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Remarks on Berry Connection in QFT, Anomalies, and Applications

by Mykola Dedushenko

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Submission summary

Authors (as registered SciPost users): Mykola Dedushenko
Submission information
Preprint Link: https://arxiv.org/abs/2211.15680v2  (pdf)
Date submitted: 2023-04-22 09:40
Submitted by: Dedushenko, Mykola
Submitted to: SciPost Physics
Ontological classification
Academic field: Physics
Specialties:
  • High-Energy Physics - Theory
  • Mathematical Physics
Approach: Theoretical

Abstract

Berry connection has been recently generalized to higher-dimensional QFT, where it can be thought of as a topological term in the effective action for background couplings. Via the inflow, this term corresponds to the boundary anomaly in the space of couplings, another notion recently introduced in the literature. In this note we address the question of whether the old-fashioned Berry connection (for time-dependent couplings) still makes sense in a QFT on $\Sigma^{(d)}\times \mathbb{R}$, where $\Sigma^{(d)}$ is a $d$-dimensional compact space and $\mathbb{R}$ is time. Compactness of $\Sigma^{(d)}$ relieves us of the IR divergences, so we only have to address the UV issues. We describe a number of cases when the Berry connection is well defined (which includes the $tt^*$ equations), and when it is not. We also mention a relation to the boundary anomalies and boundary states on the Euclidean $\Sigma^{(d)} \times \mathbb{R}_{\geq 0}$. We then work out the examples of a free 3D Dirac fermion and a 3D $\mathcal{N}=2$ chiral multiplet. Finally, we consider 3D theories on $\mathbb{T}^2\times \mathbb{R}$, where the space $\mathbb{T}^2$ is a two-torus, and apply our machinery to clarify some aspects of the relation between 3D SUSY vacua and elliptic cohomology. We also comment on the generalization to higher genus.

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Submission & Refereeing History


Reports on this Submission

Anonymous Report 2 on 2023-8-2 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:2211.15680v2, delivered 2023-08-02, doi: 10.21468/SciPost.Report.7594

Report

This work studied the question whether the conventional Berry connection in quantum mechanics is well-defined when we compactify a higher-dimensional QFT on a compact space down to a quantum mechanics. It addressed the question by classifying possible counterterms that can potentially spoil the well-defineness of the Berry connection. Then it discussed the applications of Berry connections in various examples.

Overall, it is a solid work that clarifies the use of Berry connections in higher-dimensional QFTs and points out interesting relations between Berry connections and various old results in the literature. The referee recommends the publication of this manuscript.

Requested changes

One minor comment:

1. A citation in the first paragraph on page 18 is missing.

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Report 1 by Lev Spodyneiko on 2023-7-10 (Invited Report)

  • Cite as: Lev Spodyneiko, Report on arXiv:2211.15680v2, delivered 2023-07-10, doi: 10.21468/SciPost.Report.7469

Report

The paper discussed the definition and applications of Berry connection in the framework of QFT. The author thoroughly studies complications and ambiguities related to it. The discussion smoothly evolves from a general QFT to more specific examples of supersymmetric theories. The paper provides a novel and synergetic link between different research areas and thus satisfies the publication criteria. I have some comments/suggestions/questions:

1. The IR divergences are claimed to be fixed by considering the theory on a compact space $\Sigma$. However, the effective action (say of the example in Sec. 3) does not scale with the volume of $\Sigma$. This is counter-intuitive because one naively would expect that it should scale with the number of degrees of freedom.

2. A recent paper (https://arxiv.org/abs/2305.06399v1) discussed an interpretation of a higher Thouless pump in 2+1D. They have shown that it gives the Berry connection of a mode localized on fluxon insertion. Namely, the Berry curvature of the 2+1 D system is infinite as well as the Berry curvature of the same system with fluxon insertion. However, their difference is finite and well-defined. The discussion of Sec 2 and 3 of the present paper seem to be similar because Berry curvature couples only to the holonomy of the fields on $\Sigma$. I think it is a good idea to discuss the relationship. (The mentioned paper was published after the present one, so I can't insist on it.)

3. Points 1 and 2 above, make me think that the author actually discusses higher Berry curvatures. Namely, I would call "old-fashioned" Berry curvature the one which scales with the volume. If it does not, I would expect that it is actually the Berry curvature of a single mode, or more precisely the difference of Berry curvature of the whole system with and without the mode.

4. I think it is a good idea to explicitly discuss how different issues raised in Sec 2 affect the simple example of Sec 3. For example, how the counterterm of eq. (2.6) appears in the analysis of Sec 3 when $F\ne0$? As far as I understand the other counterterm (2.9) is discussed in the last paragraph of Sec 3.1. If it is, then the author should clarify this by explicit reference to this equation.

And some minor corrections:

5. There is an empty reference [] in the first paragraph of Sec 2.4.

6. $B^{(p)}_i$ in eq. (3.48) is not defined.

Requested changes

Please address 1,4,5,6 from above, and I would appreciate the author's thoughts on 2,3.

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