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M. M. Sheikh-Jabbari

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

At least 199 records · Page 11Linked to original sources
arXiv2001

Noncommutative Open String Theories and Their Dualities

The recently found non-critical open string theories is reviewed. These open strings, noncommutative open string theories (NCOS), arise as consistent quantum theories describing the low energy theory of D-branes in a background electric B-field in the critical limit. Focusing on the D3-brane case, we construct the most general (3+1) NCOS, which is described by four parameters. We study S and T -dualities of these theories and argue the existence of a U-duality group.

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arXiv2000

Pair Production by a Constant External Field in Noncommutative QED

In this paper we study QED on the noncommutative space in the constant electro-magnetic field background. Using the explicit solutions of the noncommutative version of Dirac equation in such background, we show that there are well-defined in and out -going asymptotic states and also there is a causal Green's function. We calculate the pair production rate in this case. We show that at tree level noncommutativity will not change the pair production and the threshold electric field. We also calculate the pair production rate considering the first loop corrections. In this case we show that the threshold electric field is decreased by the noncommutativity effects.

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arXiv2000

Strong Coupling Effects in non-commutative spaces from OM Theory and supergravity

We show that a four-parameter class of 3+1 dimensional NCOS theories can be obtained by dimensional reduction on a general 2-torus from OM theory. Compactifying two spatial directions of NCOS theory on a 2-torus, we study the transformation properties under the $SO(2,2;Z)$ T-duality group. We then discuss non-perturbative configurations of non-commutative super Yang-Mills theory. In particular, we calculate the tension for magnetic monopoles and (p,q) dyons and exhibit their six-dimensional origin, and construct a supergravity solution representing an instanton in the gauge theory. We also compute the potential for a monopole-antimonopole in the supergravity approximation.

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arXiv2000

Hydrogen Atom Spectrum and the Lamb Shift in Noncommutative QED

We have calculated the energy levels of the hydrogen atom and as well the Lamb shift within the noncommutative quantum electrodynamics theory. The results show deviations from the usual QED both on the classical and on the quantum levels. On both levels, the deviations depend on the parameter of space/space noncommutativity.

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arXiv2000

Noncommutative QED and Anomalous Dipole Moments

We study QED on noncommutative spaces, NCQED. In particular we present the detailed calculation for the noncommutative electron-photon vertex and show that the Ward identity is satisfied. We discuss that in the noncommutative case moving electron will show {\it electric} dipole effects. In addition, we work out the electric and magnetic dipole moments up to one loop level. For the magnetic moment we show that noncommutative electron has an intrinsic (spin independent) magnetic moment.

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arXiv2000

On Noncommutative Open String Theories

We investigate new compactifications of OM theory giving rise to a 3+1 dimensional open string theory with noncommutative $x^0$-$x^1$ and $x^2$-$x^3$ coordinates. The theory can be directly obtained by starting with a D3 brane with parallel (near critical) electric and magnetic field components, in the presence of a RR scalar field. The magnetic parameter permits to interpolate continuously between the $x^0$-$x^1$ noncommutative open string theory and the $x^2$-$x^3$ spatial noncommutative U(N) super Yang-Mills theory. We discuss $SL(2,Z)$ transformations of this theory. Using the supergravity description of the large $N$ limit, we also compute corrections to the quark-antiquark Coulomb potential arising in the NCOS theory.

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arXiv2000

Noncommutative SO(n) and Sp(n) Gauge Theories

We study the generalization of noncommutative gauge theories to the case of orthogonal and symplectic groups. We find out that this is possible, since we are allowed to define orthogonal and symplectic subgroups of noncommutative unitary gauge transformations even though the gauge potentials and gauge transformations are not valued in the orthogonal and symplectic subalgebras of the Lie algebra of antihermitean matrices. Our construction relies on an antiautomorphism of the basic noncommutative algebra of functions which generalizes the charge conjugation operator of ordinary field theory. We show that the corresponding noncommutative picture from low energy string theory is obtained via orientifold projection in the presence of a non-trivial NSNS B-field.

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arXiv2000

Discrete Symmetries (C,P,T) in Noncommutative Field Theories

In this paper we study the invariance of the noncmmutative gauge theories under C, P and T transformations. For the noncommutative space (when only the spatial part of $θ$ is non-zero) we show that NCQED is Parity invariant. In addition, we show that under charge conjugation the theory on noncommutative $R^4_θ$ is transformed to the theory on $R^4_{-θ}$, so NCQED is a CP violating theory. The theory remains invariant under time reversal if, together with proper changes in fields, we also change $θ$ by $-θ$. Hence altogether NCQED is CPT invariant. Moreover we show that the CPT invariance holds for general noncommutative space-time.

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arXiv2000

Noncommutative Super Yang-Mills Theories with 8 Supercharges and Brane Configurations

In this paper we consider D=4 NCSYM theories with 8 supercharges. We study these theories through a proper type IIA (and M-theory) brane configuration. We find the one loop beta function of these theories and show that there is an elliptic curve describing the moduli space of the theory, which is in principle the same as the curve for the commutative counter-part of our theory. We study some other details of the dynamics by means of this brane configuration.

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arXiv1999

Closed String Brane-Like States, Brane Bound States and Noncommutative Branes

We study the mass and different RR charge distributions of the BPS (p,p-2)-brane bound states in the closed string brane-like $σ$-model. We show that such brane bound states can be realized by introducing a constant B field in the closed string theory. In addition we show that the worldvolume coordinates of these brane bound states turn out to be noncommutative.

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arXiv1999

A Note on T-Duality, Open Strings in B-field Background and Canonical Transformations

In this paper we study T-duality for open strings ending on branes with non-zero B-field on them from the point of view of canonical transformations. For the particular case of type II strings on the two torus we show that the $Sl(2,Z)_N$ transformations can be understood as a sub-class of canonical transformations on the open strings in the B-field background.

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arXiv1999

On the Deformation of $Λ$-Symmetry in B-field Background

In this note we will show that the $Λ$ symmetry, namely the U(1) symmetry of the open string sigma model which relates the B-field and the U(1) gauge field of a brane to each other, is deformed to a noncommutative version in a constant B-field background.

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arXiv1999

Supergravity and Large N Noncommutative Field Theories

We consider systems of Dp branes in the presence of a nonzero B field. We study the corresponding supergravity solutions in the limit where the branes worldvolume theories decouple from gravity. These provide dual descriptions of large N noncommutative field theories. We analyse the phase structure of the theories and the validity of the different description. We provide evidence that in the presence of a nonzero B field the worldvolume theory of D6 branes decouples from gravity. We analyse the systems of M5 branes and NS5 branes in the presence of a nonzero C field and nonzero RR fields, respectively. Finally, we study the Wilson loops (surfaces) using the dual descriptions.

hep-th↗
arXiv1999

Boundary Conditions as Dirac Constraints

In this article we show that boundary conditions can be treated as Lagrangian and Hamiltonian constraints. Using the Dirac method, we find that boundary conditions are equivalent to an infinite chain of second class constraints which is a new feature in the context of constrained systems. Constructing the Dirac brackets and the reduced phase space structure for different boundary conditions, we show why mode expanding and then quantizing a field theory with boundary conditions is the proper way. We also show that in a quantized field theory subjected to the mixed boundary conditions, the field components are noncommutative.

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arXiv1999

One Loop Renormalizability of Supersymmetric Yang-Mills Theories on Noncommutative Two-Torus

We argue that Yang-Mills theory on noncommutative torus, expressed in the Fourrier modes, is described by a gauge theory in a usual commutative space, the gauge group being a generalization of the area-preserving diffeomorphisms to the noncommutative case. In this way, performing the loop calculation in this gauge theory in the continuum limit we show that this theory is {\it one loop renormalizable}, and discuss the UV and IR limits. The moduli space of the vacua of the noncommutative super Yang-Mills theories in (2+1) dimensions is discussed.

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arXiv1999

Noncommutative Geometry From Strings and Branes

Noncommutative torus compactification of Matrix model is shown to be a direct consequence of quantization of the open strings attached to a D-membrane with a non-vanishing background $B$ field. We calculate the BPS spectrum of such a brane system using both string theory results and DBI action. The DBI action leads to a new transformation property of the compactification radii under the $SL(2,Z)_N$ transformations.

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arXiv1999

Open Strings in a B-field Background as Electric Dipoles

Studying dynamics of open strings attached to a D2-brane in a NS two form field background, we find that these open strings act as dipoles of U(1) gauge field of the brane. This provides an string theoretic description of the flux modifications needed for the DBI action on noncommutative torus.

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