24,536 research outputs found
An ansatz for a non-perturbative four-graviton amplitude in type IIB superstring theory
A natural SL(2,Z) invariant generalization of the Veneziano amplitude in type
IIB superstring theory is investigated. It includes certain perturbative and
non-perturbative (D-instanton) contributions, and it reduces to the correct
expressions in different limits. The singularities are poles in the --
channels, corresponding to the exchange of particles with a mass spectrum
coinciding with that of string states. We describe the general
structure of the associated perturbative corrections to the effective action.Comment: Version appeared in Phys.Lett. B. Discussion expanded. 7 pages,
harvma
Hyperbolic Spaces in String and M-Theory
We describe string-theory and supergravity solutions involving
symmetric spaces of constant negative curvature. Many examples of
non-supersymmetric string compactifications on hyperbolic spaces of
finite volume are given in terms of suitable cosets of the form ,
where is a discrete group. We describe in some detail the cases of
the non-compact hyperbolic spaces and , representing the fundamental
regions of and under and the Picard group, respectively.
By writing as a U(1) fibration, we obtain new solutions where
gets untwisted by T-duality to . Solutions with time-dependent dilaton field are also constructed by
starting with a solution with NS5-brane flux over . A new class of
non-supersymmetric conformal field theories can be defined via holography.Comment: 17 pages, Latex. Small additions and correction
Supersymmetric fluxbrane intersections and closed string tachyons
We consider NS-NS superstring model with several ``magnetic'' parameters
(s=1, ...,N) associated with twists mixing a compact direction with
angles in spatial 2-planes of flat 10-dimensional space. It generalizes the
Kaluza-Klein Melvin model which has single parameter . The corresponding
U-dual background is a R-R type IIA solution describing an orthogonal
intersection of flux 7-branes. Like the Melvin model, the NS-NS string
model with continuous parameters is explicitly solvable; we present its
perturbative spectrum and torus partition function explicitly for the N=2 case.
For generic (above some critical values) there are tachyons in the
winding sector. A remarkable feature of this model is that while in the Melvin
N=1 case all supersymmetry is broken, a fraction of it may be preserved for by making a special choice of the parameters . Such solvable NS-NS
models may be viewed as continuous-parameter analogs of non-compact orbifold
models. They and their U-dual R-R fluxbrane counterparts may have some
``phenomenological'' applications. In particular, in N=3 case one finds a
special 1/4 supersymmetric R-R 3-brane background. Putting Dp-branes in flat
twisted NS-NS backgrounds leads to world-volume gauge theories with reduced
amount of supersymmetry. We also discuss possible ways of evolution of unstable
backgrounds towards stable ones.Comment: 26 pages, harvmac. v3: reference added, minor changes in appendi
Exactly solvable string models of curved space-time backgrounds
We consider a new 3-parameter class of exact 4-dimensional solutions in
closed string theory and solve the corresponding string model, determining the
physical spectrum and the partition function. The background fields (4-metric,
antisymmetric tensor, two Kaluza-Klein vector fields, dilaton and modulus)
generically describe axially symmetric stationary rotating (electro)magnetic
flux-tube type universes. Backgrounds of this class include both the dilatonic
(a=1) and Kaluza-Klein (a=\sqrt 3) Melvin solutions and the uniform magnetic
field solution, as well as some singular space-times. Solvability of the string
sigma model is related to its connection via duality to a simpler model which
is a ``twisted" product of a flat 2-space and a space dual to 2-plane. We
discuss some physical properties of this model (tachyonic instabilities in the
spectrum, gyromagnetic ratios, issue of singularities, etc.). It provides one
of the first examples of a consistent solvable conformal string model with
explicit D=4 curved space-time interpretation.Comment: 54 pages, harvmac (corrected and extended version
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