369 research outputs found
Tachyon Couplings to Fermion
By fixing the internal CP factor of tachyon and massless Ramond vertex
operators in different pictures, we have shown that the internal CP factor of
the disk level S-matrix elements of two fermions and odd number of tachyon
vertex operators in the world volume of non-BPS D-branes/D-brane-anti-D-brane
is zero. We have calculated the S-matrix element of two fermions and two
tachyons which has non vanishing internal CP factor, and found the momentum
expansion of this amplitude. In the abelian case, we have compared the
two-fermion-two-tachyon coupling at low energy with the corresponding coupling
in the gauge-fixed supersymmetric tachyon DBI action. The couplings in the two
cases are exactly the same.Comment: 16 pages, latex file; V2:a missing term in the modified tachyon DBI
action adde
Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane
We study the generation of electromagnetic fields in a string-inspired
scenario associated with a rolling massive scalar field on the anti-D3
branes of KKLT de Sitter vacua. The 4-dimensional DBI type effective action
naturally gives rise to the coupling between the gauge fields and the inflaton
, which leads to the production of cosmological magnetic fields during
inflation due to the breaking of conformal invariance. We find that the
amplitude of magnetic fields at decoupling epoch can be larger than the
limiting seed value required for the galactic dynamo. We also discuss the
mechanism of reheating in our scenario and show that gauge fields are
sufficiently enhanced for the modes deep inside the Hubble radius with an
energy density greater than that of the inflaton.Comment: 4 pages and 2 eps figures, minor clarifications added and typos
correcte
Gravity on Noncommutative D-Branes
The effective action for the low energy scattering of two gravitons with a
D-brane in the presence of a constant antisytmetric field in bosonic string
theory is calculated and the modification to the standard D-brane action to
first order in is obtained.Comment: 18 pages, Latex file, accepted in Int. J. Mod. Phys.
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
We study the variation of the effective fine structure constant alpha for
Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string
theory naturally gives rise to a coupling between gauge fields and a scalar
field responsible for accelerated expansion of the universe. This leads to the
change of alpha due to a dynamical evolution of the scalar field, which can be
compatible with the recently observed cosmological data around the redshift
. We place constraints on several different DBI models
including exponential, inverse power-law and rolling massive scalar potentials.
We find that these models can satisfy the varying alpha constraint provided
that mass scales of the potentials are fine-tuned. When we adopt the mass
scales which are motivated by string theory, both exponential and inverse
power-law potentials give unacceptably large change of alpha, thus ruled out
from observations. On the other hand the rolling massive scalar potential is
compatible with the observationally allowed variation of alpha. Therefore the
information of varying alpha provides a powerful way to distinguish between a
number of string-inspired DBI dark energy models.Comment: 11 pages, 6 figure
Chiral bosonization as duality
We demonstrate that the technique of abelian bosonization through duality transformations can be extended to gauging anomalous symmetries. The example of a Dirac fermion theory is first illustrated. This idea is then also applied to bosonize a chiral fermion by gauging its chiral phase symmetry
Singular tachyon kinks from regular profiles
We demonstrate how Sen's singular kink solution of the Born-Infeld tachyon
action can be constructed by taking the appropriate limit of initially regular
profiles. It is shown that the order in which different limits are taken plays
an important role in determining whether or not such a solution is obtained for
a wide class of potentials. Indeed, by introducing a small parameter into the
action, we are able circumvent the results of a recent paper which derived two
conditions on the asymptotic tachyon potential such that the singular kink
could be recovered in the large amplitude limit of periodic solutions. We show
that this is explained by the non-commuting nature of two limits, and that
Sen's solution is recovered if the order of the limits is chosen appropriately.Comment: 7 pages, 3 figures. References adde
New Examples of Flux Vacua
Type IIB toroidal orientifolds are among the earliest examples of flux vacua.
By applying T-duality, we construct the first examples of massive IIA flux
vacua with Minkowski space-times, along with new examples of type IIA flux
vacua. The backgrounds are surprisingly simple with no four-form flux at all.
They serve as illustrations of the ingredients needed to build type IIA and
massive IIA solutions with scale separation. To check that these backgrounds
are actually solutions, we formulate the complete set of type II supergravity
equations of motion in a very useful form that treats the R-R fields
democratically.Comment: 38 pages, LaTeX; references updated; additional minor comments added;
published versio
More on WZ action of non-BPS branes
We calculate the disk level S-matrix element of one Ramond-Ramond, two gauge
fields and one tachyon vertex operators in the world volume of non-BPS branes.
We then find the momentum expansion of this amplitude and show that the
infinite tachyon/massless poles and the contact terms of this amplitude can be
reproduced by the tachyon DBI and the WZ actions, and by their higher
derivative corrections.Comment: 17 pages, no figure
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