16,216 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 $s$-$t$-$u$
channels, corresponding to the exchange of particles with a mass spectrum
coinciding with that of $(p,q)$ 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 $d=11$ supergravity solutions involving
symmetric spaces of constant negative curvature. Many examples of
non-supersymmetric string compactifications on hyperbolic spaces $H_r$ of
finite volume are given in terms of suitable cosets of the form $H_r/\Gamma$,
where $\Gamma$ is a discrete group. We describe in some detail the cases of
the non-compact hyperbolic spaces $F_2$ and $F_3$, representing the fundamental
regions of $H_2$ and $H_3$ under $SL(2,Z)$ and the Picard group, respectively.
By writing $AdS$ as a U(1) fibration, we obtain new solutions where
$AdS_{2p+1}$ gets untwisted by T-duality to ${\bf R}\times SU(p,1)/(SU(p)\times
U(1))$. Solutions with time-dependent dilaton field are also constructed by
starting with a solution with NS5-brane flux over $H_3$. 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
$b_s$ (s=1, ...,N) associated with twists mixing a compact $S^1$ direction with
angles in $N$ spatial 2-planes of flat 10-dimensional space. It generalizes the
Kaluza-Klein Melvin model which has single parameter $b$. The corresponding
U-dual background is a R-R type IIA solution describing an orthogonal
intersection of $N$ flux 7-branes. Like the Melvin model, the NS-NS string
model with $N$ continuous parameters is explicitly solvable; we present its
perturbative spectrum and torus partition function explicitly for the N=2 case.
For generic $b_s$ (above some critical values) there are tachyons in the $S^1$
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 $N
>1$ by making a special choice of the parameters $b_s$. 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

### N=2 Chern-Simons-Matter Theories Without Vortices

We study ${\cal N}=2$ Chern-Simons-matter theories with gauge group
$U_{k_1}(1)\times U_{k_2}(1)$. We find that, when $k_1+k_2=0$, the partition
function computed by localization dramatically simplifies and collapses to a
single term. We show that the same condition prevents the theory from having
supersymmetric vortex configurations. The theories include mass-deformed ABJM
theory with $U(1)_{k}\times U_{-k}(1)$ gauge group as a particular case.
Similar features are shared by a class of CS-matter theories with gauge group
$U_{k_1}(1)\times \cdots \times U_{k_N}(1)$.Comment: 17 page

### Heterotic strings in a uniform magnetic field

An exact conformal model representing a constant magnetic field background in
heterotic string theory is explicitly solved in terms of free
creation/annihilation operators. The spectrum of physical states is examined
for different possible embeddings of the magnetic U(1) subgroup. We find that
an arbitrarily small magnetic field gives rise to an infinite number of
tachyonic excitations corresponding to charged vector states of the massless
level and to higher level states with large spins and charges.Comment: 25 pages, harvmac (few comments added, misprints corrected

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