Massless D-brane Research Articles

Exoflops in two dimensions

An exoflop occurs in the gauged linear σ-model by varying the Kahler form so that a subspace appears to shrink to a point and then reemerge “outside” the original manifold. This occurs for K3 surfaces where a rational curve is “flopped” from inside to outside the K3 surface. We see that whether a rational curve contracts to an orbifold phase or an exoflop depends on whether this curve is a line or conic. We study how the D-brane category of the smooth K3 surface is described by the exoflop and, in particular, find the location of a massless D-brane in the exoflop limit. We relate exoflops to noncommutative resolutions.

Categories of massless D-branes and del Pezzo surfaces

In analogy with the physical concept of a massless D-brane, we define a notion of "Q-masslessness" for objects in the derived category. This is defined in terms of monodromy around singularities in the stringy Kahler moduli space and is relatively easy to study using spherical functors. We consider several examples in which del Pezzo surfaces and other rational surfaces in Calabi-Yau threefolds are contracted. For precisely the del Pezzo surfaces that can be written as hypersurfaces in weighted P3, the category of Q-massless objects is a "fractional Calabi-Yau" category of graded matrix factorizations.

Decompactifications and massless D-branes in hybrid models

A method of determining the mass spectrum of BPS D-branes in any phase limit of a gauged linear sigma model is introduced. A ring associated to monodromy is defined and one considers K-theory to be a module over this ring. A simple but interesting class of hybrid models with Landau-Ginzburg fibres over \( {\mathbb{P}^n} \) are analyzed using special Kähler geometry and D-brane probes. In some cases the hybrid limit is an infinite distance in moduli space and corresponds to a decompactification. In other cases the hybrid limit isat a finite distance and acquires massless D-branes. An example studied appears to correspond to a novel theory of supergravity with an SU(2) gauge symmetry where the gauge and gravitational couplings are necessarily tied to each other.

Singular spaces and generalized Poincaré complexes

We introduce a method that associates to a singular space aCW complex whose ordinary rational homology satisfiesPoincaré duality across complementary perversities as in intersectionhomology. The method is based on a homotopy theoreticprocess of spatial homology truncation, whose functoriality propertiesare investigated in detail. The resulting homology theory is notisomorphic to intersection homology and addresses certain questionsin type II string theory related to massless D-branes.The two theories satisfy an interchange of third and second plus fourthBetti number for mirror symmetric conifold transitions.Further applications of the new theory to K-theory and symmetric L-theoryare indicated.

Massless D-Branes on Calabi–Yau Threefolds and Monodromy

We analyze the link between the occurrence of massless B-type D-branes for specific values of moduli and monodromy around such points in the moduli space. This allows us to propose a classification of all massless B-type D-branes at any point in the moduli space of Calabi–Yau’s. This classification then justifies a previous conjecture due to Horja for the general form of monodromy. Our analysis is based on using monodromies around points in moduli space where a single D-brane becomes massless to generate monodromies around points where an infinite number become massless. We discuss the various possibilities within the classification.

String scattering from D-branes in type 0 theories

We derive fully covariant expressions for all two-point scattering amplitudes involving a closed string tachyon and massless strings from the Dirichlet brane in type 0 theories. The amplitude for two massless D-brane fluctuations to produce a closed string tachyon is also evaluated. We then examine in detail these string scattering amplitudes in order to extract world-volume couplings of the tachyon with itself and with massless fields on a D-brane. We find that the tachyon appears as an overall coupling function in the Born-Infeld action and conjecture the form of the function.

Decay of excited D-branes

We calculate the leading order interactions of massless D-brane excitations. Their 4-point functions are found to be identical to those found in type I theory. The amplitude for two massless D-brane fluctuations to produce a massless closed string is found to possess interesting new structure. As a function of its single kinematic invariant, it displays an infinite sequence of alternating zeros and poles. At high transverse momenta, this amplitude decays exponentially, indicating a growing effective thickness of the D-brane. This amplitude is the leading process by which non-extremal D-branes produce Hawking radiation.