Bilal Hawashin: The Nordic-walking mechanism and its explanation of deconfined pseudocriticality from Wess-Zumino-Witten theory

Many phase transitions can be understood as the spontaneous breaking of a global symmetry of the microscopic system. This is well captured within the framework initially developed by Landau, in which the extend of broken symmetry is quantified by an order parameter field. However, it is well established that not all possible phase transitions fall into the Landau paradigm of phase transitions. An exciting class of such non-Landau transitions are deconfined quantum critical points which exhibit emergent fractional excitations and gauge fields at criticality. The effective field theory describing the putative deconfined critical point of an SU(2) spin model is a (2+1)-dimensional Wess-Zumino-Witten theory with target manifold S^4. I will discuss our study of this model using the non-perturbative functional renormalisation group. Concretely, we construct an unconventional approach based on higher-order regulator insertions. We demonstrate that the phenomenology of the deconfined quantum phase transitions can be best understood in terms of a novel dynamical mechanism, termed Nordic walking. Nordic walking denotes a renormalisation group flow arising from a beta function that is flat over a range of couplings, giving rise to a logarithmic flow that is faster than the well-known walking behaviour. The Nordic-walking mechanism can thus explain weak first-order transitions, but may also play a role in high-energy physics, where it could solve hierarchy problems.

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