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How can non-Abelian fields avoid the fate of mass generation in two dimensions, where the Goldstone mechanism does not take effect? Motivated by this old question, we tell the tale of two 2D nonlinear sigma models at topological angle equal to pi: the O(3) model as an effective field theory of critical antiferromagnetic quantum spin chains and the Pruisken model for the integer quantum Hall transition. We argue that both models are infrared massless by a mechanism not appreciated so far in the literature.

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Martin Zirnbauer, University of Cologne
Seminar Room 0.03, ETP
Contact: Simon Trebst