arXiv subjects

M. M. Sheikh-Jabbari

Publications and source records attributed to M. M. Sheikh-Jabbari.

At least 163 records · Page 9Linked to original sources

arXiv2009

Tachyonic Resonance Preheating in Expanding Universe

In this paper the tachyonic resonance preheating generated from the bosonic trilinear $ϕχ^2$ interactions in an expanding Universe is studied. In $λϕ^4/4$ inflationary model the trilinear interaction, in contrast to the four-legs $ϕ^2χ^2$, breaks the conformal symmetry explicitly and the resonant source term becomes non-periodic, making the Floquet theorem inapplicable. We find that the occupation number of the produced $χ$-particles has a non-linear exponential growth with exponent $\sim x^{3/2}$, where $x$ is the conformal time. This should be contrasted with preheating from a periodic resonant source, arising for example from the four-legs $ϕ^2χ^2$ interaction, where the occupation number has a linear exponential growth. We present an analytic method to compute the interference term coming from phases accumulated in non-tachyonic scattering regions and show that the effects of the interference term causes ripples on $x^{3/2}$ curve, a result which is confirmed by numerical analysis. Studying the effects of back-reaction of the $χ$-particles, we show that tachyonic resonance preheating in our model can last long enough to transfer most of the energy from the background inflation field $ϕ$, providing an efficient model for preheating in the chaotic inflation models.

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arXiv2009

Can Seiberg-Witten Map Bypass Noncommutative Gauge Theory No-Go Theorem?

There are strong restrictions on the possible representations and in general on the matter content of gauge theories formulated on noncommutative Moyal spaces, termed as noncommutative gauge theory no-go theorem. According to the no-go theorem \cite{no-go}, matter fields in the noncommutative U(1) gauge theory can only have $\pm 1$ or zero charges and for a generic noncommutative $\prod_{i=1}^n U(N_i)$ gauge theory matter fields can be charged under at most two of the $U(N_i)$ gauge group factors. On the other hand, it has been argued in the literature that, since a noncommutative U(N) gauge theory can be mapped to an ordinary U(N) gauge theory via the Seiberg-Witten map, seemingly it can bypass the no-go theorem. In this note we show that the Seiberg-Witten map \cite{SW} can only be consistently defined and used for the gauge theories which respect the no-go theorem. We discuss the implications of these arguments for the particle physics model building on noncommutative space.

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arXiv2009

On Half-BPS States of the ABJM Theory

We analyze SU(2) invariant half-BPS states of the 3d \cN=8 or \cN=6 SCFT within the radial quantization of the ABJM theory, the theory proposed to describe N M2-branes in the R^3x C^4/Z_k background. After studying the classical moduli space of these configurations, we explicitly construct a set of gauge invariant operators involving 't Hooft monopole operators corresponding to these states. We show there is a one--to--one correspondence between the two sets carrying R-charge J and that they are labeled by Young tableaux of J boxes with a maximum of N rows. Restricting the full path integral to this half-BPS sector of the theory, we show the latter is described in terms of N fermions in a 2d harmonic potential in the sector of vanishing angular momentum. The same classification, though in the N to infinity limit, arise from the plane-wave (BMN) Matrix theory as well as the 11 dimensional LLM bubbling geometries, providing supportive evidence for the ABJM theory and/or the Matrix model.

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arXiv2008

A New Three-Algebra Representation for the {\cal N}=6 su(N)Xsu(N) Superconformal Chern-Simons Theory

Based on the realization of three-algebras in terms of algebra of matrices and four-brackets [arXiv:0807.1570] we present the notion of u(N)-based extended three-algebras, which for N=2 reproduces the Bagger-Lambert three-algebra. Using these extended three-algebras we construct an su(N)\times su(N) Chern-Simons action with explicit SO(8) invariance. The dynamical fields of this theory are eight complex valued bosonic and fermionic fields in the bi-fundamental representation of the su(N)\times su(N). For generic N the fermionic transformations, however, close only on a subclass of the states of this theory onto the 3d, {\cal N}=6 superalgebra. In this sector we deal with four complex valued scalars and fermions, our theory is closely related to the ABJM model [arXiv:0806.1218], and hence it can be viewed as the (low energy effective) theory of N M2-branes. We discuss that our three-algebra structure suggests a picture of open M2-brane stretched between any two pairs of M2-branes. We also analyze the BPS configurations of our model.

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arXiv2008

A Realization of the Cohen-Glashow Very Special Relativity

We show that the Cohen-Glashow Very Special Relativity (VSR) theory [1] can be realized as the part of the Poincaré symmetry preserved on a noncommutative Moyal plane with light-like noncommutativity. Moreover, we show that the three subgroups relevant to the Cohen-Glashow VSR can also be realized in the noncommutative space-time setting. For all these three cases the noncommutativity parameter $θ^{μν}$ should be light-like ($θ^{μν}θ_{μν}=0$). A fixed constant noncommutativity parameter respects the T(2) subgroup of Lorentz, while for the E(2) and SIM(2) cases the form of noncommutativity among the coordinates should be of linear (Lie algebra) and quadratic (quantum group) type, respectively. We discuss some physical implications of this realization of the Cohen-Glashow VSR.

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arXiv2008

An Arena for Model Building in the Cohen-Glashow Very Special Relativity

The Cohen-Glashow Very Special Relativity (VSR) algebra [arXiv:hep-ph/0601236] is defined as the part of the Lorentz algebra which upon addition of CP or T invariance enhances to the full Lorentz group, plus the space-time translations. We show that noncommutative space-time, in particular noncommutative Moyal plane, with light-like noncommutativity provides a robust mathematical setting for quantum field theories which are VSR invariant and hence set the stage for building VSR invariant particle physics models. In our setting the VSR invariant theories are specified with a single deformation parameter, the noncommutativity scale Λ_{NC}. Preliminary analysis with the available data leads to Λ_{NC}\gtrsim 1-10 TeV. This note is prepared for the Proceedings of the G27 Mathematical Physics Conference, Yerevan 2008, and is based on arXiv:0806.3699[hep-th].

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arXiv2008

Relaxed Three-Algebras: Their Matrix Representations and Implications for Multi M2-brane Theory

We argue that one can relax the requirements of the non-associative three-algebras recently used in constructing D=3, N=8 superconformal field theories, and introduce the notion of ``relaxed three-algebras''. We present a specific realization of the relaxed three-algebras in terms of classical Lie algebras with a matrix representation, endowed with a non-associative four-bracket structure which is prescribed to replace the three-brackets of the three-algebras. We show that both the so(4)-based solutions as well as the cases with non-positive definite metric find a uniform description in our setting. We discuss the implications of our four-bracket representation for the D=3, N=8 and multi M2-brane theory and show that our setup can shed light on the problem of negative kinetic energy degrees of freedom of the Lorentzian case.

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arXiv2008

Nearing Extremal Intersecting Giants and New Decoupled Sectors in N = 4 SYM

We study near-horizon limits of near-extremal charged black hole solutions to five-dimensional $U(1)^3$ gauged supergravity carrying two charges, extending the recent work of Balasubramanian et.al. We show that there are two near-horizon decoupling limits for the near-extremal black holes, one corresponding to the near-BPS case and the other for the far from BPS case. Both of these limits are only defined on the 10d IIB uplift of the 5d black holes, resulting in a decoupled geometry with a six-dimensional part (conformal to) a rotating BTZ X $S^3$. We study various aspects of these decoupling limits both from the gravity side and the dual field theory side. For the latter we argue that there should be two different, but equivalent, dual gauge theory descriptions, one in terms of the 2d CFT's dual to the rotating BTZ and the other as certain large R-charge sectors of d=4,N =4 U(N) SYM theory. We discuss new BMN-type sectors of the N=4 SYM in the $N\to\infty$ limit in which the engineering dimensions scale as $N^{3/2}$ (for the near-BPS case) and as $N^2$ (for the far from BPS case).

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arXiv2007

Electrified BPS Giants: BPS configurations on Giant Gravitons with Static Electric Field

We consider D3-brane action in the maximally supersymmetric type IIB plane-wave background. Upon fixing the light-cone gauge, we obtain the light-cone Hamiltonian which is manifestly supersymmetric. The 1/2 BPS solutions of this theory (solutions which preserve 16 supercharges) are either of the form of spherical three branes, the giant gravitons, or zero size point like branes. We then construct specific classes of 1/4 BPS solutions of this theory in which static electric field on the brane is turned on. These solutions are deformations about either of the two 1/2 BPS solutions. In particular, we study in some detail 1/4 BPS configurations with electric dipole on the three sphere giant, i.e. BIons on the giant gravitons, which we hence call BIGGons. We also study BPS configurations corresponding to turning on a background uniform constant electric field. As a result of this background electric field the three sphere giant is deformed to squashed sphere, while the zero size point like branes turn into circular or straight fundamental strings in the plane-wave background, with their tension equal to the background electric field.

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arXiv2007

Lovelock Gravity at the Crossroads of Palatini and Metric Formulations

We consider extensions of the Einstein-Hilbert Lagrangian to a general functional of metric and Riemann curvature tensor. A given such Lagrangian describes two different theories depending on considering connection and metric (Palatini formulation), or only the metric (metric formulation) as independent dynamical degrees of freedom. Equivalence of the Palatini and metric formulations at the level of equations of motion, which as we will argue is a manifestation of the Equivalence Principle, is the physical criterion that restricts form of the Lagrangians of modified gravity theories. We prove that within the class of modified gravity theories we consider, only the Lovelock gravity satisfies this requirement.

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arXiv2007

An N-tropic Solution to the Cosmological Constant Problem

Based on the assertion that the cosmological constant problem is essentially a quantum gravity problem, the framework which addresses the cosmological constant problem should also bear a picture for the ``quantum space-time''. In this talk in an attempt to address the cosmological constant problem I suggest to start with noncommutative fuzzy spheres as the toy model for the quantum space-time. In this setting, we show that the cosmological constant problem may be resolved due to the noncommutativity and ``fuzziness'' of the space and the fact that the smallest volume which could be measured in the a quantum space-time is much larger than the naively expected Planckian size. This talk is based on Ref.[1] which has appeared on the arXiv as hep-th/0605110.

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arXiv2007

Non-Abelian Magnetized Blackholes and Unstable Attractors

Fluctuations of non-Abelian gauge fields in a background magnetic flux contain tachyonic modes and hence the background is unstable. We extend these results to the cases where the background flux is coupled to Einstein gravity and show that the corresponding spherically symmetric geometries, which in the absence of a cosmological constant are of the form of Reissner-Nordstrom blackholes or the AdS_2xS^2, are also unstable. We discuss the relevance of these instabilities to several places in string theory including various string compactifications and the attractor mechanism. Our results for the latter imply that the attractor mechanism shown to work for the extremal Abelian charged blackholes, cannot be applied in a straightforward way to the extremal non-Abelian colored blackholes.

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arXiv2006

Stabilization of Compactification Volume In a Noncommutative Mini-Super-Phase-Space

We consider a class of generalized FRW type metrics in the context of higher dimensional Einstein gravity in which the extra dimensions are allowed to have different scale factors. It is shown that noncommutativity between the momenta conjugate to the internal space scale factors controls the power-law behavior of the scale factors in the extra dimensions, taming it to an oscillatory behavior. Hence noncommutativity among the internal momenta of the mini-super-\emph{phase}-space can be used to explain stabilization of the compactification volume of the internal space in a higher dimensional gravity theory.

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arXiv2006

Tiny Graviton Matrix Theory/SYM Correspondence: Analysis of BPS States

In this paper we continue analysis of the Matrix theory describing the DLCQ of type IIB string theory on AdS_5 x S^5 (and/or the plane-wave) background, i.e. the Tiny Graviton Matrix Theory (TGMT)[hep-th/0406214]. We study and classify 1/2, 1/4 and 1/8 BPS solutions of the TGMT which are generically of the form of rotating three brane giants. These are branes whose shape are deformed three spheres and hyperboloids. In lack of a classification of such ten dimensional type IIb supergravity configurations, we focus on the dual N=4 four dimensional 1/2, 1/4 and one 1/8 BPS operators and show that they are in one-to-one correspondence with the states of the same set of quantum numbers in TGMT. This provides further evidence in support of the Matrix theory.

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arXiv2006

Inherent Holography In Fuzzy Spaces and An N-tropic Approach To The Cosmological Constant Problem

In this letter we show that the noncommutative spaces, and in particular fuzzy spheres, are natural candidates which explicitly exhibit the holography, by noting that the smallest physically accessible volume is much larger that the expected Planckian size. Moreover, we show that fuzzy spheres provide us with a new approach, ''an $N$-tropic'' approach, to the cosmological constant problem, though in a Euclidean space-time.

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arXiv2006

On Classification Of The Bubbling Geometries

In this paper we classify the ten dimensional half BPS solutions of the type IIB supergravity which have SO(4) X SO(4) X U(1) isometry found by Lin-Lunin-Maldacena (LLM). Our classification is based on their asymptotic behavior and causal structure according which they fall into two classes: 1) those with R X S^3 boundary and 2) those with one dimensional light-like boundary. Each class can be divided into some subclasses depending on the asymptotic characteristics of the solutions, which in part specify the global charges defining the geometry. We analyze each of these classes in some detail and elaborate on their dual gauge theory description. In particular, we show that the Matrix Chern-Simons theory which is the gauge theory dual to the LLM geometries, can be obtained as the effective theory of spherical threebrane probes in the half BPS sector.

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arXiv2005

Extensions of AdS_5 x S^5 and the Plane-wave Superalgebras and Their Realization in the Tiny Graviton Matrix Theory

In this paper we consider all consistent extensions of the AdS_5 x S^5 superalgebra, psu(2,2|4), to incorporate brane charges by introducing both bosonic and fermionic (non)central extensions. We study the Inonu-Wigner contraction of the extended psu(2,2|4) under the Penrose limit to obtain the most general consistent extension of the plane-wave superalgebra and compare these extensions with the possible BPS (flat or spherical) brane configurations in the plane-wave background. We give an explicit realization of some of these extensions in terms of the Tiny Graviton Matrix Theory (TGMT)[hep-th/0406214] which is the 0+1 dimensional gauge theory conjectured to describe the DLCQ of strings on the AdS_5 x S^5 and/or the plane-wave background.

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arXiv2005

Noncommutativity of Space and Rotation of Polarization of Light in a Background Magnetic Field

Recently the PVLAS collaboration has reported the observation of rotation of polarization of light propagating in a background magnetic field. In this letter we explore the possibility that such a rotation is a result of noncommutativity in the background space-time. To explain the reported polarization rotation within noncommutative QED we need the noncommutativity parameter $θ\simeq (30 {\rm GeV})^{-2}$.

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