Sommerfeld Theory Colloquium (ASC)

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.

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.

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.

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.

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.

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.

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.

Sommerfeld Theory Colloquium

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.

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.