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Hyper-Kähler Geometry, Magnetic Monopoles, and Nahm's Equations

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Hyper-Kähler Geometry, Magnetic Monopoles, and Nahm's Equations Hyper-K¨ahlerGeometry, Magnetic Monopoles, and Nahm's Equations John Benjamin McCarthy Jaime Mendizabal Bruno Ricieri Souza Roso Supervised by Professor Michael Singer 3rd May 2019 Project Report Submitted for the London School of Geometry and Number Theory Abstract In this report we investigate the correspondence between SU(2) magnetic 3 monopoles and solutions to the Nahm equations on R . Further, we invest- igate a variation of this result for singular Dirac monopoles. To this end, the report starts with a review of the necessary spin geometry and hyper-K¨ahler geometry required, as well as the definition of magnetic monopoles and the Nahm equations. The final chapter consists of a presentation of Nakajima's proof of the monopole correspondence, and an investigation of how these techniques can be applied to the case of singular Dirac monopoles. i Contents Introduction 1 1 Preliminaries 3 1.1 Spin Geometry . .3 1.1.1 Clifford algebras and the Spin group . .3 1.1.2 Spin and Spinor Bundles . .6 1.1.3 Dirac Operators . .8 1.2 K¨ahlerand Hyper-K¨ahlerManifolds . 13 1.3 Hyper-K¨ahlerQuotients . 15 2 Magnetic Monopoles 18 2.1 Yang-Mills-Higgs Equations . 18 2.2 SU(2) Monopoles . 19 2.3 U(1) Monopoles . 21 2.3.1 Dirac Monopole . 22 2.3.2 General Case . 23 2.4 Moduli Space of SU(2) Monopoles . 23 3 Nahm's Equations 28 3.1 Instantons . 28 3.2 Dimensional Reduction and Nahm's Equations . 29 4 Correspondence between Monopoles and Nahm's Equations 33 4.1 SU(2) Monopoles and Nahm's Equations . 33 4.1.1 Monopoles to Solutions of Nahm's Equations . 36 4.1.2 Solutions of Nahm's Equations to Monopoles . 48 4.2 Singular Monopoles and Nahm's Equations . 58 4.2.1 b-Geometry and the case s =0 ................ 60 4.2.2 sc-Geometry and the case s > 0................ 67 4.2.3 Limit of Nahm Data . 69 References 73 ii Introduction It is a well-known observation in classical electromagnetism that all magnetic fields appear to be generated by magnetic dipoles. This is in contrast with electric fields, which may be generated by particles of a single parity. In the landmark paper [Dir31] in 1931, Paul Dirac proposed a resolution to the mystery of why electric charge is quantized by proposing the existence of magnetic monopoles. Through Dirac's work, it would be possible to prove that electric charge must be quantized provided the existence of even a single magnetic monopole in the universe. Despite this and other strong theoretical evidence for the existence of such elementary particles, none have yet been observed. Mathematically, magnetic monopoles may be modelled as solutions of the Bogo- molny equations on R3, FA = ?dAΦ: Similarly to the theory of instantons on R4 modelling other elementary particles, solutions to the Bogomolny equations are gauge-theoretic data on R3, and are hence of considerable mathematical and geometric interest independent of their physical origins. In particular, moduli spaces of solutions to equations such as the Bogomolny equations are known to, in many cases, provide powerful invariants of the un- derlying spaces, and give new tools for solving many problems in geometry and topology. This is exemplified by the famous Donaldson theory and Seiberg-Witten theory in four-manifold topology. As such, there is much interest in finding new ways of constructing solutions to these gauge-theoretic equations. The first major breakthrough in this area came with the novel ADHM construction developed by Atiyah, Drinfeld, Hitchin, and Manin in [ADHM78], in order to find solutions of the anti-self-dual Yang-Mills equations on R4 using essentially algebraic data. In [Nah83] Nahm developed, in analogy with the ADHM construction, a method for constructing solutions to the Bogomolny equations from gauge-theoretic data over an interval. In [Hit83] Hitchin showed that this technique can in fact be used to find all magnetic monopoles, and considerable work was done by many others around this time to extend these results to other situations, in particular with varying choices of gauge group. 1 The work of Nahm and Hitchin involved passing through intermediate algebraic- geometric data, spectral curves in twistor spaces. In [Nak93] Nakajima gave a purely differential-geometric proof of the correspondence of Hitchin, phrased in language that is readily generalisable to the theory of Nahm transforms. In this report, we will review the correspondence between magnetic monopoles and solutions of the Nahm equations through the eyes of Nakajima's differential geometric techniques, and investigate a variation of this result for singular mag- netic monopoles of the simplest type { so called Dirac monopoles. In Chapter 1 we recall the necessary preliminaries for these constructions, in- cluding elementary spin geometry and the basics of hyper-K¨ahlergeometry. In Chapter 2 we describe in more detail the definition of a magnetic mono- pole and the construction of the hyper-K¨ahlermoduli space of solutions to the Bogomolny equations. In Chapter 3 the Nahm equations and their relation to the anti-self-duality equations on R4 are described. Finally, in Chapter 4 we present (part of) the proof of the correspondence of magnetic monopoles with the Nahm equations as shown by Nakajima, as well as an investigation of the correspondence for singular Dirac monopoles. This involves the theory of b-geometry and scattering calculus, and a mild amalgamation of these concepts applicable in this case. The authors wish to thank Professor Michael Singer for providing guidance in learning the material presented in this report, and for contributing most of the ideas in determining the correspondence between singular magnetic monopoles and appropriate Nahm data. 2 Chapter 1 Preliminaries In this chapter we will give some preliminary results which will be necessary for the development of the project. 1.1 Spin Geometry Here we will lay out the essential definitions and some useful results about Spin geometry and Dirac operators. Many of these concepts can be studied in greater generality, but we will often restrict ourselves to the case we are interested in. We will go over many of the results without proofs, specially at the beginning. A more detailed and rigorous exposition can be found Lawson and Michelsohn's Spin Geometry [LM89], whose notation we follow closely, and in Friedrich's Dirac Operators in Riemannian Geometry [Fri00]. 1.1.1 Clifford algebras and the Spin group Definition 1.1.1. Let (V; q) be a vector space with a quadratic form. Its Clifford algebra is defined as Cl(V; q) := T (V )=Iq(V ); where 1 M T (V ) = V ⊗k k=0 is the tensor algebra of V and Iq(V ) is the ideal of T (V ) generated by elements of the form v ⊗ v + q(v) for v 2 V . The product of two elements '; 2 Cl(V; q) will be denoted by '· , or simply by ' . 3 In other words, the Clifford algebra of a vector space is the algebra generated by V under the relations v2 = −q(v)1 for v 2 V . Equivalently, if q(v; w) = 1 2 (q(v+w)−q(v)−q(w)) is the polarisation of the quadratic form (if the underlying field has characteristic different from 2), then we have the relations v · w + w · v = −2q(v; w): It can be shown that Cl(V; q) is isomorphic to V•V as a vector space (it will be isomorphic as an algebra precisely when q ≡ 0). In particular, V will be naturally embedded as a vector space. From now on, for simplicity, we will take our vector space to be finite dimen- sional and real, and we will take the quadratic form to be positive definite. This means that (V; q) will be isomorphic to Rn with the standard metric, for some n. Whenever we take (V; q) to be explicitly Rn with the standard metric, we will write Cl(n) for the Clifford algebra. It will be useful to consider as well the complexi- fication of the Clifford algebra, which will be denoted by Cl(V; q) := Cl(V; q) ⊗ C. Firstly, let us consider Cl×(V; q) = f' 2 Cl(V; q): 9'−1 such that ''−1 = '−1' = 1g; which has a group structure. This group is, in fact, a Lie group of dimension 2n, and its Lie algebra can be identified with Cl(V; q), with the Lie bracket being the commutator [x; y] = xy − yx. Furthermore, analogously to matrix Lie groups, the adjoint representation on the Lie algebra is given by conjugation. We can now define the Spin group as a subgroup of Cl×(V; q). Definition 1.1.2. Given (V; q), we define the Spin group as Spin(V; q) := fv1v2 ··· v2k 2 Cl(V; q): vi 2 V and q(vi) = 1g: That is, it is the subgroup of Cl×(V; q) formed by products of an even amount of unit vectors. It forms a group, since multiplying two elements clearly gives another element of the group, and the element v2k ··· v2v1 is the inverse of v1v2 ··· v2k. Again, if (V; q) is taken to be explicitly Rn with the standard metric, then we denote the Spin group by Spin(n). The Spin group acts on Cl(V; q) through the adjoint representation, and can be shown that this action actually preserves V ⊂ Cl(V; q). This gives a group homomorphism ξ : Spin(V; q) ! SO(V; q). It turns out that this is actually a covering. Theorem 1.1.3. Given (V; q), the map ξ : Spin(V; q) ! SO(V; q) is a 2-to-1 cov- ering.
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