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Relative Anomalies in (2+1)D Symmetry Enriched Topological States
by Maissam Barkeshli, Meng Cheng
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Submission summary
Authors (as registered SciPost users):  Maissam Barkeshli · Meng Cheng 
Submission information  

Preprint Link:  https://arxiv.org/abs/1906.10691v2 (pdf) 
Date accepted:  20191217 
Date submitted:  20191211 01:00 
Submitted by:  Barkeshli, Maissam 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
Certain patterns of symmetry fractionalization in topologically ordered phases of matter are anomalous, in the sense that they can only occur at the surface of a higher dimensional symmetryprotected topological (SPT) state. An important question is to determine how to compute this anomaly, which means determining which SPT hosts a given symmetryenriched topological order at its surface. While special cases are known, a general method to compute the anomaly has so far been lacking. In this paper we propose a general method to compute relative anomalies between different symmetry fractionalization classes of a given (2+1)D topological order. This method applies to all types of symmetry actions, including anyonpermuting symmetries and general spacetime reflection symmetries. We demonstrate compatibility of the relative anomaly formula with previous results for diagnosing anomalies for $\mathbb{Z}_2^{\bf T}$ spacetime reflection symmetry (e.g. where timereversal squares to the identity) and mixed anomalies for $U(1) \times \mathbb{Z}_2^{\bf T}$ and $U(1) \rtimes \mathbb{Z}_2^{\bf T}$ symmetries. We also study a number of additional examples, including cases where spacetime reflection symmetries are intertwined in nontrivial ways with unitary symmetries, such as $\mathbb{Z}_4^{\bf T}$ and mixed anomalies for $\mathbb{Z}_2 \times \mathbb{Z}_2^{\bf T}$ symmetry, and unitary $\mathbb{Z}_2 \times \mathbb{Z}_2$ symmetry with nontrivial anyon permutations.
Author comments upon resubmission
Regarding the referee's first question, indeed this was discussed in the paper by Etingof, Nikshych, and Ostrik (see https://arxiv.org/pdf/0909.3140.pdf, Proposition 8.12 ). The formulas derived by ENO apply to the case where the symmetry actions do not permute the anyons. The generalization to the case where anyons are permuted has not been proven, as far as we are aware.
Regarding the referee's second comment, we have included the discussion of the Abelian case summarized by the referee at the end of Sec. VI of our revised version.
Finally, we have made the additional grammatical changes suggested by the referee.
List of changes
1. Added a short discussion on the relation between the relative anomaly formula for Z2^T symmetry for Abelian anyons and the arf invariant discussed in the paper of Lee and Tachikawa (2018) .
2. Fixed a typo in Table I and clarified the theories it applies to.
3. Fixed some minor typos and grammatical issues mentioned by the referee.
Published as SciPost Phys. 8, 028 (2020)