Sommerfeld Theory Colloquium (ASC)

Latest Episodes:

Harmony of Scattering Amplitudes and Form Factors May 22, 2014

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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.

Cascade of phase transitions near Quantum Critical Point May 08, 2014

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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.

Status of the Electroweak Standard Model Jan 16, 2014

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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.

Near-Pristine Gas at High Redshifts: First Stars, Big-Bang Nucleosynthesis, and Limits on Dark Radiation Dec 06, 2013

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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.

Ultrahigh Energy Cosmic Rays: probe of extreme particle physics Dec 04, 2013

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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

The History behind the Brout-Englert-Mechanism and the Standard Model Nov 28, 2013

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

Real Time Imaging of Thermal and Quantum Fluctuations Nov 21, 2013

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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.

Quantum Gravity with Anisotropic Scaling and the Multicritical Universe Oct 24, 2013

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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.

Negative absolute temperatures for mobile particles Jul 11, 2013

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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.

Exact, Broken and Approximate Symmetries Jul 04, 2013

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