18,461 research outputs found
Wess-Zumino Term in Tachyon Effective Action
We show that the source of RR field computed from the boundary state
describing the decay of a non-BPS brane is reproduced by a particular form of
the Wess-Zumino term in the tachyon effective action. We also obtain a simple
expression of the S-charge associated with rolling tachyons.Comment: 10 pages, lanlma
Two Centered Black Holes and N=4 Dyon Spectrum
The exact spectrum of dyons in a class of N=4 supersymmetric string theories
is known to change discontinuously across walls of marginal stability. We show
that the change in the degeneracy across the walls of marginal stability can be
accounted for precisely by the entropy of two centered small black holes which
disappear as we cross the walls of marginal stability.Comment: LaTeX file, 12 pages; v3: added footnote 2 regarding overall sign of
the index, expanded footnote 3, added reference
Cosmological Solution from D-brane motion in NS5-Branes background
We study dynamics of a D3-brane propagating in the vicinity of k coincident
NS5 branes. We show that when is small, there exists a regime in which
dynamics of the D-brane is governed by Dirac-Born-Infeld action while higher
order derivative in the expansion can not be neglected. This leads to a
restriction on how fast scalar field may roll. We analyze the motion of a
rolling scalar field in this regime, and extend the analysis to cosmological
systems obtained by coupling this type of field theory to four dimensinal
gravity. It also leads to some FRW cosmologies, some of which are related to
those obtained with tachyon matter.Comment: 15 pages, To appear in International Journal of Modern Physics
Tachyon condensation on brane sphalerons
We consider a sphaleron solution in field theory that provides a toy model
for unstable D-branes of string theory. We investigate the tachyon condensation
on a Dp-brane. The localized modes, including a tachyon, arise in the spectrum
of a sphaleron solution of a \phi^4 field theory on M^{p+1}\times S^1. We use
these modes to find a multiscalar tachyon potential living on the sphaleron
world-volume. A complete cancelation between brane tension and the minimum of
the tachyon potential is found as the size of the circle becomes small.Comment: To appear in JHEP, 13 pages, 2 eps figures, minor changes and
references adde
From Fundamental Strings to Small Black Holes
We give evidence in favour of a string/black hole transition in the case of
BPS fundamental string states of the Heterotic string. Our analysis goes beyond
the counting of degrees of freedom and considers the evolution of dynamical
quantities in the process. As the coupling increases, the string states
decrease their size up to the string scale when a small black hole is formed.
We compute the absorption cross section for several fields in both the black
hole and the perturbative string phases. At zero frequency, these cross
sections can be seen as order parameters for the transition. In particular, for
the scalars fixed at the horizon the cross section evolves to zero when the
black hole is formed.Comment: 36 pages, 4 figures, argument about minimally coupled scalar
absorption cross section clarifie
Cosmology in scalar tensor theory and asymptotically de-Sitter Universe
We have investigated the cosmological scenarios with a four dimensional
effective action which is connected with multidimensional, supergravity and
string theories. The solution for the scale factor is such that initially
universe undergoes a decelerated expansion but in late times it enters into the
accelerated expansion phase. Infact, it asymptotically becomes a de-Sitter
universe. The dilaton field in our model is a decreasing function of time and
it becomes a constant in late time resulting the exit from the scalar tensor
theory to the standard Einstein's gravity. Also the dilaton field results the
existence of a positive cosmological constant in late times.Comment: 7 pages, Revtex Style, 6 Postscript figure
Rare Decay Modes of Quarter BPS Dyons
The degeneracy of quarter BPS dyons in N=4 supersymmetric string theories is
known to jump across walls of marginal stability on which a quarter BPS dyon
can decay into a pair of half BPS dyons. We show that as long as the electric
and magnetic charges of the original dyon are primitive elements of the charge
lattice, the subspaces of the moduli space on which a quarter BPS dyon becomes
marginally unstable against decay into a pair of quarter BPS dyons or a half
BPS dyon and a quarter BPS dyon are of codimension two or more. As a result any
pair of generic points in the moduli space can be connected by a path avoiding
these subspaces and there is no jump in the spectrum associated with these
subspaces.Comment: LaTeX file, 9 pages; v2: a minor logical error corrected with no
change in the result
How Do Black Holes Predict the Sign of the Fourier Coefficients of Siegel Modular Forms?
Single centered supersymmetric black holes in four dimensions have
spherically symmetric horizon and hence carry zero angular momentum. This leads
to a specific sign of the helicity trace index associated with these black
holes. Since the latter are given by the Fourier expansion coefficients of
appropriate meromorphic modular forms of Sp(2,Z) or its subgroup, we are led to
a specific prediction for the signs of a subset of these Fourier coefficients
which represent contributions from single centered black holes only. We
explicitly test these predictions for the modular forms which compute the index
of quarter BPS black holes in heterotic string theory on T^6, as well as in Z_N
CHL models for N=2,3,5,7.Comment: LaTeX file, 17 pages, 1 figur
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