arXiv subjects

T. Daniel Brennan

Publications and source records attributed to T. Daniel Brennan.

At least 19 recordsLinked to original sources

arXiv2026

Line Defect Quantum Numbers & Anomalies

We explore the connection between the global symmetry quantum numbers of line defects and 't Hooft anomalies. Relative to local (point) operators, line defects may transform projectively under both internal and spacetime symmetries. This phenomenon is known as symmetry fractionalization, and in general it signals the presence of certain discrete 't Hooft anomalies. We describe this in detail in the context of free Maxwell theory in four dimensions. This understanding allows us to deduce the 't Hooft anomalies of non-Abelian gauge theories with renormalization group flows into Maxwell theory by analyzing the fractional quantum numbers of dynamical magnetic monopoles. We illustrate this method in $SU(2)$ gauge theories with matter fermions in diverse representations of the gauge group. For adjoint matter, we uncover a mixed anomaly involving the 0-form and 1-form symmetries, extending previous results. For $SU(2)$ QCD with fundamental fermions, the 't Hooft anomaly for the 0-form symmetries that is encoded by the fractionalization patterns of lines in the Maxwell phase is a consequence of the familiar perturbative (triangle) anomaly.

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arXiv2026

Generalized Families of QFTs

RG flows and IR phases of QFTs can be constrained by generalized symmetries and their anomalies. Broken symmetries act on the space of coupling constants of families of theories, and can also have IR-constraining family anomalies. We generalize family anomaly considerations to cases of broken generalized/categorical symmetries, including higher-group and non-invertible symmetries. We consider the anomaly inflow and SymTFTs of such generalized families of QFTs, and their implications for RG flows and constraints on the IR phases. As examples, we apply family anomalies to study the IR phases of $4d$ QCD-like theories deformed by irrelevant, multi-fermion interactions.

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arXiv2025

Consequences of Symmetry Fractionalization without 1-Form Global Symmetries

We study the fractionalization of 0-form global symmetries on line operators in theories without 1-form global symmetries. The projective transformation properties of line operators are renormalization group invariant, and we derive constraints which are similar to the consequences of exact 1-form symmetries. For instance, symmetry fractionalization can lead to exact selection rules for line operators in twisted sectors, and in theories with 't Hooft anomalies involving the fractionalization class, these selection rules can further imply that certain twisted sectors have exact finite-volume vacuum degeneracies. Along the way, we define topological operators on open codimension-1 manifolds, which we call `disk operators', that provide a convenient way of encoding the projective action of 0-form symmetries on lines. In addition, we discuss the possible ways symmetry fractionalization can be matched along renormalization group flows.

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arXiv2025

Revisiting Scattering Enhancement from the Aharonov-Bohm Effect

We revisit the problem of a charged particle scattering off of an Aharonov-Bohm cosmic string. A classic computation gave an infinite total scattering cross section, leading to a Callan-Rubakov-like enhancement which can have important implications on baryon number asymmetry in the early universe. However, unlike the Callan-Rubakov effect, the Aharonov-Bohm interaction is topological and thus it is surprising that it leads to such a dramatic dynamical effect for single particle scattering. We reexamine this old problem through the modern lens of generalized global symmetries by embedding Aharanov-Bohm strings in a discrete gauge theory. We show that the scattering cross section is suppressed by the core size and there is thus no Callan-Rubakov-like enhancement.

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arXiv2024

Monopole Catalyzed Baryogenesis with a $θ$ angle

Monopoles are generally expected in Grand Unified Theories (GUTs) where they can catalyze baryon decay at an unsuppressed rate by the Callan-Rubakov effect. For the first time, we show this catalysis effect can generate the observed baryon asymmetry at GeV scale temperatures. We study the minimal SU(5) GUT model and demonstrate that monopoles-fermion scattering with a $CP$-violating $θ$-term leads to realistic baryogenesis even when $θ\lesssim 10^{-10}$ is below the neutron EDM bound, potentially detectable in the future measurements. Our calculation also shows that to generate the observed baryon asymmetry, the abundance of the monopoles is below the current experiential bounds.

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arXiv2024

A New Solution to the Callan Rubakov Effect

In this paper we study the scattering of massive fermions off of smooth, spherically symmetric monopoles in $4d$ $SU(2)$ gauge theory. We propose a complete physical picture of the monopole-fermion interaction which encompasses all angular momentum modes. We show that as an in-going fermion scatters off a monopole, it excites trapped $W$-bosons in the monopole core by a version of the Witten effect so that the monopole can accrue charge and transform into a dyon at parametrically low energies. The imparted electric charge is then protected from decay by an emergent $\mathbb{Z}_N$ generalized global symmetry, creating a stable dyon. At sufficiently low energies, the scattered fermion can be trapped by the dyon's electrostatic potential, forming a bound state, which can decay into spherically symmetric fermion modes subject to the preserved $\mathbb{Z}_N$ global symmetry. We propose that monopole-fermion scattering can be described in this way without needing to add ``new'' states to the Hilbert space, thereby eliminating a long standing confusion in the Callan Rubakov effect.

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arXiv2024

A SymTFT for Continuous Symmetries

Symmetry is a powerful tool for studying dynamics in QFT: it provides selection rules, constrains RG flows, and often simplifies analysis. Currently, our understanding is that the most general form of symmetry is described by categorical symmetries which can be realized via Symmetry TQFTs or ``SymTFTs." In this paper, we show how the framework of the SymTFT, which is understood for discrete symmetries (i.e. finite categorical symmetries), can be generalized to continuous symmetries. In addition to demonstrating how $U(1)$ global symmetries can be incorporated into the paradigm of the SymTFT, we apply our formalism to study cubic $U(1)$ anomalies in $4d$ QFTs, describe the $\mathbb{Q}/\mathbb{Z}$ non-invertible chiral symmetry in $4d$ theories, and conjecture the SymTFT for general continuous $G^{(0)}$ global symmetries.

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arXiv2024

Callan-Rubakov Effect and Higher Charge Monopoles

In this paper we study the interaction between magnetic monopoles and massless fermions. In the low energy limit, the monopole's magnetic field polarizes the fermions into purely in-going and out-going modes. Unitarity requires that the UV fermion-monopole interaction leads to non-trivial IR boundary conditions that relate the in-going to out-going modes. These non-trivial boundary conditions lead to what is known as the Callan-Rubakov effect. Here we derive the effective boundary condition by explicitly integrating out the UV degrees of freedom for the general class of spherically symmetric $SU(N)$ monopoles coupled to massless fermions of arbitrary representation. We then show that the boundary conditions preserve continuous symmetries without ABJ-type anomalies. As an application we explicitly derive the boundary conditions for the stable, spherically symmetric monopoles associated to the $SU(5)$ Georgi-Glashow model and comment on the relation to baryon number violation.

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arXiv2024

Anomalies of 4d $Spin_G$ Theories

We consider 't Hooft anomalies of four-dimensional gauge theories whose fermion matter content admits $Spin_G(4)$ generalized spin structure, with $G$ either gauged or a global symmetry. We discuss methods to directly compute $w_2\cup w_3$ 't Hooft anomalies involving Stiefel-Whitney classes of gauge and flavor symmetry bundles that such theories can have on non-spin manifolds, e.g. $M_4=\mathbb{CP}^2$. Such anomalies have been discussed for $SU(2)$ gauge theory with adjoint fermions, where they were shown to give an effect that was originally found in the Donaldson-Witten topological twist of ${\cal N}=2$ SYM theory. We directly compute these anomalies for a variety of theories, including general $G$ gauge theories with adjoint fermions, $SU(2)$ gauge theory with fermions in general representations, and $Spin(N)$ gauge theories with fundamental matter. We discuss aspects of matching these and other 't Hooft anomalies in the IR phase where global symmetries are spontaneously broken, in particular for general $G_{\rm gauge}$ theory with $N_f$ adjoint Weyl fermions. For example, in the case of $N_f=2$ we discuss anomaly matching in the IR phase consisting of $h^\vee _{G_{\rm gauge}}$ copies of a $\mathbb{CP}^1$ non-linear sigma model, including for the $w_2w_3$ anomalies when formulated with $Spin_{SU(2)_{\rm global}}(4)$ structure.

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arXiv2024

Constraints on Symmetry Preserving Gapped Phases from Coupling Constant Anomalies

In this note, we will characterize constraints on the possible IR phases of a given QFT by anomalies in the space of coupling constants. We will give conditions under which a coupling constant anomaly cannot be matched by a continuous family of symmetry preserving gapped phases, in which case the theory is either gapless, or exhibits spontaneous symmetry breaking or a phase transition. We additionally demonstrate examples of theories with coupling constant anomalies which can be matched by a family of symmetry preserving gapped phases without a phase transition and comment on the interpretation of our results for the spontaneous breaking of "$(-1)$-form global symmetries."

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arXiv2024

Anomaly Enforced Gaplessness for Background Flux Anomalies and Symmetry Fractionalization

Anomalous symmetries are known to strongly constrain the possible IR behavior along any renormalization group (RG) flow. Recently, the extension of the notion of symmetry in QFT has provided new types of anomalies with a corresponding new class of constraints on RG flows. In this paper, we derive the constraints imposed on RG flows from anomalies that can only be activated in the presence of specific background fluxes even though they do not necessarily correspond to a symmetry. We show that such anomalies can only be matched by gapped theories that exhibit either spontaneous symmetry breaking or symmetry fractionalization. In addition, we exhibit previously unstudied examples of these flux background anomalies that arise in $4d$ QCD and $4d$ SUSY QCD.

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arXiv2024

Coupling a Cosmic String to a TQFT

A common framework of particle physics consists of two sectors of particles, such as the Standard Model and a dark sector, with some interaction between them. In this work, we initiate the study of a qualitatively different setup in which one of the sectors is a topological quantum field theory (TQFT). Instead of particles, the physics of a TQFT only manifests itself in non-trivial spacetime topologies. Topological defects provide a natural place to investigate such effects. In particular, we consider two possible ways in which axionic cosmic strings can interact with a Zn TQFT. One of them, by extending the structure of the axion coupling, leads to specific predictions for the localized degrees of freedom on the cosmic string, which can in turn effect their evolution and leave observable signals. The second approach, by gauging a discrete subgroup of the axionic shift symmetry, leads to dramatic changes in the string spectrum. We stress that the scenario considered here should be regarded as a plausible way for new physics to arise since it can be the low energy effective field theory for quite generic scenarios at high energies. To demonstrate this point and further illustrate the physical implications, we constructed such UV completions for both of the cases of couplings to TQFTs. The detailed prediction for observable signals of such scenarios needs further investigation. At the same time, our results demonstrate that there are rich new phenomena in this scenario.

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arXiv2024

Anomaly Enforced Gaplessness and Symmetry Fractionalization for $Spin_G$ Symmetries

Symmetries and their anomalies give strong constraints on renormalization group (RG) flows of quantum field theories. Recently, the identification of a theory's global symmetries with its topological sector has provided additional constraints on RG flows to symmetry preserving gapped phases due to mathematical results in category and topological quantum field theory. In this paper, we derive constraints on RG flows from $\mathbb{Z}_2$-valued pure- and mixed-gravitational anomalies that can only be activated on non-spin manifolds. We show that such anomalies cannot be matched by a unitary, symmetry preserving gapped phase without symmetry fractionalization. In particular, we discuss examples that commonly arise in $4d$ gauge theories with fermions.

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arXiv2023

Introduction to Generalized Global Symmetries in QFT and Particle Physics

Generalized symmetries (also known as categorical symmetries) is a newly developing technique for studying quantum field theories. It has given us new insights into the structure of QFT and many new powerful tools that can be applied to the study of particle phenomenology. In these notes we give an exposition to the topic of generalized/categorical symmetries for high energy phenomenologists although the topics covered may be useful to the broader physics community. Here we describe generalized symmetries without the use of category theory and pay particular attention to the introduction of discrete symmetries and their gauging.

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arXiv2021

Index-like Theorem for Massless Fermions in Spherically Symmetric Monopole Backgrounds

In this paper we study massless fermions coupled to spherically symmetric $SU(N)$ monopoles without Yukawa couplings between the Higgs and fermion fields. The corresponding Dirac operator is not Fredholm and the associated eigenfunctions are not $L^2$-normalizable. Here we derive a formula for the dimension of the plane-wave normalizable kernel of such a Dirac operator for fermions of any representation of $SU(N)$ in the presence of any spherically symmetric monopole background. Notably, our results also apply to fermions coupled to monopoles that preserve non-abelian gauge symmetry.

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arXiv2021

Axions, Higher-Groups, and Emergent Symmetry

Axions, periodic scalar fields coupled to gauge fields through the instanton density, have a rich variety of higher-form global symmetries. These include a two-form global symmetry, which measures the charge of axion strings. As we review, these symmetries typically combine into a higher-group, a kind of non-abelian structure where symmetries that act on operators of different dimensions, such as points, lines, and strings, are mixed. We use this structure to derive model independent constraints on renormalization group flows that realize theories of axions at long distances. These give universal inequalities on the energy scales where various infrared symmetries emerge. For example, we show that in any UV completion of axion-Yang-Mills, the energy scale at which axion strings can decay is always larger than the mass scale of charged particles.

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arXiv2020

Wrapped Fivebranes Redux

The near-horizon region of Neveu-Schwarz fivebranes provides interesting examples of gauge/gravity duality. We revisit the structure of wrapped and/or intersecting fivebranes using the tools of null-gauged WZW models in worldsheet string theory, revealing the effective geometry of the fivebrane throat in a variety of examples. Variant gaugings yield linear dilaton fivebrane throats with AdS3 caps, providing a wealth of information about the near-BPS structure of the corresponding spacetime CFT duals.

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arXiv2020

Defining the $T \overline{T}$ Deformation on $\mathrm{AdS}_2$

We show that the $T \overline{T}$ deformation of two-dimensional quantum field theory on $\mathrm{AdS}_2$ is well-defined and solvable at the quantum level. Flow equations for the energy spectrum and partition function are derived in analogy with the flat space case. As a non-trivial check, we perturbatively compute the deformed energy spectrum for the case of a free scalar field. We analyze the high energy density of states of the deformed theory and find a Hagedorn growth of states.

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