arXiv subjects

Mohammad R. Garousi

Publications and source records attributed to Mohammad R. Garousi.

At least 109 records · Page 6Linked to original sources

arXiv2007

On Wess-Zumino terms of Brane-Antibrane systems

We calculate the disk level S-matrix element of one closed string RR field, two open string tachyons and one gauge field in type II superstring theory. An expansion for the S-matrix element has been found that its four leading order terms are reproduced exactly by the symmetric trace tachyon DBI and the Wess-Zumino actions of D-brane-anti-D-brane systems. Using this consistency, we have also found the first higher derivative correction to the some of the WZ terms.

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arXiv2007

Entropy Function for Non-extremal D1D5 and D2D6NS5-branes

We apply the entropy function formalism to non-extremal D1D5 and D2D6NS5-branes whose throat approximation is given by the Schwarzschild black hole in AdS_3\times S^3\times T^4 and AdS_3\times S^2\times S^1\times T^4, respectively. We find the Bekenstein-Hawking entropy and the (alpha')^3R^4 corrections from the value of the entropy function at its saddle point. While the higher derivative terms have no effect on the temperature, they decrease the value of the entropy.

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arXiv2007

On Attractor Mechanism and Entropy Function for Non-extremal Black Holes/Branes

We examine in details the entropy function formalism for non-extremal D3, M2, and M5-branes that their throat approximation is given by Schwarzschild black hole in AdS_{p+2} \times S^{D-(p+2)}. We show that even though there is no attractor mechanism in the non-extremal black holes/branes, the entropy function formalism does work and the entropy is given by the entropy function at its saddle point.

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arXiv2007

Superstring Scattering from O-planes

We write the vertex operators of massless NS-NS and RR states of Type II superstring theory in the presence of Orientifold p-planes. They include the usual vertex operators of Type II theory and their images. We then calculate the two-point functions of these vertex operators at the projective plane PR_2 level. We show that the result can be written in the Veneziano-type formulae, with the same kinematic factor that appears in the D_p-branes amplitudes. While the scattering amplitudes with the usual vertex operators are not gauge invariant, the above amplitudes are invariant. From the amplitude describing scattering of two NS-NS states off the O-plane, we find the low energy effective action of O-planes. The result shows a relative factor 2^{p-6} between couplings to O-planes and to D-branes at (α')^2 order.

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arXiv2006

Non-abelian expansion of S-matrix elements and non-abelian tachyon DBI action

We apply the prescription proposed in hep-th/0307197 for non-abelian expansion of S-matrix elements, to the S-matrix element of four tachyons and one gauge field in superstring theory. We show that the leading order terms of the expansion are in perfect agreement with the non-abelian generalization of the tachyon DBI action in which the tachyon potential is $V(T)=1+πα' m^2T^2+\frac{1}{2!}(πα' m^2T^2)^2+...$ where $m^2=-1/(2α')$ is the mass of tachyon. This calculation fixes the coefficient of $T^4$ in the potential without on-shell ambiguity.

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arXiv2006

Tachyon Tunnelling in D-brane-anti-D-brane

Using the tachyon DBI action proposal for the effective theory of non-coincident D$_p$-brane-anti-D$_p$-brane system, we study the decay of this system in the tachyon channel. We assume that the branes separation is held fixed, i.e. no throat formation, and then find the bounce solution which describe the decay of the system from false to the true vacuum of the tachyon potential. We shall show that due to the non-standard form of the kinetic term in the effective action, the thin wall approximation for calculating the bubble nucleation rate gives a result which is independent of the branes separation. This unusual result might indicate that the true decay of this metastable system should be via a solution that represents a throat formation as well as the tachyon tunneling.

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arXiv2005

Closed String S-matrix Elements in Open String Field Theory

We study the S-matrix elements of the gauge invariant operators corresponding to on-shell closed strings, in open string field theory. In particular, we calculate the tree level S-matrix element of two ${\it arbitrary}$ closed strings, and the S-matrix element of one closed string and two open strings. By mapping the world-sheet of these amplitudes to the upper half $z$-plane, and by evaluating explicitly the correlators in the ghost part, we show that these S-matrix elements are ${\it exactly}$ identical to the corresponding disk level S-matrix elements in perturbative string theory.

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arXiv2005

What is needed of a tachyon if it is to be the dark energy?

We study a dark energy scenario in the presence of a tachyon field $ϕ$ with potential $V(ϕ)$ and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity $λ=-M_pV_ϕ/V^{3/2}$. If $λ$ is a constant, which corresponds to an inverse square potential $V(ϕ) \propto ϕ^{-2}$, there exists one stable critical point that gives an acceleration of the universe at late times. When $λ\to 0$ asymptotically, we can have a viable dark energy scenario in which the system approaches an ``instantaneous'' critical point that dynamically changes with $λ$. If $|λ|$ approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where $|λ|$ is smaller than of order unity.

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arXiv2005

D-brane anti-D-brane effective action and brane interaction in open string channel

We construct the effective action of a $D_p$-brane-anti-$D_p$-brane system by making use of the non-abelian extension of tachyonic DBI action. We succeed the construction by restricting the Chan-Paton factors of two non-BPS $D_p$-branes in the action to the Chan-Paton factors of a $D_p\bar{D}_p$ system. For the special case that both branes are coincident, the action reduces to the one proposed by A. Sen. \\The effective $D_p\bar{D}_p$ potential indicates that when branes separation is larger than the string length scale, there are two minima in the tachyon direction. As branes move toward each other under the gravitational force, the tachyon tunneling from false to true vacuum may make a bubble formation followed by a classical evolution of the bubble. On the other hand, when branes separation is smaller than the string length scale, the potential shows one maximum and one minimum. In this case, a homogeneous tachyon rolling in real time makes an attractive potential for the branes distance. This classical force is speculated to be the effective force between the two branes.

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arXiv2004

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 $\tilde{z} \lesssim 3$. 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.

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arXiv2004

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 $ϕ$ 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 $ϕ$, 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.

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arXiv2004

Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua

We investigate a string-inspired scenario associated with a rolling massive scalar field on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the massive scalar field on the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of the warp factor of the anti-D3 brane, it is possible to obtain the required level of amplitude of density perturbations. We study the spectra of scalar and tensor perturbations generated during the rolling scalar inflation and show that our scenario satisfies the observational constraint coming from the Cosmic Microwave Background anisotropies and other observational data. We also implement the negative cosmological constant arising from the stabilization of the modulus fields in the KKLT vacua and find that this leads to a successful reheating in which the energy density of the scalar field effectively scales as a pressureless dust. The present dark energy can be also explained in our scenario provided that the potential energy of the massive rolling scalar does not exactly cancel with the amplitude of the negative cosmological constant at the potential minimum.

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arXiv2004

S-matrix elements and covariant tachyon action in type 0 theory

We evaluate the sphere level S-matrix element of two tachyons and two massless NS states, the S-matrix element of four tachyons, and the S-matrix element of two tachyons and two Ramon-Ramond vertex operators, in type 0 theory. We then find an expansion for theses amplitudes that their leading order terms correspond to a covariant tachyon action. To the order considered, there are no $T^4$, $T^2(\prt T)^2$, $T^2H^2$, nor $T^2R$ tachyon couplings, whereas, the tachyon couplings $F\bF T$ and $T^2F^2$ are non-zero.

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arXiv2003

S-matrix elements and off-shell tachyon action with non-abelian gauge symmetry

We propose that there is a unique expansion for the string theory S-matrix elements of tachyons that corresponds to non-abelian tachyon action. For those S-matrix elements which, in their expansion, there are the Feynman amplitudes resulting from the non-abelian kinetic term, we give a prescription on how to find the expansion. The gauge invariant action is an $α'$ expanded action, and the tachyon mass $m$ which appears as coefficient of many different couplings, is arbitrary. We then analyze in details the S-matrix element of four tachyons and the S-matrix element of two tachyons and two gauge fields, in both bosonic and superstring theories, in favor of this proposal. In the superstring theory, the leading terms of the non-abelian gauge invariant couplings are in agreement with the symmetrised trace of the direct non-abelian generalization of the tachyonic Born-Infeld action in which the tachyon potential is consistent with $V(T)=e^{πα' m^2T^2}$. In the bosonic theory, on the other hand, the leading terms are those appear in superstring case as well as some other gauge invariant couplings which spoils the symmetrised trace prescription. These latter terms are zero in the abelian case.

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arXiv2003

Slowly varying tachyon and tachyon potential

We show that the scattering amplitude of four open string scalars or tachyons on the world-volume of a D$_p$-brane in the bosonic string theory can be written in a universal form. The difference between this amplitude and the corresponding amplitude in the superstring theory is in an extra tachyonic pole. We show that in an $α'$ expansion and for slowly varying fields, the amplitude is consistent with the tachyonic DBI action in which the even part of the tachyon potential is $V(T)=e^{-(\sqrtπ T/α)^2}$ with $α=1$ for bosonic theory and $α=\sqrt{2}$ for superstring theory.

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arXiv2003

Off-shell extension of S-matrix elements and tachyonic effective actions

We show that the on-shell S-matrix elements of four open string massless scalars, two scalars and two tachyons, and four open string tachyons in the super string theory can be written in a unique form. We then propose an off-shell extension for the S-matrix element of four scalars which is consistent, in the low energy limit, with the Dirac-Born-Infeld effective action. Using a similar off-shell extension for the S-matrix element of two scalars and two tachyons and for the S-matrix element of four tachyons, we show that they are fully consistent with the tachyonic DBI action.

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arXiv2003

On-shell S-matrix and tachyonic effective actions

We evaluate various disk level four-point functions involving the massless scalar and tachyon vertex operators in the presence of background B-flux in superstring theory. By studying these amplitudes in specific limits, we find couplings of two scalars with two tachyons, and couplings of four tachyons on the world-volume of non-BPS D-branes of superstring theory. They are fully consistent with the non-commutative tachyonic Dirac-Born-Infeld effective action. They also fix the coefficient of $T^4$ term in the expansion of the tachyon potential around its maximum.

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arXiv2002

Gauge Invariant Operators and Closed String Scattering in Open String Field Theory

Using the recent proposal for the observables in open string field theory, we explicitly compute the coupling of closed string tachyon and massless states with the open string states up to level two. Using these couplings, we then calculate the tree level S-matrix elements of two closed string tachyons or two massless states in the open string field theory. Up to some contact terms, the results reproduce exactly the corresponding amplitudes in the bosonic string theory.

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