Sommerfeld Theory Colloquium (ASC)

Michael Haack
  • 4.5 (2)
  • SCIENCE

The Arnold Sommerfeld Center for Theoretical Physics organizes regular colloquia about topics of current interest in the field of theoretical physics.

  1. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    Majorana fermions are spatially localized superpositions of electron and hole excitations in the middle of a superconducting energy gap. These unusual particles have been predicted to occur at the interface between a magnetic and superconducting electrode, in contact with a topological insulator (such as a Bi crystal or a HgTe quantum well). A single qubit can be encoded nonlocally in a pair of spatially separated Majorana fermions. Such Majorana qubits are in demand as building blocks of a topological quantum computer, but direct experimental tests of the nonlocality remain elusive. We propose a method to probe the nonlocality by means of crossed Andreev reflection, which is the injection of an electron into one bound state followed by the emission of a hole by the other bound state. The resulting splitting of a Cooper pair by the Majorana qubit produces a pair of excitations that are maximally entangled in the momentum (rather than the spin) degree of freedom, and might be used as "flying qubits" in quantum information processing.

    1h 9m

  2. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    To illuminate how electroweak symmetry breaking shapes the physical world, we investigate what the world would be like in the absence of electroweak symmetry breaking at the usual scale, whether by the conventional Higgs mechanism or by any of its alternatives, including dynamical symmetry breaking and higher-dimensional formulations. Many interesting charac- teristics of the models stem from the fact that the effective strength of the weak interactions is much closer to that of the residual strong interactions than in the real world. The Higgs- free models not only provide informative contrasts to the real world, but also lead us to consider intriguing issues in the application of field theory to the real world.

    1h 17m

  3. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    In this talk I will describe the light-front formulation of maxi- mally supersymmetric theories. This is a formalism where only the physical degrees of freedom is used. This means that the SuperPoincaré invariance is non-linearly realized, a fact we use to build the models. In this formalism the N=4 Yang-Mills and the N=8 Supergravity are treated in a very similar fashion and the close relationship between them is obvious. I will also show that the exceptional symmetry E_7(7) is very naturally connec- ted to the N=8 SuperPoincaré algebra. This formalism is an al- ternative to the ordinary covariant formalisms and is very use- ful to investigate the UV properties of these models.

    1h 10m

  4. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    In this talk I present a simple derivation of an old result of Kochen and Specker, which is apparently unrelated to the famous work of Bell on hidden variables, but is presumably equally important. Kochen and Specker showed in 1967 that quantum mechanics cannot be embedded into a classical stochastic theory, provided the quantum theoretical probability distributions are repro- duced and one additional highly desirable property is satisfied. This showed in a striking manner what were the difficulties in implementing the Einstein programme of a `complete' version of quantum mechanics.

    1h 7m

  5. FEB 4 · VIDEO

    Sommerfeld Theory Colloquium

    Are there plausible extensions of the Standard Model that could lead to early discoveries at the LHC? To address this general question on a concrete example, I will consider a class of minimal models with an extra massive neutral gauge boson Z'. I will first review different theoretical motivations for extending the SM gauge group with an extra U(1) factor, possibly broken near the TeV scale. I will then discuss the interplay between the bounds from electroweak precision tests and direct searches at the Tevatron, to identify the early LHC discovery potential. I will finally comment on the peculiar features of models where the Z' couples non-universally to lepton flavors and of string models with intersecting or magnetized branes.

    1h 16m

  6. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    Based on a calculation of the shear viscosity η and the entropy density s of certain strongly coupled field theories via the AdS/CFT-correspondence, Kovtun, Son and Starinets conjectured that their ratio is bounded below by the universal number ħ/4πkB, a bound that appears to hold for all existing fluids. The substances that come closest to being perfect in the sense of a minimum value of η/s are the quark gluon plasma and ultracold atoms at infinite scattering length, the so called unitary Fermi gas. The talk provides an introduction to the origin and interpretation of this bound from a simple physics perspective. Moreover, quantitative results for η/s are presented in the normal phase of the unitary Fermi gas. They are consistent with the Kovtun, Son and Starinets bound and are also in good agreement with experiments.

    1h 14m

  7. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    The "Inverse Ising Problem" refers to finding the parameters (the Jij's and the hi's) in an Ising model given the first and second moments (the magnetizations mi and the correlation functions cij). This is of considerable interest in machine learning and data analysis whenever the data set and the number of variables is large, but the values taken by the variables can be taken to be "high" and "low". The maximum entropy distributions with given first and second moments then has the Ising form where the hi's and Jij's are Lagrange parameters. Several perturbative methods to solve the inverse Ising problem approximately have been proposed in the last few years. I will give a survey of the situation, with a focus on what we have know about the applicability of these methods to data such as gene expression and recordings from many neurons, where an underlying exact description is surely not of the Ising form. This is work with John Hertz and Yasser Roudi (Frontiers Comp Neuroscience, 2009) and work in progress.

    1h 4m

  8. FEB 4 · VIDEO

    Arnold Sommerfeld Theory Colloquium

    The next generation of surveys, e.g. the Dark Energy Survey, PanSTARRS, LSST, Euclid and others, aim to study the nature of Dark Energy and alternatives. The talk will discuss how the Dark Energy paradigm evolved over the past 20 years, and the cosmic probes which will help us to test it. In particular the surveys rely on accurate of photometric redshifts for the determination of cosmological quantities such as Dark Energy parameters and neutrino masses. The talk will describe photometric redshift methods and their impact on analysing galaxy clustering and weak lensing data and on the derived cosmological parameters.

    1h 10m
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The Arnold Sommerfeld Center for Theoretical Physics organizes regular colloquia about topics of current interest in the field of theoretical physics.

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