Sommerfeld Theory Colloquium (ASC)

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Lattice gauge theory insights Jun 10, 2026

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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.

Two-Dimensional Melting Transition: New Algorithms, New Insights Jun 10, 2026

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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.

Precision Measurements of the Cosmic Microwave Background Jun 10, 2026

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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.

The development of twistor geometry for the description of fundamental physics Jun 10, 2026

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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.

Topological Superconductivity and Unconventional pairing in Oxide Interfaces Jun 10, 2026

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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.

Phenomenological and Cosmological Implications of String Theory Models Jun 10, 2026

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Sommerfeld Theory Colloquium

Cosmology with Type Ia Supernovae: where do we stand today? Jun 10, 2026

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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.

Higher Spin --- CFT duality Jun 10, 2026

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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.

Extreme Light and Quantum Fields Jun 10, 2026

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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.

Turbulence without Linear Instability Jun 10, 2026

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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.

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