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Ivancevic_Applied-Diff-Geom

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742 <strong>Applied</strong> <strong>Diff</strong>erential <strong>Geom</strong>etry: A Modern IntroductionThen we introduce arbitrary local coordinates u s , (s = 1, . . . , r), on themoduli space M, and describe a map f : M → X by functions a i (u),a D,j (u). We require f to be such that f ∗ (ω h ) = 0; this precisely ensuresthat locally, if we pick u i = a i , then a D,j must be of the form in (4.232)with some holomorphic function F. Then we take the metric on M to bethe one whose Kähler form is f ∗ (ω), i.e.,(ds) 2 = Im ∑ s,t,i∂a D,i∂u s∂ā i∂ū t dus dū t .If again we arrange a, a D as a 2r−component column vector v, then theformalism is invariant under transformations v → Mv + c, with M amatrix in Sp(2r, R) and c a constant vector. Again, considerations involvingthe charges will eventually require that M be in Sp(2r, Z) and imposerestrictions on c.Physical Interpretation via DualitySo far we have seen that the spin zero component of the N = 2 multiplethas a Kähler metric of a very special sort, constructed using a distinguishedset of coordinate systems. This rigid structure is related by N = 2 supersymmetryto the natural linear structure of the gauge field. We have foundthat, for the spin zero component, the distinguished parametrization is notcompletely unique; there is a natural family of parameterizations relatedby SL(2, R). How does this SL(2, R) (which will actually be reduced toSL(2, Z)) act on the gauge fields?SL(2, R) is generated by the transformationsT b =( 1 b0 1), and S =( 0 1−1 0with real b. The former acts as a D → a D + ba, a → a; this acts triviallyon the distinguished coordinate a, and can be taken to act trivially on thegauge field. By inspection of (4.218), the effect of a D → a D + ba on thegauge kinetic energy is just to shift the θ angle by 2πb; in the Abeliantheory, this has no effect until magnetic monopoles (or at least non–trivialU(1) bundles) are considered. Once that is done, the allowed shifts in theθ angle are by integer multiples of 2π; that is why b must be integral andgives essentially our first derivation of the reduction to SL(2, Z).The remaining challenge is to understand what S means in terms of thegauge fields. We will see that it corresponds to electric-magnetic duality. To),

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