Sommerfeld Theory Colloquium (ASC)

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, 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.

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, 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.

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.

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.

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.

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.