Sommerfeld Theory Colloquium (ASC) Michael Haack

 Wissenschaft

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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Thinking positively: the numerical quantum mechanical bootstrap
I will describe recent developments on the (numerical) computation of energy levels of various systems by the quantum mechanical bootstrap. The main way the bootstrap works is by using constraints that arise from positive matrices. Part of the goal is to turn the bootstrap problem into a problem that can be solved by semidefinite programming methods. I will describe how this method leads to solutions of the spectrum of various systems and will describe some additional applications of this way of solving problems to the study of quantum spin chains.

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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. 
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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 higherdimensional 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.

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Arnold Sommerfeld Theory Colloquium
In this talk I will describe the lightfront 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 nonlinearly realized, a fact we use to build the models. In this formalism the N=4 YangMills 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.

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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.

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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 nonuniversally to lepton flavors and of string models with intersecting or magnetized branes.