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1. Codimension-2 defects and higher symmetries in (3+1)D topological phases

   https://doi.org/10.21468/SciPostPhys.14.4.065  

   Barkeshli, Maissam; Chen, Yu-An; Huang, Sheng-Jie; Kobayashi, Ryohei; Tantivasadakarn, Nathanan; Zhu, Guanyu (January 2023, SciPost Physics)

   (3+1)D topological phases of matter can host a broad class of non-trivial topological defects of codimension-1, 2, and 3, of which the well-known point charges and flux loops are special cases. The complete algebraic structure of these defects defines a higher category, and can be viewed as an emergent higher symmetry. This plays a crucial role both in the classification of phases of matter and the possible fault-tolerant logical operations in topological quantum error-correcting codes. In this paper, we study several examples of such higher codimension defects from distinct perspectives. We mainly study a class of invertible codimension-2 topological defects, which we refer to as twist strings. We provide a number of general constructions for twist strings, in terms of gauging lower dimensional invertible phases, layer constructions, and condensation defects. We study some special examples in the context of \mathbb{Z}_2 ℤ 2 gauge theory with fermionic charges, in \mathbb{Z}_2 \times \mathbb{Z}_2 ℤ 2 × ℤ 2 gauge theory with bosonic charges, and also in non-Abelian discrete gauge theories based on dihedral ( D_n D n ) and alternating ( A_6 A 6 ) groups. The intersection between twist strings and Abelian flux loops sources Abelian point charges, which defines an H^4 H 4 cohomology class that characterizes part of an underlying 3-group symmetry of the topological order. The equations involving background gauge fields for the 3-group symmetry have been explicitly written down for various cases. We also study examples of twist strings interacting with non-Abelian flux loops (defining part of a non-invertible higher symmetry), examples of non-invertible codimension-2 defects, and examples of the interplay of codimension-2 defects with codimension-1 defects. We also find an example of geometric, not fully topological, twist strings in (3+1)D A_6 A 6 gauge theory. 

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2. S-duality in $$ T\overline{T} $$-deformed CFT

   https://doi.org/10.1007/JHEP05(2023)140  

   Benjamin, Nathan; Collier, Scott; Kruthoff, Jorrit; Verlinde, Herman; Zhang, Mengyang (May 2023, Journal of High Energy Physics)

   A bstract $$ T\overline{T} $$ T T ¯ deformed conformal field theories can be reformulated as worldsheet theories of non-critical strings. We use this correspondence to compute and study the $$ T\overline{T} $$ T T ¯ deformed partition sum of a symmetric product CFT. We find that it takes the form of a partition sum of a second quantized string theory with a worldsheet given by the product of the seed CFT and a gaussian sigma model with the two-torus as target space. We show that deformed symmetric product theory admits a natural UV completion that exhibits a strong weak coupling ℤ 2 duality that interchanges the momentum and winding numbers and maps the $$ T\overline{T} $$ T T ¯ -coupling λ to its inverse 1/ λ . The ℤ 2 duality is part of a full O(2, 2, ℤ)-duality group that includes a PSL(2, ℤ) acting on the complexified $$ T\overline{T} $$ T T ¯ coupling. The duality symmetry eliminates the appearance of complex energies at strong coupling for all seed CFTs with central charge c ≤ 6. 

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3. Non-planar corrections in ABJM theory from quantum M2 branes

   https://doi.org/10.1007/JHEP11(2024)056  

   Giombi, Simone; Kurlyand, Stefan A; Tseytlin, Arkady A (November 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> The quantization of semiclassical strings in AdS spacetimes yields predictions for the strong-coupling behaviour of the scaling dimensions of the corresponding operators in the planar limit of the dual gauge theory. Finding non-planar corrections requires computing string loops (corresponding to torus and higher genus surfaces), which is a challenging task. It turns out that in the case of theU_k(N) ×U_−k(N) ABJM theory there is an alternative approach: one may semiclassically quantize M2 branes in AdS₄×S⁷/ℤ_kwhich are wrapped around the 11d circle of radius 1/k=λ/N. Such M2 branes are the M-theory generalization of the strings in AdS × CP³. In this work, we show that by expanding in large M2 brane tensionT₂~$$ \sqrt{kN} $$ $\sqrt{\mathrm{kN}}$for fixedk, followed by an expansion in largek, we can predict the largeλasymptotics of the non-planar corrections to the dimensions of the dual ABJM operators. As a specific example, we consider the M2 brane configuration that generalizes the long folded string with large spin in AdS₄, and compute the 1-loop correction to its energy. This calculation allows us to determine non-planar corrections to the universal scaling function or cusp anomalous dimension. We extend our analysis to the semiclassical M2 branes that generalize the “short” and “long” circular strings with two equal angular momenta in CP³. The “short” M2 brane corresponds to a dual operator whose dimension at strong coupling scales as ∆ ∼λ^1/4+ …, and we derive the leading non-planar correction to it. 

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4. The Weak Gravity Conjecture and axion strings

   https://doi.org/10.1007/JHEP11(2021)004  

   Heidenreich, Ben; Reece, Matthew; Rudelius, Tom (November 2021, Journal of High Energy Physics)

   A bstract Strong (sublattice or tower) formulations of the Weak Gravity Conjecture (WGC) imply that, if a weakly coupled gauge theory exists, a tower of charged particles drives the theory to strong coupling at an ultraviolet scale well below the Planck scale. This tower can consist of low-spin states, as in Kaluza-Klein theory, or high-spin states, as with weakly-coupled strings. We provide a suggestive bottom-up argument based on the mild p -form WGC that, for any gauge theory coupled to a fundamental axion through a θF ∧ F term, the tower is a stringy one. The charge-carrying string states at or below the WGC scale gM Pl are simply axion strings for θ , with charged modes arising from anomaly inflow. Kaluza-Klein theories evade this conclusion and postpone the appearance of high-spin states to higher energies because they lack a θF ∧ F term. For abelian Kaluza-Klein theories, modified arguments based on additional abelian groups that interact with the Kaluza-Klein gauge group sometimes pinpoint a mass scale for charged strings. These arguments reinforce the Emergent String and Distant Axionic String Conjectures. We emphasize the unproven assumptions and weak points of the arguments, which provide interesting targets for further work. In particular, a sharp characterization of when gauge fields admit θF ∧ F couplings and when they do not would be immensely useful for particle phenomenology and for clarifying the implications of the Weak Gravity Conjecture. 

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5. Notes on two-dimensional pure supersymmetric gauge theories

   https://doi.org/10.1007/JHEP04(2021)261  

   Gu, Wei; Sharpe, Eric; Zou, Hao (April 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract In this note we study IR limits of pure two-dimensional supersymmetric gauge theories with semisimple non-simply-connected gauge groups including SU( k )/ℤ k , SO(2 k )/ℤ 2 , Sp(2 k )/ℤ 2 , E 6 /ℤ 3 , and E 7 /ℤ 2 for various discrete theta angles, both directly in the gauge theory and also in nonabelian mirrors, extending a classification begun in previous work. We find in each case that there are supersymmetric vacua for precisely one value of the discrete theta angle, and no supersymmetric vacua for other values, hence supersymmetry is broken in the IR for most discrete theta angles. Furthermore, for the one distinguished value of the discrete theta angle for which supersymmetry is unbroken, the theory has as many twisted chiral multiplet degrees of freedom in the IR as the rank. We take this opportunity to further develop the technology of nonabelian mirrors to discuss how the mirror to a G gauge theory differs from the mirror to a G / K gauge theory for K a subgroup of the center of G . In particular, the discrete theta angles in these cases are considerably more intricate than those of the pure gauge theories studied in previous papers, so we discuss the realization of these more complex discrete theta angles in the mirror construction. We find that discrete theta angles, both in the original gauge theory and their mirrors, are intimately related to the description of centers of universal covering groups as quotients of weight lattices by root sublattices. We perform numerous consistency checks, comparing results against basic group-theoretic relations as well as with decomposition, which describes how two-dimensional theories with one-form symmetries (such as pure gauge theories with nontrivial centers) decompose into disjoint unions, in this case of pure gauge theories with quotiented gauge groups and discrete theta angles. 

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