sensitive way to search for these signatures of new physics. I will talk about how the experiments are done and give an update on the current and future status of such torsion balance experiments in the Eot-Wash group.

Jan 16, 2020

Sommerfeld Theory Colloquium, Several new techniques are currently being employed to probe the strong gravitational fi�eld in the vicinity of black holes. Long baselineinterferometry at sub-millimeter wavelengths sets constraints on the silhouette of the black holes in the Galactic center (SgrA*) and M87. Stars which get tidally disrupted as they orbit too close to a single black hole are being discovered at cosmological distances. Electromagnetic counterparts of black hole binaries in galaxy mergers are being identifi�ed, and can be used to calibrate the rate of gravitational wave sources. Most interestingly, the recoil induced by the anisotropic emission of gravitational waves in the �nal plunge of binaries leaves unusual imprints on their host galaxies. Finally, the lecture will describe new constraints on the contribution of primordial black holes to the dark matter.

Jan 16, 2020

Sommerfeld Theory Colloquium, Three-dimensional Einstein gravity has no local dynamical degree of freedom. Yet, it is far from being trivial when the cosmological constant is negative. (i) It admits black hole solutions. (ii) It easily allows for consistently interacting and tractable higher-spin extensions. (iii) It possesses remarkable asymptotic properties at infi�nity where an infi�nite-dimensional symmetry algebra emerges. These features make three-dimensional gravity a perfect "theoretical laboratory" in which to explore the conceptual issues related to (i), (ii) and (iii) in a simpler context. The talk will not only discuss three-dimensional gravity assuming no previous knowledge on the subject, but will also provide access to recent work where new results on points (i), (ii) and (iii) are developed.

Jan 16, 2020

Sommerfeld Theory Colloquium, In this seminar 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. These 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.

Jan 16, 2020

Sommerfeld Theory Colloquium, In the standard picture of a quantum phase transition, a single quantum critical point separates the phases at zero temperature. Here we show that the two-dimensional case is considerably more complex. Instead of the single point separating the antiferromagnet from the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterized by a superposition of d-wave superconductivity and a quadrupole density wave (QDW), which builds a checkerboard pattern with a period incommensurate with that of the original spin density wave. At moderate temperatures both orders co-exist over comparatively large distances but thermal fluctuations destroy the long-range order. Below a critical temperature the fluctuations are less essential and super- conductivity becomes stable. Applying a magnetic field destroys the superconductivity but establishes QDW. In addition to these phases we obtain also a charge density wave (CDW) arising as a result of interaction of electrons with superconducting fluctuations. This phase is possible when the superconductivity is destroyed by either thermal fluctuations or a magnetic field. The results of our theory can serve as explanation of recent experiments on cuprates performed with the help of STM, NMR, hard and resonant soft X-ray scattering, sound propagation, and other techniques.

Jan 16, 2020

Sommerfeld Theory Colloquium, Einstein is well known for his rejection of quantum mechanics in the form it emerged from the work of Heisenberg, Born and Schrodinger in 1926. Much less appreciated are the many seminal contributions he made to quantum theory prior to his �final scientifi�c verdict, that the theory was at best incomplete. In this talk I present an overview of Einsteins many conceptual breakthroughs and place them in historical context. I argue that Einstein, much more than Planck, introduced the concept of quantization of energy in atomic mechanics. Einstein proposed the photon, the fi�rst force-carrying particle discovered for a fundamental interaction, and put forward the notion of wave-particle duality, based on sound statistical arguments 14 years before De Broglies work. He was the fi�rst to recognize the intrinsic randomness in atomic processes, and introduced the notion of transition probabilities, embodied in the A and B coeffi�cients for atomic emission and absorption. He also preceded Born in suggesting the interpretation of wave fi�elds as probability densities for particles, photons, in the case of the electromagnetic �field. Finally, stimulated by Bose, he introduced the notion of indistinguishable particles in the quantum sense and derived the condensed phase of bosons, which is one of the fundamental states of matter at low temperatures. His work on quantum statistics in turn directly stimulated Schrodinger towards his discovery of the wave equation of quantum mechanics. It was only due to his rejection of the �final theory that he is not generally recognized as the most central �figure in this historic achievement of human civilization.

Jan 16, 2020

Sommerfeld Theory Colloquium, Since the emergence of information theory in the middle of the 20th century, this new scientific field has been deeply entangled with statistical physics, as attested from the beginning by the use of entropy to quantify information content. The talk will explore the major impact of these transdisciplinary exchanges, with a particular focus on the important new research topic called "compressed sensing". Starting from the observation that interesting signals can be compressed, and thus are sparse in some representation, compressed sensing aims at acquiring data directly in a compressed way, using then computational methods to reconstruct the original signal. It opens the way to faster, less destructive, and more e�ective signal acquisition, with possible applications in many branches of science, from magnetic resonance imaging to astronomy, tomography, or gene interaction network reconstruction. The talk will describe the spectacular progress that can be made using various statistical physics ideas, from spin glass theory to crystal nucleation.

Jan 16, 2020

Sommerfeld Theory Colloquium, With the discovery of the Higgs boson and the determination of its mass, the Standard Model is complete and its parameters are now known and over-constrained. I will review the status and future directions in precision electroweak physics both at high and low en- ergies. There is strong evidence that the Standard Model is correct at the level of quantum corrections, and that in the absence of major conspiracies, any new physics beyond it is either significantly heavier than the electroweak scale or very weakly coupled.

Jan 16, 2020

Sommerfeld Theory Colloquium, In this seminar, I shall describe recent work by our group on iden- tifying pockets of gas at high redshift that have undergone mini- mum processing through stars. The chemical composition of such gas still bears the imprints of the first few generations of stars that formed only a few hundred million years after the Big Bang, and thereby gives us clues to the physical properties of these still mys- terious objects which heralded the so-called ‘epoch of reionisation’. Near-pristine gas at high redshift is also the astrophysical environ- ment where the primordial abundance of deuterium can be measured most precisely. I will show how determinations of the cosmic den- sity of baryons from Big-Bang Nucleosynthesis and from the Cosmic Microwave Background have now reached comparable precision, in both cases of order of a few percent. The excellent agreement be- tween these two measures at widely different cosmic epochs places interesting limits on the existence of relativistic particles beyond the standard model of physics.

Jan 16, 2020

Sommerfeld Theory Colloquium, Remarkably, a strong candidate for Dark Matter exists within the Standard Model. Theoretical arguments suggest that QCD forces in the flavor-singlet sector may be strong enough that the H-dibaryon is a deeply-bound, compact state which is absolutely stable with a mass

Jan 16, 2020

Sommerfeld Theory Colloquium, I will describe the process of how to award Nobel Prizes and then give the history behind this year’s prize.

Jan 16, 2020

Sommerfeld Theory Colloquium, Tremendous progresses have been achieved in the last decade in real- ising and manipulating stable and controllable quantum systems, and these made possible to experimentally study fundamental questions posed in the early days of quantum mechanics. We shall theoretical discuss recent cavity QED experiments on non-demolition quantum measurements. While they nicely illustrate postulates of quantum mechanics and the possibility to implement efficient quantum state manipulations, these experiments pose a few questions such as: What does it mean to observe a progressive wave function collapse in real time? How to describe it? What do we learn from them? Their analysis will allow us one hand to link these experiments to basics notions of probability or information theory, and on the other hand to touch upon notions of quantum noise. As an illustration, we shall look at quantum systems in contact with a heat bath subject to quan- tum transitions between energy levels upon absorption or emission of energy quanta. Isolating the two indispensable mechanisms in com- petition, we shall describe the main physical features of thermally activated quantum jumps.

Jan 16, 2020

Sommerfeld Theory Colloquium, The problem of understanding how gravity fits together with other fundamental interactions of matter has been at the forefront of the- oretical research for many decades, leading to the rich framework of string theory and M-theory. In this framework, many fundamen- tal questions are being resolved, but many remain quite mysterious, suggesting that search for novel concepts may be well justified. I review the recent concept of multicritical gravity with Lifshitz-type anisotropic scaling, and its applications in areas ranging from par- ticle phenomenology beyond the standard model to non-relativistic versions of the holographic AdS/CFT correspondence.

Jan 16, 2020

Sommerfeld Theory Colloquium, Absolute temperature, that is the fundamental temperature scale in thermodynamics, is usually bound to be positive. Under special con- ditions, however, negative temperatures - where high-energy states are more occupied than low-energy states - are also possible. In this talk, I will present a negative temperature state for motional degrees of freedom: By tailoring the Bose-Hubbard Hamiltonian we exper- imentally created an attractively interacting ensemble of ultracold bosons, which is stable against collapse for arbitrary atom numbers. In this negative temperature state, the quasi-momentum distribu- tion develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Nega- tive temperatures imply negative pressures and open up new param- eter regimes for cold atoms, enabling fundamentally new many-body states and counterintuitive effects such as Carnot engines above unity efficiency. In addition, this system enabled us to study the dynam- ics of the phase transition from Mott insulator to superfluid and to experimentally investigate how fast phase coherence can spread.

Jan 16, 2020

Sommerfeld Theory Colloquium

Jan 16, 2020

Sommerfeld Theory Colloquium

Jan 16, 2020

Sommerfeld Theory Colloquium, A single potential impurity can drastically change a low-temperature transport of strongly interacting particles in one dimension - a sys- tem which is known as the Luttinger liquid. An arbitrary weak backscattering of fermions from the impurity totally destroys their zero-temperature current while even a very strong backscattering of bosons makes no impact on their flow. On the other hand, a more complicated impurity (like a quantum dot or a double-barrier struc- ture with a resonant level) can preserve an ideal resonant conductance of fermions, or conversely lead (in a different geometry) to an ideal (i.e. infinite) resonant resistance. I emphasize the role of a so-called duality in these transport effects. The duality was related to the integrability of the Luttinger liquid with an impurity. I will show that - surprisingly - duality survives the addition of a non-local and retarded interaction (like electron-phonon) which almost certainly destroys the integrability.

Jan 16, 2020

Sommerfeld Theory Colloquium, The restricted three body problem has an intriguing dynamics. Glo- bal surfaces of section are a tool to reduce the study of the dynamics on a three dimensional energy hypersurface to the study of an area preserving map of a two dimensional surface. The existence of such a global surface of section is far from obvious. So far mostly per- turbative methods have been used to prove its existence. However, recently developed global tools originating from Gromov-Witten the- ory combined with the fact that energy hypersurfaces are of contact type can be applied to produce new global surfaces of section. In this talk I explain the restricted three body problem, the notion of a global surface of section and the new results we can obtain by bringing holomorphic curves and the contact form into play.

Jan 16, 2020

Sommerfeld Theory Colloquium, Ultra-fast optical spectroscopy is a powerful tool for the observation of dynamical processes in several kind of materials. The basic time- resolved optical experiment is the so-called “pump-probe”: a first light pulse, the “pump”, resonantly triggers a photo-induced process. The subsequent system evolution can be monitored, for example, by the time–dependent transmission changes of a delayed “probe” pulse. The pump pulse photon energy, spectral width and peak intensity cre- ates a certain density of electron-hole pairs in a more or less localized region of space. After the creation of the initial carrier density the time evolution of the single-particle and many-particle excitations is now governed by a non-trivial interplay between electron-electron and electron-phonon scatterings. In this talk I will present a novel approach based on the merging of Non-Equilibrium Green’s func- tion theory and Density Functional Theory to investigate the carrier dynamics following a pump excitation. The case of bulk Silicon, a paradigmatic indirect gap semiconductor, is studied by using the Baym-Kadanoff equations. Both the electron-electron (e-e) and elec- tronphonon (e-p) self-energies are calculated fully Ab-Initio by using a semi-static GW approximation in the e-e case and a Fan self-energy in the e-p case. By using the generalized Baym-Kadanoff ansatz the two-time evolution is replaced by the only dynamics on the macro- scopic time axis. The enormous numerical difficulties connected with a real-time simulation of realistic systems is overcomed by using a completed collision approximation that further simplifies the mem- ory effects connected to the time evolution. The carrier dynamics is shown to reduce in such a way to have stringent connections to the well-known equilibrium electron-electron and electron-phonon self- energies. This link allows to use general arguments to motivate the relative balance between the e-e and e-p scattering channels on the basis of the carrier energies.

Jan 16, 2020

Sommerfeld Theory Colloquium, In Regensburg there exists a large Lattice-QCD group (SFB/TR-55) working in many fields, ranging from the development of energy efficient super-computers to specialized Lattice studies of SU(N) gauge theories with N>3 for matching to AdS/CFT predictions. The bulk of the work is focused on hadron phenomenology [generalized parton distributions (GPDs), distribution amplitudes (DAs), transverse momentum dependent parton distributions (TMDs)] and QCD thermodynamics, especially in a magnetic background field. (Like a chemical potential a B-field affects quark and gluon degrees of freedom differently thus allowing to investigate their connection. In contrast to the chemical potential, however, it is easy to implement on the lattice. The aim of the talk is to give an overview and to possibly identify topics of mutual interest.

Jan 16, 2020

Sommerfeld Theory Colloquium, In the next few years new data from ground and space will push the limit of cosmological observations to new frontiers. Combining CMB, weak lensing, redshift clustering and supernovae data, we will be able to constrain the properties of dark energy and its interactions from the background to the non-linear level. In this talk I discuss these methods and the future expected constraints on dark energy and modified gravity.

Jan 16, 2020

Sommerfeld Theory Colloquium, If systems characterized by slow (algebraic) relaxation are prepared in an out-of-equilibrium initial state, one can observe a "physical aging regime" in the ensuing approach to equilibrium that is governed by broken time translation invariance and non-trivial, often universal scaling laws. Dynamical systems near a critical point constitute prototypical and now well-understood examples. Indeed, measuring critical exponents in the intermediate aging rather than the asymptotic stationary temporal regime is now a standard numerical tool. In this talk, I will first apply these concepts to simple driven lattice gases that relax towards non-equilibrium stationary systems displaying generic scale invariance. The expected simple aging behavior in the two-time density auto-correlation function is verified through Monte Carlo simulations in one, two, and three dimensions. Next I shall address the continuous non-equilibrium phase transition in driven Ising lattice gases in two dimensions. Whereas the temporal scaling of the density auto-correlation function in the non-equilibrium steady state does not allow a precise measurement of the associated critical exponents, these can be accurately determined from the aging scaling of the two-time auto-correlations and the order parameter evolution following a quench to the critical point. In the second part of the talk, I will present numerical results for the non-equilibrium relaxation kinetics of interacting magnetic flux lines in disordered type-II superconductors at low temperatures and low magnetic fields, represented by means of a three-dimensional elastic line model. Investigating the vortex density and height auto-correlations as well as the flux line mean-square displacement allows us to carefully disentangle different relaxation mechanisms (e.g., vortex line fluctuations and positional relaxation), and to assess their relative impact on the kinetics of dilute vortex matter at low temperatures. We observe the emergence of genuine glassy dynamics, caused by the competing effects of vortex pinning and long-range repulsive interactions between the flux lines. We contrast the effects of random point-like pinning centers and correlated columnar defects. We also compare data from Monte Carlo simulations with results from Langevin molecular dynamics.

Jan 16, 2020

Sommerfeld Theory Colloquium, The decay of a false vacuum of unbroken B-L, the difference of baryon and lepton number, is an intriguing and testable mechanism to gen- erate the initial conditions of the hot early universe. If B-L is broken at the grand unification scale, the false vacuum phase yields hybrid inflation, ending in tachyonic preheating. The dynamics of the B-L breaking Higgs field and thermal processes produce an abundance of heavy neutrinos whose decays generate entropy, baryon asymmetry and dark matter.

Jan 16, 2020

Sommerfeld Theory Colloquium, This lecture is aimed at a rather broad audience including under- graduates. It will discuss scaling arguments, what they can achieve (Kolmogorov’s -5/3 law, etc.) and what they miss (fractals, Etc).

Jan 16, 2020

Sommerfeld Theory Colloquium, Lorentz non-invariance in quantum field theory is reconsidered as well as its interplay with gravity, string theory, diffeomorphism invariance and changing reference frames. We clarify these issues, and argue that Lorentz violation is always an environmental effect. We provide a holographic view of the breaking, and its connection to Horava- Lifshitz type of gravitational theories. We also argue that dissipation is always a signal of Lorentz violation. We provide calculations of dissipation at strong coupling.

Jan 16, 2020

Sommerfeld Theory Colloquium, The physics of elementary particles is governed by symmetries. A particular symmetry stands out: the one between left and right, called parity. Its breaking in beta decay created a bombshell more than fifty years ago, and ultimately led to the creation of the Standard Model of particle interactions, whose final crowning confirmation is to be provided by the Large Hadron Collider (LHC) at CERN. The Standard Model is based on the premise of parity being broken al- ways, at all energies. I argue, on the contrary, that in nature left-right symmetry is fundamental, and that at high energies of the LHC one could actually see its restoration in full glory. I show how this would probe the nature of neutrino, through the spectacular signatures of lepton number violation.

Jan 16, 2020

Sommerfeld Theory Colloquium, Reflection positivity is a useful tool in statistical mechanics and con- densed matter physics. A recent application is to the determination of the possible distortions of the hexagonal graphene lattice. Other applications, such as to potential theory, the flux-phase problem, Peierls instability and stripe formation, will also be given.

Jan 16, 2020

Sommerfeld Theory Colloquium, Much of condensed matter physics is concerned with understanding how different kinds of order emerge from interactions between a large number of simple constituents. In ordered phases such as crystals, magnets, and superfluids, the order is understood through ”symme- try breaking”: in a crystal, for example, the continuous symmetries of space under rotations and translations are not reflected in the ground state. A major discovery of the 1980s was that electrons confined to two dimensions and in a strong magnetic field exhibit a completely different, ”topological” type of order that underlies the quantum Hall effect. In the past few years, we have learned that topological order also occurs in some three-dimensional materials, dubbed ”topological in- sulators”, in zero magnetic field. Spin-orbit coupling, an intrinsic property of all solids, drives the formation of the topological state. This talk will explain what topological order means, how topologi- cal were predicted and discovered, and how they realize the ”axion electrodynamics” studied by particle physicists in the 1980s. Some possible applications of these new materials are discussed in closing.

Jan 16, 2020

Sommerfeld Theory Colloquium, Conformal interfaces are the long-distance limits of domain walls. They play a role in condensed-matter physics, and could be also rel- evant for theories of gravity. I will review some recently-developed techniques for analyzing their properties, and will discuss their pos- sible applications.

Jan 16, 2020

Sommerfeld Theory Colloquium, In collisions of ultra-intense laser-pulse with relativistic electrons as well as in ultra relativistic heavy ion collisions at RHIC and at LHC it is possible to probe critical acceleration a=mc^3/hbar. The behavior of a particle undergoing critical acceleration challenges the limits of the current understanding of basic interactions: little is known about this physics frontier; both classical and quantum physics will need further development in order to be able to address this newly accessible area of physics. The problem of critical acceleration is closely connected to strong field particle production, Mach's Principle, Unruh and Hawking radiation.

Jan 16, 2020

Sommerfeld Theory Colloquium, In this lecture I propose to discuss some issues concerning the founda- tions of quantum mechanics and its interplay with space-time physics. I start by clarifying the distinction between ’realistic theories’ and ’probabilistic theories’ of Nature and sketch how the latter can often be viewed as ’deformations’ of the former. I then briefly recall some of the intriguing features of atomistic Quantum Mechanics, which belongs to the second class of theories. I attempt to describe, in con- ceptual terms, what it is that Quantum Mechanics predicts about Nature when appropriate experiments are done. I try to sketch some implications of this discussion for our views of space and time. I will conclude by sketching some recent results on the ’effective quantum dynamics’ of Open Systems, in particular on ’Quantum Brownian Motion’.

Jan 16, 2020

Quantum Criticality, High Tc Superconductivity and the AdS/CFT Correspondence of String Theory , The central mystery in quantum matter is the general nature of mat- ter formed from fermions. The methods of many body quantum physics fail and one can only rely on the phenomenological Fermi- liquid and BCS theories. However, in heavy fermion systems and cuprates one deals with non Fermi-liquid quantum critical metals, and to understand the superconductivity one needs to understand these normal states first. Remarkably, it might well be that the mathematics of string theory is capable of describing such states of fermion matter. The AdS/CFT correspondence translates this problem into an equivalent general-relativity problem involving the propagation of classical fields in an Anti-de-Sitter space-time with a black hole in its center. This development started with the demon- stration that AdS/CFT predicts correctly the low viscosity of the quark-gluon plasma of the Brookhaven heavy ion collider. In 2007 it was realized that it could have relevance to high Tc superconductors but only last year the focus shifted to the way AdS/CFT processes fermions, creating much excitement: it appears that both emergent heavy Fermi-liquids and non Fermi-liquids can be gravitationally encoded, as well as holographic superconductors having suggestive traits in common with the real life high Tc variety.

Jan 16, 2020

Imaging Astrophysical Turbulent Convection and Dynamo Action in the Sun, The Sun is a most remarkable object: it is filled with vibrantly evolv- ing magnetic fields, well-mixed hierarchically-arranged turbulent con- vective cells, and also poorly-mixed sunspots that persist with im- punity. Solar variability has direct consequences for the earth and space weather, an important reason to develop an appreciation for the physics of the solar cycle (dynamo). Direct observation of the solar subsurface is impossible due to the high degree of optical scattering by the partially ionized plasma that inhabits the near-surface layers of the Sun. The deepest part of the Sun visible to us, known as the photosphere (also the solar surface), appears as a roiling, turbulent, radiative, magnetized, convecting plasma. At first glance, it would seem therefore that subtle ques- tions relating to the subsurface constitution of the Sun seem com- pletely unanswerable and the interior properties unknowable. How- ever, analogous to geoseismology, a great deal can be gleaned about the internal structure and dynamics of the Sun by carefully observ- ing and analyzing the surface wavefield. This has been made possible over the last few decades through the development and application of techniques of helioseismology. In this talk I will outline some of the major results in this area over the last two decades and discuss some recent developments pertaining to the properties of turbulence in the deep-convection zone of the Sun.

Jan 16, 2020

Scenarios of Baryogenesis and "Experimental" Search for Cosmic Antimatter , In connection with the existing and forthcoming missions dedicated to search for cosmic antimatter, the models of baryogenesis leading to an efficient creation of astronomically large antimatter objects are reviewed. It is argued that such objects may be abundant in the universe and even in the Galaxy. Their observational signatures and the prospects for discovery are discussed.

Jan 16, 2020

Testing Strings at the LHC?, A theory with such a mathematical beauty cannot be wrong: this was one of the main arguments in favor of string theory, which unifies all known physical theories of fundamental interactions in a single coherent description of the universe. But no one has ever observed strings, not even indirectly, neither the space of extra dimensions where they live. However, there are good reasons to believe that the ``hidden" dimensions of string theory may be much larger than what we thought in the past and they become within experimental reach in the near future, together with the strings themselves. In my talk, I will give an elementary introduction of this framework and describe the main experimental predictions.

Jan 16, 2020

Anderson Localization: Looking Forward, Localization of the eigenfunctions of quantum particles in a random potential was discovered by P.W. Anderson more than 50 years ago. In spite of its respectable maturity and rather intensive theoretical and experimental studies this field is by far not exhausted. Anderson localization was originally discovered in connection with spin relaxation and charge transport in disordered conductors. Later this phenomenon was observed for light, microwaves, sound, and more recently for cold atoms. Moreover, it became clear that the domain of applicability of the concept of localization is much broader. For example, it provides an adequate framework for discussing the transition between integrable and chaotic behavior in quantum systems. We will discuss current understanding of the Anderson localization and its manifestation in different physical situations.

Jan 16, 2020

The Massive Black Hole and Nuclear Star Cluster of the Milky Way, Evidence has been accumulating for several decades that many galaxies harbor central mass concentrations that may be in the form of black holes with masses between a few million to a few billion time the mass of the Sun. I will discuss measurements over the last two decades, employing adaptive optics imaging and spectroscopy on large ground-based telescopes that prove the existence of such a massive black hole in the Center of our Milky Way, beyond any reasonable doubt. These data also provide key insights into its properties and environment. Future interferometric studies of the Galactic Center black hole promise to be able to test gravity in its strong field limit. I will also briefly discuss the cosmological evolution of massive black holes.

Jan 16, 2020

Testing fundamental physics with CMB, The Planck mission was launched successfully in May last year. I will give a summary of the scientific aims of the Planck mission and a brief overview of its current status. I will also place the Planck mission in context with ground and suborbital CMB experiments and other probes of early universe cosmology.

Jan 16, 2020

Gravitational Waves from Coalescing Binary Black Holes: Theoretical and Experimental Challenges, A network of ground-based interferometric gravitational wave detectors (LIGO/VIRGO/GEO/...) is currently taking data near its planned sensitivity. Coalescing black hole binaries are among the most promising, and most exciting, gravitational wave sources for these detectors. The talk will review the theoretical and experimental challenges that must be met in order to successfully detect gravitational waves from coalescing black hole binaries, and to be able to reliably measure the physical parameters of the source (masses, spins, ...).

Jan 16, 2020

Testing the Dark Energy Paradigm and Beyond, The next generation of surveys, e.g. the Dark Energy Survey, PanSTARRS, LSST, Euclid and others, aim to study the nature of Dark Energy and alternatives. The talk will discuss how the Dark Energy paradigm evolved over the past 20 years, and the cosmic probes which will help us to test it. In particular the surveys rely on accurate of photometric redshifts for the determination of cosmological quantities such as Dark Energy parameters and neutrino masses. The talk will describe photometric redshift methods and their impact on analysing galaxy clustering and weak lensing data and on the derived cosmological parameters.

Jan 16, 2020

The Inverse Ising Problem, The "Inverse Ising Problem" refers to finding the parameters (the Jij's and the hi's) in an Ising model given the first and second moments (the magnetizations mi and the correlation functions cij). This is of considerable interest in machine learning and data analysis whenever the data set and the number of variables is large, but the values taken by the variables can be taken to be "high" and "low". The maximum entropy distributions with given first and second moments then has the Ising form where the hi's and Jij's are Lagrange parameters. Several perturbative methods to solve the inverse Ising problem approximately have been proposed in the last few years. I will give a survey of the situation, with a focus on what we have know about the applicability of these methods to data such as gene expression and recordings from many neurons, where an underlying exact description is surely not of the Ising form. This is work with John Hertz and Yasser Roudi (Frontiers Comp Neuroscience, 2009) and work in progress.

Jan 16, 2020

The search for a perfect fluid: is string-theory relevant for ultracold atoms?, Based on a calculation of the shear viscosity η and the entropy density s of certain strongly coupled field theories via the AdS/CFT-correspondence, Kovtun, Son and Starinets conjectured that their ratio is bounded below by the universal number ħ/4πkB, a bound that appears to hold for all existing fluids. The substances that come closest to being perfect in the sense of a minimum value of η/s are the quark gluon plasma and ultracold atoms at infinite scattering length, the so called unitary Fermi gas. The talk provides an introduction to the origin and interpretation of this bound from a simple physics perspective. Moreover, quantitative results for η/s are presented in the normal phase of the unitary Fermi gas. They are consistent with the Kovtun, Son and Starinets bound and are also in good agreement with experiments.

Jan 16, 2020

Z' models and the early LHC, Are there plausible extensions of the Standard Model that could lead to early discoveries at the LHC? To address this general question on a concrete example, I will consider a class of minimal models with an extra massive neutral gauge boson Z'. I will first review different theoretical motivations for extending the SM gauge group with an extra U(1) factor, possibly broken near the TeV scale. I will then discuss the interplay between the bounds from electroweak precision tests and direct searches at the Tevatron, to identify the early LHC discovery potential. I will finally comment on the peculiar features of models where the Z' couples non-universally to lepton flavors and of string models with intersecting or magnetized branes.

Jan 16, 2020

A simple proof of the Kochen-Specker theorem on the problem of hidden variables, In this talk I present a simple derivation of an old result of Kochen and Specker, which is apparently unrelated to the famous work of Bell on hidden variables, but is presumably equally important. Kochen and Specker showed in 1967 that quantum mechanics cannot be embedded into a classical stochastic theory, provided the quantum theoretical probability distributions are repro- duced and one additional highly desirable property is satisfied. This showed in a striking manner what were the difficulties in implementing the Einstein programme of a `complete' version of quantum mechanics.

Jan 16, 2020

Maximal Supersymmetry, Light-Front Dynamics and Exceptional Symmetries, In this talk I will describe the light-front formulation of maxi- mally supersymmetric theories. This is a formalism where only the physical degrees of freedom is used. This means that the SuperPoincaré invariance is non-linearly realized, a fact we use to build the models. In this formalism the N=4 Yang-Mills and the N=8 Supergravity are treated in a very similar fashion and the close relationship between them is obvious. I will also show that the exceptional symmetry E_7(7) is very naturally connec- ted to the N=8 SuperPoincaré algebra. This formalism is an al- ternative to the ordinary covariant formalisms and is very use- ful to investigate the UV properties of these models.

Jan 16, 2020

Gedanken World without Higgs Fields, To illuminate how electroweak symmetry breaking shapes the physical world, we investigate what the world would be like in the absence of electroweak symmetry breaking at the usual scale, whether by the conventional Higgs mechanism or by any of its alternatives, including dynamical symmetry breaking and higher-dimensional formulations. Many interesting charac- teristics of the models stem from the fact that the effective strength of the weak interactions is much closer to that of the residual strong interactions than in the real world. The Higgs- free models not only provide informative contrasts to the real world, but also lead us to consider intriguing issues in the application of field theory to the real world.

Jan 16, 2020

How to detect Majorana fermions in topological insulators, Majorana fermions are spatially localized superpositions of electron and hole excitations in the middle of a superconducting energy gap. These unusual particles have been predicted to occur at the interface between a magnetic and superconducting electrode, in contact with a topological insulator (such as a Bi crystal or a HgTe quantum well). A single qubit can be encoded nonlocally in a pair of spatially separated Majorana fermions. Such Majorana qubits are in demand as building blocks of a topological quantum computer, but direct experimental tests of the nonlocality remain elusive. We propose a method to probe the nonlocality by means of crossed Andreev reflection, which is the injection of an electron into one bound state followed by the emission of a hole by the other bound state. The resulting splitting of a Cooper pair by the Majorana qubit produces a pair of excitations that are maximally entangled in the momentum (rather than the spin) degree of freedom, and might be used as "flying qubits" in quantum information processing.