Bernd Schroers | Heriot-Watt University (original) (raw)
Papers by Bernd Schroers
Physical Review Letters, 2002
Many two-dimensional physical systems have symmetries which are mathematically described by quant... more Many two-dimensional physical systems have symmetries which are mathematically described by quantum groups (quasi-triangular Hopf algebras). In this letter we introduce the concept of a spontaneously broken Hopf symmetry and show that it provides an effective tool for analysing a wide variety of phases exhibiting many distinct confinement phenomena.
Physics Letters B, 1995
The scale invariance of the O(3) sigma model can be broken by gauging a U (1) subgroup of the O(3... more The scale invariance of the O(3) sigma model can be broken by gauging a U (1) subgroup of the O(3) symmetry and including a Maxwell term for the gauge field in the Lagrangian. Adding also a suitable potential one obtains a field theory of Bogomol'nyi type with topological solitons. These solitons are stable against rescaling and carry magnetic flux which can take arbitrary values in some finite interval. The soliton mass is independent of the flux, but the soliton size depends on it. However, dynamically changing the flux requires infinite energy, so the flux, and hence the soliton size, remains constant during time evolution.
Inspired by soliton models, we propose a description of static particles in terms of Riemannian 4... more Inspired by soliton models, we propose a description of static particles in terms of Riemannian 4-manifolds with self-dual Weyl tensor. For electrically charged particles, the 4-manifolds are non-compact and asymptotically fibred by circles over physical 3-space. This is akin to the Kaluza-Klein description of electromagnetism, except that we exchange the roles of magnetic and electric fields, and only assume the bundle structure asymptotically, away from the core of the particle in question. We identify the Chern class of the circle bundle at infinity with minus the electric charge and the signature of the 4-manifold with the baryon number. Electrically neutral particles are described by compact 4-manifolds. We illustrate our approach by studying the Taub-NUT manifold as a model for the electron, the Atiyah-Hitchin manifold as a model for the proton, CP^2 with the Fubini-Study metric as a model for the neutron, and S^4 with its standard metric as a model for the neutrino.
European Physical Journal C, 1994
We study the intermediate-and long-range forces between moving and spinning Skyrmions, employing ... more We study the intermediate-and long-range forces between moving and spinning Skyrmions, employing two different methods. One uses a relativised product ansatz for the Skyrme fields, the other models Skyrmions as triplets of scalar dipoles. The methods lead to the same finite-dimensional Lagrangian dynamical system which may be interpreted as a point-particle approximation to Skyrmion dynamics. We discuss in detail the dynamics in the so-called attractive channel and the interaction between well-separated and rapidly spinning Skyrmions, and point out the resemblance between the latter and the one-pion exchange potential in nuclear physics.
Nuclear Physics B, 1998
Magnetic monopoles in Yang-Mills-Higgs theory with a non-abelian unbroken gauge group are classif... more Magnetic monopoles in Yang-Mills-Higgs theory with a non-abelian unbroken gauge group are classified by holomorphic charges in addition to the topological charges familiar from the abelian case. As a result the moduli spaces of monopoles of given topological charge are stratified according to the holomorphic charges. Here the physical consequences of the stratification are explored in the case where the gauge group SU (3) is broken to U (2). The description due to A. Dancer of the moduli space of charge two monopoles is reviewed and interpreted physically in terms of non-abelian magnetic dipole moments. Semi-classical quantisation leads to dyonic states which are labelled by a magnetic charge and a representation of the subgroup of U (2) which leaves the magnetic charge invariant (centraliser subgroup). A key result of this paper is that these states fall into representations of the semi-direct product U (2) ⋉ R 4 . The combination rules (Clebsch-Gordan coefficients) of dyonic states can thus be deduced. Electric-magnetic duality properties of the theory are discussed in the light of our results, and supersymmetric dyonic BPS states which fill the SL(2, Z)-orbit of the basic massive W -bosons are found.
We explore in detail the role in euclidean 3d quantum gravity of quantum Born reciprocity or `sem... more We explore in detail the role in euclidean 3d quantum gravity of quantum Born reciprocity or `semidualisation'. The latter is an algebraic operation defined using quantum group methods that interchanges position and momentum. Using this we are able to clarify the structural relationships between the effective non-commutative geometries that have been discussed in the context of 3d gravity. We show that the spin model based on D(U(su_2)) for quantum gravity without cosmological constant is the semidual of a quantum particle on a three-sphere, while the bicrossproduct (DSR) model is the semidual of a quantum particle on hyperbolic space. We show further how the different models are all specific limits of q-deformed models with q=e^{-\hbar \sqrt{-\Lambda}/m_p}, where m_p is the Planck mass and \Lambda is the cosmological constant, and argue that semidualisation interchanges m_p and l_c, where l_c is the cosmological length scale l_c=1/\sqrt{|\Lambda|}. We investigate the physics of semidualisation by studying representation theory. In both the spin model and its semidual we show that irreducible representations have a physical picture as solutions of a respectively non-commutative/curved wave equation. We explain, moreover, that the q-deformed model, at a certain algebraic level, is self-dual under semidualisation.
Proposals that quantum gravity gives rise to non-commutative spacetime geometry and deformations ... more Proposals that quantum gravity gives rise to non-commutative spacetime geometry and deformations of Poincare symmetry are examined in the context of (2+1)-dimensional quantum gravity. The results are expressed in five lessons, which summarise how the gravitational constant, Planck's constant and the cosmological constant enter the non-commutative and non-cocommutative structures arising in (2+1)-dimensional quantum gravity. It is emphasised that the much studied bicrossproduct kappa-Poincare algebra does not arise directly in (2+1)-dimensional quantum gravity.
These notes summarise a talk surveying the combinatorial or Hamiltonian quantisation of three dim... more These notes summarise a talk surveying the combinatorial or Hamiltonian quantisation of three dimensional gravity in the Chern-Simons formulation, with an emphasis on the role of quantum groups and on the way the various physical constants (c,G,\Lambda,\hbar) enter as deformation parameters. The classical situation is summarised, where solutions can be characterised in terms of model spacetimes (which depend on c and \Lambda), together with global identifications via elements of the corresponding isometry groups. The quantum theory may be viewed as a deformation of this picture, with quantum groups replacing the local isometry groups, and non-commutative spacetimes replacing the classical model spacetimes. This point of view is explained, and open issues are sketched.
In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions, the phase space is the mod... more In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions, the phase space is the moduli space of flat G-connections on a two-dimensional surface, where G is a typically non-compact Lie group which depends on the signature of space-time and the cosmological constant. For Euclidean signature and vanishing cosmological constant, G is the three-dimensional Euclidean group. For this case the Poisson structure of the moduli space is given explicitly in terms of a classical r-matrix. It is shown that the quantum R-matrix of the quantum double D(SU(2)) provides a quantization of that Poisson structure.
Nuclear Physics B, 1995
Baby Skyrmions are topological solitons in a (2+1)-dimensional field theory which resembles the S... more Baby Skyrmions are topological solitons in a (2+1)-dimensional field theory which resembles the Skyrme model in important respects. We apply some of the techniques and approximations commonly used in discussions of the Skyrme model to the dynamics of baby Skyrmions and directly test them against numerical simulations. Specifically we study the effect of spin on the shape of a single baby Skyrmion, the dependence of the forces between two baby Skyrmions on the baby Skyrmions' relative orientation and the forces between two baby Skyrmions when one of them is spinning.
We present a systematic approach to the linearised Yang-Mills-Higgs equations in the background o... more We present a systematic approach to the linearised Yang-Mills-Higgs equations in the background of a 't Hooft-Polyakov monopole and use it to unify and extend previous studies of their spectral properties. We show that a quaternionic formulation allows for a compact and efficient treatment of the linearised equations in the BPS limit of vanishing Higgs self-coupling, and use it to study both scattering and bound states. We focus on the sector of vanishing generalised angular momentum and analyse it numerically, putting zero-energy bound states, Coulomb bound states and infinitely many Feshbach resonances into a coherent picture. We also consider the linearised Yang-Mills-Higgs equations with non-vanishing Higgs self-coupling and confirm the occurrence of Feshbach resonances in this situation.
Journal of High Energy Physics, 2003
Gauge theories in 2+1 dimensions whose gauge symmetry is spontaneously broken to a finite group e... more Gauge theories in 2+1 dimensions whose gauge symmetry is spontaneously broken to a finite group enjoy a quantum group symmetry which includes the residual gauge symmetry. This symmetry provides a framework in which fundamental excitations (electric charges) and topological excitations (magnetic fluxes) can be treated on equal footing. In order to study symmetry breaking by both electric and magnetic condensates we develop a theory of symmetry breaking which is applicable to models whose symmetry is described by a quantum group (quasitriangular Hopf algebra). Using this general framework we investigate the symmetry breaking and confinement phenomena which occur in (2+1)-dimensional gauge theories. Confinement of particles is linked to the formation of string-like defects. Symmetry breaking by an electric condensate leads to magnetic confinement and vice-versa. We illustrate the general formalism with examples where the symmetry is broken by electric, magnetic and dyonic condensates.
Journal of Physics A-mathematical and General, 1999
A Fourier transform S is defined for the quantum double D(G) of a finite group G. Acting on chara... more A Fourier transform S is defined for the quantum double D(G) of a finite group G. Acting on characters of D(G), S and the central ribbon element of D(G) generate a unitary matrix representation of the group SL(2,Z). The characters form a ring over the integers under both the algebra multiplication and its dual, with the latter encoding the fusion rules of D(G). The Fourier transform relates the two ring structures. We use this to give a particularly short proof of the Verlinde formula for the fusion coefficients.
In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions the phase space of gravity ... more In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions the phase space of gravity is the moduli space of flat G-connections, where G is a typically non-compact Lie group which depends on the signature of space-time and the cosmological constant. For Euclidean signature and vanishing cosmological constant, G is the three-dimensional Euclidean group. For this case the Poisson structure of the moduli space is given explicitly in terms of a classical r-matrix. It is shown that the quantum R-matrix of the quantum double D(SU(2)) provides a quantisation of that Poisson structure.
We use factorisations of the local isometry groups arising in 3d gravity for Lorentzian and Eucli... more We use factorisations of the local isometry groups arising in 3d gravity for Lorentzian and Euclidean signatures and any value of the cosmological constant to construct associated bicrossproduct quantum groups via semidualisation. In this way we obtain quantum doubles of the Lorentz and rotation groups in 3d, as well as kappa-Poincare algebras whose associated r-matrices have spacelike, timelike and lightlike deformation parameters. We confirm and elaborate the interpretation of semiduality proposed in [13] as the exchange of the cosmological length scale and the Planck mass in the context of 3d quantum gravity. In particular, semiduality gives a simple understanding of why the quantum double of the Lorentz group and the kappa-Poincare algebra with spacelike deformation parameter are both associated to 3d gravity with vanishing cosmological constant, while the kappa-Poincare algebra with a timelike deformation parameter can only be associated to 3d gravity if one takes the Planck mass to be imaginary.
Nuclear Physics B, 1995
The deuteron is described as a quantum state on a ten-dimensional manifold M 10 of Skyrme fields ... more The deuteron is described as a quantum state on a ten-dimensional manifold M 10 of Skyrme fields of degree two, which are obtained by calculating the holonomy of SU (2) instantons. The manifold M 10 includes both toroidal configurations of minimal energy and configurations which are approximately the product of two Skyrmions in the most attractive relative orientation. The quantum Hamiltonian is of the form −∆ + V , where ∆ is the covariant Laplace operator on M 10 and V is the potential which M 10 inherits from the Skyrme potential energy functional. Quantum states are complexvalued functions on the double cover of M 10 satisfying certain constraints. There is a unique bound state with the quantum numbers of the deuteron, and its binding energy is approximately 6 MeV. Some of the deuteron's electrostatic and magnetostatic properties are also calculated and compared with experiment. A fundamental challenge in particle physics is to understand nuclear forces from first principles. So far, there is no quantitative understanding of nuclear binding starting with quarks and QCD, but one may attempt to investigate nuclei in terms of an effective low energy theory, like the Skyrme model . In the Skyrme model, nucleons are solitons, and the parameters of the model are fixed so that the masses of the nucleon and the delta resonance are in agreement with experiment. All of nuclear physics can in principle then be derived from the Skyrme model (assuming that it is at least approximately right).
Nuclear Physics B, 2005
We study the action of the mapping class group of an oriented genus g surface with n punctures an... more We study the action of the mapping class group of an oriented genus g surface with n punctures and a disc removed on a Poisson algebra which arises in the combinatorial description of Chern-Simons gauge theory when the gauge group is a semidirect product G ⋉ g * . We prove that the mapping class group acts on this algebra via Poisson isomorphisms and express the action of Dehn twists in terms of an infinitesimally generated G-action. We construct a mapping class group representation on the representation spaces of the associated quantum algebra and show that Dehn twists can be implemented via the ribbon element of the quantum double D(G) and the exchange of punctures via its universal R-matrix.
European Physical Journal C, 1995
The Skyrme model can be generalised to a situation where static fields are maps from one Riemanni... more The Skyrme model can be generalised to a situation where static fields are maps from one Riemannian manifold to another. Here we study a Skyrme model where physical space is two-dimensional euclidean space and the target space is the two-sphere with its standard metric. The model has topological soliton solutions which are exponentially localised. We describe a superposition procedure for solitons in our model and derive an expression for the interaction potential of two solitons which only involves the solitons' asymptotic fields. If the solitons have topological degree 1 or 2 there are simple formulae for their interaction potentials which we use to prove the existence of solitons of higher degree. We explicitly compute the fields and energy distributions for solitons of degrees between one and six and discuss their geometrical shapes and binding energies.
Classical and Quantum Gravity, 2003
In the formulation of (2+1)-dimensional gravity as a Chern-Simons gauge theory, the phase space i... more In the formulation of (2+1)-dimensional gravity as a Chern-Simons gauge theory, the phase space is the moduli space of flat Poincar\'e group connections. Using the combinatorial approach developed by Fock and Rosly, we give an explicit description of the phase space and its Poisson structure for the general case of a genus g oriented surface with punctures representing particles and a boundary playing the role of spatial infinity. We give a physical interpretation and explain how the degrees of freedom associated with each handle and each particle can be decoupled. The symmetry group of the theory combines an action of the mapping class group with asymptotic Poincar\'e transformations in a non-trivial fashion. We derive the conserved quantities associated to the latter and show that the mapping class group of the surface acts on the phase space via Poisson isomorphisms.
Nuclear Physics B, 2009
We study, in detail, the supersymmetric quantum mechanics of charge-(1,1) monopoles in N=2 supers... more We study, in detail, the supersymmetric quantum mechanics of charge-(1,1) monopoles in N=2 supersymmetric Yang-Mills-Higgs theory with gauge group SU(3) spontaneously broken to U(1) x U(1). We use the moduli space approximation of the quantised dynamics, which can be expressed in two equivalent formalisms: either one describes quantum states by Dirac spinors on the moduli space, in which case the Hamiltonian is the square of the Dirac operator, or one works with anti-holomorphic forms on the moduli space, in which case the Hamiltonian is the Laplacian acting on forms. We review the derivation of both formalisms, explicitly exhibit their equivalence and derive general expressions for the supercharges as differential operators in both formalisms. We propose a general expression for the total angular momentum operator as a differential operator, and check its commutation relations with the supercharges. Using the known metric structure of the moduli space of charge-(1,1) monopoles we show that there are no quantum bound states of such monopoles in the moduli space approximation. We exhibit scattering states and compute corresponding differential cross sections.
Physical Review Letters, 2002
Many two-dimensional physical systems have symmetries which are mathematically described by quant... more Many two-dimensional physical systems have symmetries which are mathematically described by quantum groups (quasi-triangular Hopf algebras). In this letter we introduce the concept of a spontaneously broken Hopf symmetry and show that it provides an effective tool for analysing a wide variety of phases exhibiting many distinct confinement phenomena.
Physics Letters B, 1995
The scale invariance of the O(3) sigma model can be broken by gauging a U (1) subgroup of the O(3... more The scale invariance of the O(3) sigma model can be broken by gauging a U (1) subgroup of the O(3) symmetry and including a Maxwell term for the gauge field in the Lagrangian. Adding also a suitable potential one obtains a field theory of Bogomol'nyi type with topological solitons. These solitons are stable against rescaling and carry magnetic flux which can take arbitrary values in some finite interval. The soliton mass is independent of the flux, but the soliton size depends on it. However, dynamically changing the flux requires infinite energy, so the flux, and hence the soliton size, remains constant during time evolution.
Inspired by soliton models, we propose a description of static particles in terms of Riemannian 4... more Inspired by soliton models, we propose a description of static particles in terms of Riemannian 4-manifolds with self-dual Weyl tensor. For electrically charged particles, the 4-manifolds are non-compact and asymptotically fibred by circles over physical 3-space. This is akin to the Kaluza-Klein description of electromagnetism, except that we exchange the roles of magnetic and electric fields, and only assume the bundle structure asymptotically, away from the core of the particle in question. We identify the Chern class of the circle bundle at infinity with minus the electric charge and the signature of the 4-manifold with the baryon number. Electrically neutral particles are described by compact 4-manifolds. We illustrate our approach by studying the Taub-NUT manifold as a model for the electron, the Atiyah-Hitchin manifold as a model for the proton, CP^2 with the Fubini-Study metric as a model for the neutron, and S^4 with its standard metric as a model for the neutrino.
European Physical Journal C, 1994
We study the intermediate-and long-range forces between moving and spinning Skyrmions, employing ... more We study the intermediate-and long-range forces between moving and spinning Skyrmions, employing two different methods. One uses a relativised product ansatz for the Skyrme fields, the other models Skyrmions as triplets of scalar dipoles. The methods lead to the same finite-dimensional Lagrangian dynamical system which may be interpreted as a point-particle approximation to Skyrmion dynamics. We discuss in detail the dynamics in the so-called attractive channel and the interaction between well-separated and rapidly spinning Skyrmions, and point out the resemblance between the latter and the one-pion exchange potential in nuclear physics.
Nuclear Physics B, 1998
Magnetic monopoles in Yang-Mills-Higgs theory with a non-abelian unbroken gauge group are classif... more Magnetic monopoles in Yang-Mills-Higgs theory with a non-abelian unbroken gauge group are classified by holomorphic charges in addition to the topological charges familiar from the abelian case. As a result the moduli spaces of monopoles of given topological charge are stratified according to the holomorphic charges. Here the physical consequences of the stratification are explored in the case where the gauge group SU (3) is broken to U (2). The description due to A. Dancer of the moduli space of charge two monopoles is reviewed and interpreted physically in terms of non-abelian magnetic dipole moments. Semi-classical quantisation leads to dyonic states which are labelled by a magnetic charge and a representation of the subgroup of U (2) which leaves the magnetic charge invariant (centraliser subgroup). A key result of this paper is that these states fall into representations of the semi-direct product U (2) ⋉ R 4 . The combination rules (Clebsch-Gordan coefficients) of dyonic states can thus be deduced. Electric-magnetic duality properties of the theory are discussed in the light of our results, and supersymmetric dyonic BPS states which fill the SL(2, Z)-orbit of the basic massive W -bosons are found.
We explore in detail the role in euclidean 3d quantum gravity of quantum Born reciprocity or `sem... more We explore in detail the role in euclidean 3d quantum gravity of quantum Born reciprocity or `semidualisation'. The latter is an algebraic operation defined using quantum group methods that interchanges position and momentum. Using this we are able to clarify the structural relationships between the effective non-commutative geometries that have been discussed in the context of 3d gravity. We show that the spin model based on D(U(su_2)) for quantum gravity without cosmological constant is the semidual of a quantum particle on a three-sphere, while the bicrossproduct (DSR) model is the semidual of a quantum particle on hyperbolic space. We show further how the different models are all specific limits of q-deformed models with q=e^{-\hbar \sqrt{-\Lambda}/m_p}, where m_p is the Planck mass and \Lambda is the cosmological constant, and argue that semidualisation interchanges m_p and l_c, where l_c is the cosmological length scale l_c=1/\sqrt{|\Lambda|}. We investigate the physics of semidualisation by studying representation theory. In both the spin model and its semidual we show that irreducible representations have a physical picture as solutions of a respectively non-commutative/curved wave equation. We explain, moreover, that the q-deformed model, at a certain algebraic level, is self-dual under semidualisation.
Proposals that quantum gravity gives rise to non-commutative spacetime geometry and deformations ... more Proposals that quantum gravity gives rise to non-commutative spacetime geometry and deformations of Poincare symmetry are examined in the context of (2+1)-dimensional quantum gravity. The results are expressed in five lessons, which summarise how the gravitational constant, Planck's constant and the cosmological constant enter the non-commutative and non-cocommutative structures arising in (2+1)-dimensional quantum gravity. It is emphasised that the much studied bicrossproduct kappa-Poincare algebra does not arise directly in (2+1)-dimensional quantum gravity.
These notes summarise a talk surveying the combinatorial or Hamiltonian quantisation of three dim... more These notes summarise a talk surveying the combinatorial or Hamiltonian quantisation of three dimensional gravity in the Chern-Simons formulation, with an emphasis on the role of quantum groups and on the way the various physical constants (c,G,\Lambda,\hbar) enter as deformation parameters. The classical situation is summarised, where solutions can be characterised in terms of model spacetimes (which depend on c and \Lambda), together with global identifications via elements of the corresponding isometry groups. The quantum theory may be viewed as a deformation of this picture, with quantum groups replacing the local isometry groups, and non-commutative spacetimes replacing the classical model spacetimes. This point of view is explained, and open issues are sketched.
In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions, the phase space is the mod... more In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions, the phase space is the moduli space of flat G-connections on a two-dimensional surface, where G is a typically non-compact Lie group which depends on the signature of space-time and the cosmological constant. For Euclidean signature and vanishing cosmological constant, G is the three-dimensional Euclidean group. For this case the Poisson structure of the moduli space is given explicitly in terms of a classical r-matrix. It is shown that the quantum R-matrix of the quantum double D(SU(2)) provides a quantization of that Poisson structure.
Nuclear Physics B, 1995
Baby Skyrmions are topological solitons in a (2+1)-dimensional field theory which resembles the S... more Baby Skyrmions are topological solitons in a (2+1)-dimensional field theory which resembles the Skyrme model in important respects. We apply some of the techniques and approximations commonly used in discussions of the Skyrme model to the dynamics of baby Skyrmions and directly test them against numerical simulations. Specifically we study the effect of spin on the shape of a single baby Skyrmion, the dependence of the forces between two baby Skyrmions on the baby Skyrmions' relative orientation and the forces between two baby Skyrmions when one of them is spinning.
We present a systematic approach to the linearised Yang-Mills-Higgs equations in the background o... more We present a systematic approach to the linearised Yang-Mills-Higgs equations in the background of a 't Hooft-Polyakov monopole and use it to unify and extend previous studies of their spectral properties. We show that a quaternionic formulation allows for a compact and efficient treatment of the linearised equations in the BPS limit of vanishing Higgs self-coupling, and use it to study both scattering and bound states. We focus on the sector of vanishing generalised angular momentum and analyse it numerically, putting zero-energy bound states, Coulomb bound states and infinitely many Feshbach resonances into a coherent picture. We also consider the linearised Yang-Mills-Higgs equations with non-vanishing Higgs self-coupling and confirm the occurrence of Feshbach resonances in this situation.
Journal of High Energy Physics, 2003
Gauge theories in 2+1 dimensions whose gauge symmetry is spontaneously broken to a finite group e... more Gauge theories in 2+1 dimensions whose gauge symmetry is spontaneously broken to a finite group enjoy a quantum group symmetry which includes the residual gauge symmetry. This symmetry provides a framework in which fundamental excitations (electric charges) and topological excitations (magnetic fluxes) can be treated on equal footing. In order to study symmetry breaking by both electric and magnetic condensates we develop a theory of symmetry breaking which is applicable to models whose symmetry is described by a quantum group (quasitriangular Hopf algebra). Using this general framework we investigate the symmetry breaking and confinement phenomena which occur in (2+1)-dimensional gauge theories. Confinement of particles is linked to the formation of string-like defects. Symmetry breaking by an electric condensate leads to magnetic confinement and vice-versa. We illustrate the general formalism with examples where the symmetry is broken by electric, magnetic and dyonic condensates.
Journal of Physics A-mathematical and General, 1999
A Fourier transform S is defined for the quantum double D(G) of a finite group G. Acting on chara... more A Fourier transform S is defined for the quantum double D(G) of a finite group G. Acting on characters of D(G), S and the central ribbon element of D(G) generate a unitary matrix representation of the group SL(2,Z). The characters form a ring over the integers under both the algebra multiplication and its dual, with the latter encoding the fusion rules of D(G). The Fourier transform relates the two ring structures. We use this to give a particularly short proof of the Verlinde formula for the fusion coefficients.
In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions the phase space of gravity ... more In the Chern-Simons formulation of Einstein gravity in 2+1 dimensions the phase space of gravity is the moduli space of flat G-connections, where G is a typically non-compact Lie group which depends on the signature of space-time and the cosmological constant. For Euclidean signature and vanishing cosmological constant, G is the three-dimensional Euclidean group. For this case the Poisson structure of the moduli space is given explicitly in terms of a classical r-matrix. It is shown that the quantum R-matrix of the quantum double D(SU(2)) provides a quantisation of that Poisson structure.
We use factorisations of the local isometry groups arising in 3d gravity for Lorentzian and Eucli... more We use factorisations of the local isometry groups arising in 3d gravity for Lorentzian and Euclidean signatures and any value of the cosmological constant to construct associated bicrossproduct quantum groups via semidualisation. In this way we obtain quantum doubles of the Lorentz and rotation groups in 3d, as well as kappa-Poincare algebras whose associated r-matrices have spacelike, timelike and lightlike deformation parameters. We confirm and elaborate the interpretation of semiduality proposed in [13] as the exchange of the cosmological length scale and the Planck mass in the context of 3d quantum gravity. In particular, semiduality gives a simple understanding of why the quantum double of the Lorentz group and the kappa-Poincare algebra with spacelike deformation parameter are both associated to 3d gravity with vanishing cosmological constant, while the kappa-Poincare algebra with a timelike deformation parameter can only be associated to 3d gravity if one takes the Planck mass to be imaginary.
Nuclear Physics B, 1995
The deuteron is described as a quantum state on a ten-dimensional manifold M 10 of Skyrme fields ... more The deuteron is described as a quantum state on a ten-dimensional manifold M 10 of Skyrme fields of degree two, which are obtained by calculating the holonomy of SU (2) instantons. The manifold M 10 includes both toroidal configurations of minimal energy and configurations which are approximately the product of two Skyrmions in the most attractive relative orientation. The quantum Hamiltonian is of the form −∆ + V , where ∆ is the covariant Laplace operator on M 10 and V is the potential which M 10 inherits from the Skyrme potential energy functional. Quantum states are complexvalued functions on the double cover of M 10 satisfying certain constraints. There is a unique bound state with the quantum numbers of the deuteron, and its binding energy is approximately 6 MeV. Some of the deuteron's electrostatic and magnetostatic properties are also calculated and compared with experiment. A fundamental challenge in particle physics is to understand nuclear forces from first principles. So far, there is no quantitative understanding of nuclear binding starting with quarks and QCD, but one may attempt to investigate nuclei in terms of an effective low energy theory, like the Skyrme model . In the Skyrme model, nucleons are solitons, and the parameters of the model are fixed so that the masses of the nucleon and the delta resonance are in agreement with experiment. All of nuclear physics can in principle then be derived from the Skyrme model (assuming that it is at least approximately right).
Nuclear Physics B, 2005
We study the action of the mapping class group of an oriented genus g surface with n punctures an... more We study the action of the mapping class group of an oriented genus g surface with n punctures and a disc removed on a Poisson algebra which arises in the combinatorial description of Chern-Simons gauge theory when the gauge group is a semidirect product G ⋉ g * . We prove that the mapping class group acts on this algebra via Poisson isomorphisms and express the action of Dehn twists in terms of an infinitesimally generated G-action. We construct a mapping class group representation on the representation spaces of the associated quantum algebra and show that Dehn twists can be implemented via the ribbon element of the quantum double D(G) and the exchange of punctures via its universal R-matrix.
European Physical Journal C, 1995
The Skyrme model can be generalised to a situation where static fields are maps from one Riemanni... more The Skyrme model can be generalised to a situation where static fields are maps from one Riemannian manifold to another. Here we study a Skyrme model where physical space is two-dimensional euclidean space and the target space is the two-sphere with its standard metric. The model has topological soliton solutions which are exponentially localised. We describe a superposition procedure for solitons in our model and derive an expression for the interaction potential of two solitons which only involves the solitons' asymptotic fields. If the solitons have topological degree 1 or 2 there are simple formulae for their interaction potentials which we use to prove the existence of solitons of higher degree. We explicitly compute the fields and energy distributions for solitons of degrees between one and six and discuss their geometrical shapes and binding energies.
Classical and Quantum Gravity, 2003
In the formulation of (2+1)-dimensional gravity as a Chern-Simons gauge theory, the phase space i... more In the formulation of (2+1)-dimensional gravity as a Chern-Simons gauge theory, the phase space is the moduli space of flat Poincar\'e group connections. Using the combinatorial approach developed by Fock and Rosly, we give an explicit description of the phase space and its Poisson structure for the general case of a genus g oriented surface with punctures representing particles and a boundary playing the role of spatial infinity. We give a physical interpretation and explain how the degrees of freedom associated with each handle and each particle can be decoupled. The symmetry group of the theory combines an action of the mapping class group with asymptotic Poincar\'e transformations in a non-trivial fashion. We derive the conserved quantities associated to the latter and show that the mapping class group of the surface acts on the phase space via Poisson isomorphisms.
Nuclear Physics B, 2009
We study, in detail, the supersymmetric quantum mechanics of charge-(1,1) monopoles in N=2 supers... more We study, in detail, the supersymmetric quantum mechanics of charge-(1,1) monopoles in N=2 supersymmetric Yang-Mills-Higgs theory with gauge group SU(3) spontaneously broken to U(1) x U(1). We use the moduli space approximation of the quantised dynamics, which can be expressed in two equivalent formalisms: either one describes quantum states by Dirac spinors on the moduli space, in which case the Hamiltonian is the square of the Dirac operator, or one works with anti-holomorphic forms on the moduli space, in which case the Hamiltonian is the Laplacian acting on forms. We review the derivation of both formalisms, explicitly exhibit their equivalence and derive general expressions for the supercharges as differential operators in both formalisms. We propose a general expression for the total angular momentum operator as a differential operator, and check its commutation relations with the supercharges. Using the known metric structure of the moduli space of charge-(1,1) monopoles we show that there are no quantum bound states of such monopoles in the moduli space approximation. We exhibit scattering states and compute corresponding differential cross sections.