498 research outputs found
Quantum States, Thermodynamic Limits and Entropy in M-Theory
We discuss the matching of the BPS part of the spectrum for (super)membrane,
which gives the possibility of getting membrane's results via string
calculations. In the small coupling limit of M--theory the entropy of the
system coincides with the standard entropy of type IIB string theory (including
the logarithmic correction term). The thermodynamic behavior at large coupling
constant is computed by considering M--theory on a manifold with topology
. We argue that the finite temperature
partition functions (brane Laurent series for ) associated with BPS
brane spectrum can be analytically continued to well--defined functionals.
It means that a finite temperature can be introduced in brane theory, which
behaves like finite temperature field theory. In the limit (point
particle limit) it gives rise to the standard behavior of thermodynamic
quantities.Comment: 7 pages, no figures, Revtex style. To be published in the Physical
Review
On solvable models of type IIB superstring in NS-NS and R-R plane wave backgrounds
We consider type IIB string in the two plane-wave backgrounds which may be
interpreted as special limits of the AdS_3 x S^3 metric supported by either the
NS-NS or R-R 3-form field. The NS-NS plane-wave string model is equivalent to a
direct generalization of the Nappi-Witten model, with its spectrum being
similar to that of strings in constant magnetic field. The R-R model can be
solved in the light-cone gauge, where the Green-Schwarz action describes 4
massive and 4 massless copies of free bosons and fermions. We find the spectra
of the two string models and study the asymptotic density of states. We also
discuss a more general class of exactly solvable plane-wave models with reduced
supersymmetry which is obtained by adding twists in two spatial 2-planes.Comment: 36 pages, harvmac. v2: discussion of equivalence of the supergravity
parts of the spectra of the NS-NS and R-R models added in sect.5.3; v3: added
remark on periodicity of the NS-NS spectrum; v4: minor correction in sect.6.
A modified Vogel\u27s medium for crossings, mating-type tests and the isolation of female-sterile mutants of Neurospora crassa.
A modified Vogel\u27s medium for crossing, mating-type tests and the isolation of female-sterile mutants of Neurospora crassa
A New Cosmological Scenario in String Theory
We consider new cosmological solutions with a collapsing, an intermediate and
an expanding phase. The boundary between the expanding (collapsing) phase and
the intermediate phase is seen by comoving observers as a cosmological past
(future) horizon. The solutions are naturally embedded in string and M-theory.
In the particular case of a two-dimensional cosmology, space-time is flat with
an identification under boost and translation transformations. We consider the
corresponding string theory orbifold and calculate the modular invariant
one-loop partition function. In this case there is a strong parallel with the
BTZ black hole. The higher dimensional cosmologies have a time-like curvature
singularity in the intermediate region. In some cases the string coupling can
be made small throughout all of space-time but string corrections become
important at the singularity. This happens where string winding modes become
light which could resolve the singularity. The new proposed space-time casual
structure could have implications for cosmology, independently of string
theory.Comment: 28 pages, 3 figures; v2: Added new subsection relating
two-dimensional model to BTZ black hole, typos corrected and references
added; v3: minor corrections, PRD versio
Generalised chiral null models and rotating string backgrounds
We consider an extension of a special class of conformal sigma models
(`chiral null models') which describe extreme supersymmetric string solutions.
The new models contain both `left' and `right' vector couplings and should
correspond to non-BPS backgrounds. In particular, we discuss a conformal
six-dimensional model which is a combination of fundamental string and 5-brane
models with two extra couplings representing rotations in orthogonal planes. If
the two rotational parameters are independent the resulting background is found
to be either singular or not asymptotically flat. The non asymptotically flat
solution has a regular short distance limit described by a `twisted' product of
SL(2,R) and SU(2) WZW theories with two twist parameters mixing the isometric
Euler angles of SU(2) with a null direction of SL(2,R).Comment: 12 pages, late
Magnetic flux tube models in superstring theory
Superstring models describing curved 4-dimensional magnetic flux tube
backgrounds are exactly solvable in terms of free fields. We first consider the
simplest model of this type (corresponding to `Kaluza-Klein' Melvin
background). Its 2d action has a flat but topologically non-trivial
10-dimensional target space (there is a mixing of angular coordinate of the
2-plane with an internal compact coordinate). We demonstrate that this theory
has broken supersymmetry but is perturbatively stable if the radius R of the
internal coordinate is larger than R_0=\sqrt{2\a'}. In the Green-Schwarz
formulation the supersymmetry breaking is a consequence of the presence of a
flat but non-trivial connection in the fermionic terms in the action. For R <
R_0 and the magnetic field strength parameter q > R/2\a' there appear
instabilities corresponding to tachyonic winding states. The torus partition
function Z(q,R) is finite for R > R_0 (and vanishes for qR=2n, n=integer). At
the special points qR=2n (2n+1) the model is equivalent to the free superstring
theory compactified on a circle with periodic (antiperiodic) boundary condition
for space-time fermions. Analogous results are obtained for a more general
class of static magnetic flux tube geometries including the a=1 Melvin model.Comment: 28 pages, harvmac. Minor changes, final version to appear in NP
(Twisted) Toroidal Compactification of pp-Waves
The maximally supersymmetric type IIB pp-wave is compactified on spatial
circles, with and without an auxiliary rotational twist. All spatial circles of
constant radius are identified. Without the twist, an S compactification
can preserve 24, 20 or 16 supercharges. compactifications can preserve
20, 18 or 16 supercharges; compactifications can preserve 18 or 16
supercharges and higher compactifications preserve 16 supercharges. The
worldsheet theory of this background is discussed. The T-dual and
decompactified type IIA and M-theoretic solutions which preserve 24
supercharges are given. Some comments are made regarding the AdS parent and the
CFT description.Comment: 22 pages REVTeX 4 and AMSLaTeX. v3: References and a paragraph on
nine dimensional Killing spinors were added. v4: A few typos corrected and a
footnote was modifie
Decay Modes of Intersecting Fluxbranes
Just as the single fluxbrane is quantum mechanically unstable to the
nucleation of a locally charged spherical brane, so intersecting fluxbranes are
unstable to various decay modes. Each individual element of the intersection
can decay via the nucleation of a spherical brane, but uncharged spheres can
also be nucleated in the region of intersection. For special values of the
fluxes, however, intersecting fluxbranes are supersymmetric, and so are
expected to be stable. We explicitly consider the instanton describing the
decay modes of the two--element intersection (an F5-brane in the string theory
context), and show that in dimensions greater than four the action for the
decay mode of the supersymmetric intersection diverges. This observation allows
us to show that stable intersecting fluxbranes should also exist in type 0A
string theory.Comment: 19 pages, 6 figures. References adde
On the structure of composite black p-brane configurations and related black holes
We comment on the structure of intersecting black p-brane solutions in string
theory explaining how known solutions can be obtained from Schwarzschild
solution simply by sequences of boosts and dualities. This implies, in
particular, that dimensional reduction in all internal world-volume directions
including time leads to a metric (related by analytic continuation to a
cosmological metric) which does not depend on p-brane charges, i.e. is the same
as the metric following by reduction from a higher-dimensional `neutral'
Schwarzschild black hole.Comment: 6 pages, harvmac. Misprints corrected, version to appear in Physics
Letter
N=2 Sigma Models for Ramond-Ramond Backgrounds
Using the U(4) hybrid formalism, manifestly N=(2,2) worldsheet supersymmetric
sigma models are constructed for the Type IIB superstring in Ramond-Ramond
backgrounds. The Kahler potential in these N=2 sigma models depends on four
chiral and antichiral bosonic superfields and two chiral and antichiral
fermionic superfields. When the Kahler potential is quadratic, the model is a
free conformal field theory which describes a flat ten-dimensional target space
with Ramond-Ramond flux and non-constant dilaton. For more general Kahler
potentials, the model describes curved target spaces with Ramond-Ramond flux
that are not plane-wave backgrounds. Ricci-flatness of the Kahler metric
implies the on-shell conditions for the background up to the usual four-loop
conformal anomaly.Comment: 19 pages harvma
- âŠ