arXiv subjects

Andrew B. Royston

Publications and source records attributed to Andrew B. Royston.

At least 19 recordsLinked to original sources

arXiv2025

Framework for the Forced Soliton Equation: Regularization, Numerical Solutions, and Perturbation Theory

The forced soliton equation is the starting point for semiclassical computations with solitons away from the small momentum transfer regime. This paper develops necessary analytical and numerical tools for analyzing solutions to the forced soliton equation in the context of two-dimensional models with kinks. Results include a finite degree of freedom regularization of soliton sector physics based on periodic and anti-periodic lattice models, a detailed analysis of numerical solutions, and the development of perturbation theory in the soliton momentum transfer to mass ratio Delta P/M. Numerical solutions at large transfer, greater or similar to the soliton mass, are capable of exhibiting, in a smooth and controlled fashion, extreme phenomena such as soliton-antisoliton pair creation and superluminal collective coordinate velocities, which we investigate.

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arXiv2022

Cut-Off Kinks

We answer the question: If a vacuum sector Hamiltonian is regularized by an energy cutoff, how is the one-kink sector Hamiltonian regularized? We find that it is not regularized by an energy cutoff, indeed normal modes of all energies are present in the kink Hamiltonian, but rather the decomposition of the field into normal mode operators yields coefficients which lie on a constrained surface that forces them to become small for energies above the cutoff. This explains the old observation that an energy cutoff of the kink Hamiltonian leads to an incorrect one-loop kink mass. To arrive at our conclusion, we impose that the regularized kink sector Hamiltonian is unitarily equivalent to the regularized vacuum sector Hamiltonian. This condition implies that the two regularized Hamiltonians have the same spectrum and so guarantees that the kink Hamiltonian yields the correct kink mass.

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arXiv2022

Supersymmetry of the D3/D5 Defect Field Theory

Four-dimensional N=4 super Yang-Mills, with a codimension-one defect breaking half of the supersymmetry, arises as the field theory description of the D3/D5 intersection in the holographic limit. This is one of the earliest, most extensively studied, and commonly used systems in holography. In this note we give the full R-symmetry-covariant supersymmetry variations for this system. We also provide the supercurrents and compute the algebra of the corresponding supercharges, obtaining the full set of central charges. We show that magnetically charged finite-energy field configurations preserving half of the supersymmetry are solutions to a new form of the extended Bogomolny equations, in which the defect fields play the role of jumping data for the Nahm-like part of the equations. In the appendices, we explain the connection between our results and the superspace-based formulations in the literature.

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arXiv2021

Simulating Magnetic Monopole-Defect Dynamics

We present simulations of one magnetic monopole interacting with multiple magnetic singularities. Three-dimensional plots of the energy density are constructed from explicit solutions to the Bogomolny equation obtained by Blair, Cherkis, and Durcan. Animations follow trajectories derived from collective coordinate mechanics on the multi-centered Taub--NUT monopole moduli space. We supplement our numerical results with a complete analytic treatment of the single-defect case.

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arXiv2020

The Forced Soliton Equation and Semiclassical Soliton Form Factors

We show that the leading semiclassical behavior of soliton form factors at arbitrary momentum transfer is controlled by solutions to a new wave-like integro-differential equation that describes solitons undergoing acceleration. We work in the context of two-dimensional linear sigma models with kink solitons for concreteness, but our methods are purely semiclassical and generalizable.

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arXiv2020

Revisiting Soliton Contributions to Perturbative Amplitudes

It is often said that soliton contributions to perturbative processes in QFT are exponentially suppressed by a form factor. We provide a new derivation of this form factor for a class of scalar theories with generic soliton moduli. The derivation treats the soliton momenta relativistically and is valid at leading order in momentum transfer. The computation reduces to a matrix element in the quantum mechanics on the soliton moduli space. We investigate the conditions under which the latter leads to suppression. Extending this framework to instanton-solitons in five-dimensional Yang-Mills theory leaves open the possibility that such contributions will not be suppressed.

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arXiv2020

Accelerating Solitons

We present the saddle-point approximation for the effective Hamiltonian of the quantum kink in two-dimensional linear sigma models to all orders in the time-derivative expansion. We show how the effective Hamiltonian can be used to obtain semiclassical soliton form factors, valid at momentum transfers of order the soliton mass. Explicit results, however, hinge on finding an explicit solution to a new wave-like partial differential equation, with a time-dependent velocity and a forcing term that depend on the solution. In the limit of small momentum transfer, the effective Hamiltonian reduces to the expected form, namely H = (P^2 + M^2)^(1/2), where M is the one-loop corrected soliton mass, and soliton form factors are given in terms of Fourier transforms of the corresponding classical profiles.

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arXiv2019

Nonabelian Probes In Holography

We find the range of parameters for which the open string physics on probe Dq-branes in the near-horizon geometry of Dp-branes decouples from gravity, and is well-approximated by a (q+1)-dimensional supersymmetric Yang-Mills-Higgs theory on a rigid curved spacetime. We study the vacua of these theories, which include moduli spaces of instantons, monopoles, and vortices. This intricate structure is made possible through couplings to the background Ramond-Ramond flux. The probe brane theories we study provide holographic descriptions of defects in dual field theories.

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arXiv2019

Wall Crossing from Dirac Zeromodes

We explore the physics of two-body decay of BPS states using semiclassical analysis to construct explicit solutions that illustrate the main features of wall crossing, for both ordinary and framed BPS states. In particular we recover the primitive wall-crossing formula from an asymptotic analysis of certain Dirac-type operators on monopole moduli spaces. Along the way we give an asymptotic metric for the moduli space of singular monopoles, analogous to the Gibbons-Manton and Lee-Weinberg-Yi metrics for the moduli space of smooth monopoles, and we find evidence for the existence of stable non-BPS boundstates. Our discussion applies to four-dimensional N = 2 super-Yang-Mills theories with general gauge group and general 't Hooft defects.

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arXiv2017

Holography for field theory solitons

We extend a well-known D-brane construction of the AdS/dCFT correspondence to non-abelian defects. We focus on the bulk side of the correspondence and show that there exists a regime of parameters in which the low-energy description consists of two approximately decoupled sectors. The two sectors are gravity in the ambient spacetime, and a six-dimensional supersymmetric Yang-Mills theory. The Yang-Mills theory is defined on a rigid AdS_4 x S^2 background and admits sixteen supersymmetries. We also consider a one-parameter deformation that gives rise to a family of Yang-Mills theories on asymptotically AdS_4 x S^2 spacetimes, which are invariant under eight supersymmetries. With future holographic applications in mind, we analyze the vacuum structure and perturbative spectrum of the Yang-Mills theory on AdS_4 x S^2, as well as systems of BPS equations for finite-energy solitons. Finally, we demonstrate that the classical Yang-Mills theory has a consistent truncation on the two-sphere, resulting in maximally supersymmetric Yang-Mills on AdS_4.

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arXiv2016

Some tree-level string amplitudes in the NSR formalism

We calculate tree level scattering amplitudes for open strings using the NSR formalism. We present a streamlined symmetry-based and pedagogical approach to the computations, which we first develop by checking two-, three-, and four-point functions involving bosons and fermions. We calculate the five-point amplitude for massless gluons and find agreement with an earlier result by Brandt, Machado and Medina. We then compute the five-point amplitudes involving two and four fermions respectively, the general form of which has not been previously obtained in the NSR formalism. The results nicely confirm expectations from the supersymmetric $F^4$ effective action. Finally we use the prescription of Kawai, Lewellen and Tye (KLT) to compute the amplitudes for the closed string sector.

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arXiv2016

L^2-Kernels Of Dirac-Type Operators On Monopole Moduli Spaces

We state some mathematical predictions concerning the kernels of Dirac-type operators on moduli spaces of (singular) monopoles in R^3. These predictions follow from the semiclassical interpretation of physical results on spaces of (framed) BPS states in d=4, N=2 gauge theories.

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arXiv2016

Semiclassical framed BPS states

We provide a semiclassical description of framed BPS states in four-dimensional N = 2 super Yang-Mills theories probed by 't Hooft defects, in terms of a supersymmetric quantum mechanics on the moduli space of singular monopoles. Framed BPS states, like their ordinary counterparts in the theory without defects, are associated with the L^2 kernel of certain Dirac operators on moduli space, or equivalently with the L^2 cohomology of related Dolbeault operators. The Dirac/Dolbeault operators depend on two Cartan-valued Higgs vevs. We conjecture a map between these vevs and the Seiberg-Witten special coordinates, consistent with a one-loop analysis and checked in examples. The map incorporates all perturbative and nonperturbative corrections that are relevant for the semiclassical construction of BPS states, over a suitably defined weak coupling regime of the Coulomb branch. We use this map to translate wall crossing formulae and the no-exotics theorem to statements about the Dirac/Dolbeault operators. The no-exotics theorem, concerning the absence of nontrivial SU(2)_R representations in the BPS spectrum, implies that the kernel of the Dirac operator is chiral, and further translates into a statement that all L^2 cohomology of the Dolbeault operator is concentrated in the middle degree. Wall crossing formulae lead to detailed predictions for where the Dirac operators fail to be Fredholm and how their kernels jump. We explore these predictions in nontrivial examples. This paper explains the background and arguments behind the results announced in a short accompanying note.

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arXiv2016

Parameter counting for singular monopoles on R^3

We compute the dimension of the moduli space of gauge-inequivalent solutions to the Bogomolny equation on R^3 with prescribed singularities corresponding to the insertion of a finite number of 't Hooft defects. We do this by generalizing the methods of C. Callias and E. Weinberg to the case of R^3 with a finite set of points removed. For a special class of Cartan-valued backgrounds we go further and construct an explicit basis of L^2-normalizable zero-modes. Finally we exhibit and study a two-parameter family of spherically symmetric singular monopoles, using the dimension formula to provide a physical interpretation of these configurations. This paper is the first in a series of three on singular monopoles, where we also explore the role they play in the contexts of intersecting D-brane systems and four-dimensional N=2 super Yang-Mills theories.

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arXiv2016

Brane bending and monopole moduli

We study intersecting brane systems that realize a class of singular monopole configurations in four-dimensional Yang-Mills-Higgs theory. Singular monopoles are solutions to the Bogomolny equation on R^3 with a prescribed number of singularities corresponding to the insertion of 't Hooft defects. We use the brane construction to motivate a recent conjecture on the conditions for which the moduli space of solutions is non-empty. We also show how branes provide physical intuition for various aspects of the dimension formula derived in {arXiv:1404.5616}, including the contribution to the dimension from the defects and its invariance under Weyl reflections of the 't Hooft charges. Along the way we uncover and illustrate new dynamical phenomena for the brane systems, including a description of smooth monopole extraction and bubbling from 't Hooft defects.

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arXiv2014

Instanton-soliton loops in 5D super-Yang-Mills

Soliton contributions to perturbative processes in QFT are controlled by a form factor, which depends on the soliton size. We provide a demonstration of this fact in a class of scalar theories with generic moduli spaces. We then argue that for instanton-solitons in 5D super-Yang-Mills theory the analogous form factor does not lead to faster-than-any-power suppression in the perturbative coupling. We also discuss the implications of such contributions for the UV behavior of maximally supersymmetric Yang-Mills in 5D and its relation to the (2,0) CFT in 6D. This is a contribution to the proceedings of the "String Math 2013'" conference and is a condensed version of results appearing in 1404.0016 and 1403.5017.

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arXiv2014

Scalar Soliton Quantization with Generic Moduli

We canonically quantize multi-component scalar field theories in the presence of solitons. This extends results of Tomboulis to general soliton moduli spaces. We derive the quantum Hamiltonian, discuss reparameterization invariance and explicitly show how, in the semiclassical approximation, the dynamics of the full theory reduce to quantum mechanics on the soliton moduli space. We emphasize the difference between the semiclassical approximation and a truncation of the dynamical variables to moduli. Both procedures produce quantum mechanics on moduli space, but the two Hamiltonians are generically different.

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arXiv2012

Successes and Failures of a More Comprehensive Hard Wall AdS/QCD

We explore corrections to the "hard wall" gravity dual of QCD with two-form tensor fields. These fields correspond to a dimension three quark bilinear in QCD, which generates states with 1(--) and 1(+-) quantum numbers, or omega/rho-like and h1/b1-like mesons, respectively. We include new interaction terms, which render the model complete up to dimension six. We find that breaking chiral symmetry induces modifications to the spectrum, by mixing the vector current and the new two-tensor operator. This breaks the degeneracy of the 1(--) and 1(+-) spectra. We also compute the (b1->omega + pi) decay rate and D/S ratio, and the couplings of excited rho states to two pions. We then perform a comprehensive new fit to QCD data. We obtain a value for the magnetic susceptibility of the QCD quark condensate which is consistent with phenomenological estimates. These results allow us to evaluate the success of dimension counting and UV matching in this model.

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