Past String Theory Seminar

9 March 2015
12:00
Tudor Dimofte
Abstract
While the Higgs branch of a 3d N=4 gauge theory is protected from quantum corrections and its metric is easily computable, the Coulomb branch suffers both perturbative and nonperturbative corrections, and has long remained mysterious. I will present a construction of the Coulomb branch as a complex manifold, and (in principle) as a hyperkahler manifold. In particular, holomorphic functions on the Coulomb branch come from vevs of monopole operators in a chiral ring, and it turns out that this ring has a simple, quasi-abelian description. Applying the construction to linear quiver gauge theories, one finds new descriptions of singular monopole moduli spaces. I may also touch upon relations to equivariant vortex counting, geometric representation theory, and symplectic duality.
  • String Theory Seminar
2 March 2015
12:00
George Papadopoulos
Abstract

I shall demonstrate, under some mild assumptions, that the symmetry group of  extreme, Killing, supergravity horzions contains an sl(2, R) subalgebra.  The proof requires a generalization of the  Lichnerowicz theorem for non-metric connections. The techniques developed can also be applied in the classification
of AdS and Minkowski flux backgrounds.
 

  • String Theory Seminar
23 February 2015
12:00
Abstract
The spectrum of BPS states in four-dimensional gauge theories and string vacua with N=2 supersymmetry is well-known to be jump across certain walls in moduli space, where bound states can decay. In this talk I will survey how the discontinuity can be understood in terms of the supersymmetric quantum mechanics of mutually non-local point particles. This physical picture 
suggests that, at any point in moduli space, the BPS spectrum can be viewed as a sum of bound states of absolutely stable `single-centered' constituents. This idea appears to be vindicated in the context of quiver moduli spaces. Finally, I shall explain how the discontinuous BPS indices can be combined into a `new' supersymmetric index, a function which sums up multi-particle state contributions and is continuous across the wall.
  • String Theory Seminar
16 February 2015
12:00
Sakura Schafer-Nameki
Abstract

I will discuss how singular fibers in higher codimension in elliptically fibered Calabi-Yau fourfolds can be studied using Coulomb branch phases for d=3 supersymmetric gauge theories. This approach gives an elegent description of the generalized Kodaira fibers in terms of combinatorial/representation-theoretic objects called "box graphs", including the network of flops connecting distinct small resolutions. For physics applications, this approach can be used to constrain the possible matter spectra and possible U(1) charges (models with higher rank Mordell Weil group) for F-theory GUTs.

  • String Theory Seminar
9 February 2015
12:00
Charles Strickland-Constable
Abstract

Motivated by the study of supersymmetric backgrounds with non-trivial fluxes, we provide a formulation of supergravity in the language of generalised geometry, as first introduced by Hitchin, and its extensions. This description both dramatically simplifies the equations of the theory by making the hidden symmetries manifest, and writes the bosonic sector geometrically as a direct analogue of Einstein gravity. Further, a natural analogue of special holonomy manifolds emerges and coincides with the conditions for supersymmetric backgrounds with flux, thus formulating these systems as integrable geometric structures.
 

  • String Theory Seminar
2 February 2015
12:00
John Dixon
Abstract

The chiral scalar superfield has interesting BRST cohomology, but the relevant cohomology objects all  have spinor indices. So they cannot occur in an action. They need to be coupled to a chiral dotted spinor superfield. Until now, this has been very problematic, since no sensible action for a chiral dotted spinor superfield was known.  The most obvious such action contains higher derivatives and tachyons.

Now,  a sensible  action has been found. When coupled to the cohomology, this action removes the supersymmetry charge from the theory while maintaining the rigidity and power of supersymmetry.The simplest example of this phenomenon has exactly the fermion content of the Leptons or the Quarks.  The mechanism has the potential to get around the cosmological constant problem, and also the problem of the sum rules of spontaneously broken supersymmetry.

  • String Theory Seminar
26 January 2015
12:00
Philip Candelas
Abstract

This is a report on an ongoing project to construct Calabi-Yau manifolds for which the Hodge numbers $(h^{11}, h^{21})$ are both relatively small. These manifolds are, in a sense, the simplest Calabi-Yau manifolds. I will report on joint work with Volker Braun, Andrei Constantin, Rhys Davies, Challenger Mishra and others.

  • String Theory Seminar

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