Sommerfeld Theory Colloquium (ASC)

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.

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.

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.

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.

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.

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.

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.

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.

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.