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Nathan Seiberg

Publications and source records attributed to Nathan Seiberg.

At least 109 records · Page 6Linked to original sources
arXiv2003

Superstrings in Graviphoton Background and N=1/2+3/2 Supersymmetry

Motivated by Ooguri and Vafa, we study superstrings in flat R^4 in a constant self-dual graviphoton background. The supergravity equations of motion are satisfied in this background which deforms the N=2 d=4 flat space super-Poincare algebra to another algebra with eight supercharges. A D-brane in this space preserves a quarter of the supercharges; i.e. N=1/2 supersymmetry is realized linearly, and the remaining N=3/2 supersymmetry is realized nonlinearly. The theory on the brane can be described as a theory in noncommutative superspace in which the chiral fermionic coordinates $θ^α$ of N=1 d=4 superspace are not Grassman variables but satisfy a Clifford algebra.

hep-th↗
arXiv2003

D-brane Decay in Two-Dimensional String Theory

We consider unstable D0-branes of two dimensional string theory, described by the boundary state of Zamolodchikov and Zamolodchikov [hep-th/0101152] multiplied by the Neumann boundary state for the time coordinate $t$. In the dual description in terms of the $c=1$ matrix model, this D0-brane is described by a matrix eigenvalue on top of the upside down harmonic oscillator potential. As suggested by McGreevy and Verlinde [hep-th/0304224], an eigenvalue rolling down the potential describes D-brane decay. As the eigenvalue moves down the potential to the asymptotic region it can be described as a free relativistic fermion. Bosonizing this fermion we get a description of the state in terms of a coherent state of the tachyon field in the asymptotic region, up to a non-local linear field redefinition by an energy-dependent phase. This coherent state agrees with the exponential of the closed string one-point function on a disk with Sen's marginal boundary interaction for $t$ which describes D0-brane decay.

hep-th↗
arXiv2003

Noncommutative Superspace, N=1/2 Supersymmetry, Field Theory and String Theory

We deform the standard four dimensional $\N=1$ superspace by making the odd coordinates $θ$ not anticommuting, but satisfying a Clifford algebra. Consistency determines the other commutation relations of the coordinates. In particular, the ordinary spacetime coordinates $x$ cannot commute. We study chiral superfields and vector superfields and their interactions. As in ordinary noncommutative field theory, a change of variables allows us to express the gauge interactions in terms of component fields which are subject to standard gauge transformation laws. Unlike ordinary noncommutative field theories, the change of the Lagrangian is a polynomial in the deformation parameter. Despite the deformation, the noncommutative theories still have an antichiral ring with all its usual properties. We show how these theories with precisely this deformation arise in string theory in a graviphoton background.

hep-th↗
arXiv2003

Chiral Rings and Phases of Supersymmetric Gauge Theories

We solve for the expectation values of chiral operators in supersymmetric U(N) gauge theories with matter in the adjoint, fundamental and anti-fundamental representations. A simple geometric picture emerges involving a description by a meromorphic one-form on a Riemann surface. The equations of motion are equivalent to a condition on the integrality of periods of this form. The solution indicates that all semiclassical phases with the same number of U(1) factors are continuously connected.

hep-th↗
arXiv2003

Phases of N=1 Supersymmetric Gauge Theories and Matrices

N=1 supersymmetric U(N) gauge theory with adjoint matter $Φ$ and a polynomial superpotential $\Tr W(Φ)$ has been much studied recently. The classical theory has several vacua labeled by integers $(N_1,N_2,...,N_k)$, with the classical unbroken gauge group $\prod_i U(N_i)$. Quantum mechanically, each classical vacuum leads to $\prod_i N_i$ different vacua. As the parameters of $W(Φ)$ are varied, these vacua change in a continuous (and holomorphic) fashion. We find that vacua associated with $(N_1,N_2,...,N_k)$ can be continuously transformed to vacua with $(\tilde N_1,\tilde N_2,...,\tilde N_k)$, thus leading to a new kind of duality. Traditional order parameters, like the Wilson loop and 't Hooft loop, sometimes distinguish different phases. We also find phases that are not distinguished by conventional order parameters. The whole picture of the phase diagram is reminiscent of the phase diagram of $M$-theory.

hep-th↗
arXiv2002

The Challenging Cosmic Singularity

In this talk we discuss the cosmic singularity. We motivate the need to correct general relativity in the study of singularities, and mention the kind of corrections provided by string theory. We review how string theory resolves time-like singularities with two examples. Then, a simple toy model with lightlike singularities is presented, and studied in classical string theory. It turns out that classical string theory cannot resolve these singularities, and therefore better understanding of the full quantum theory is needed. The implications of this result for the Ekpyrotic/Cyclic Model are discussed. We end by mentioning the known suggestions for explaining the cosmological singularity.

gr-qc↗
arXiv2002

Adding Fundamental Matter to ``Chiral Rings and Anomalies in Supersymmetric Gauge Theory''

We consider a supersymmetric U(N) gauge theory with matter fields in the adjoint, fundamental and anti-fundamental representations. As in the framework which was put forward by Dijkgraaf and Vafa, this theory can be described by a matrix model. We analyze this theory along the lines of [F. Cachazo, M. Douglas, N.S. and E. Witten, ``Chiral Rings and Anomalies in Supersymmetric Gauge Theory'' hep-th/0211170] and show the equivalence of the gauge theory and the matrix model. In particular, the anomaly equations in the gauge theory is identified with the loop equations in the matrix model.

hep-th↗
arXiv2002

Chiral Rings and Anomalies in Supersymmetric Gauge Theory

Motivated by recent work of Dijkgraaf and Vafa, we study anomalies and the chiral ring structure in a supersymmetric U(N) gauge theory with an adjoint chiral superfield and an arbitrary superpotential. A certain generalization of the Konishi anomaly leads to an equation which is identical to the loop equation of a bosonic matrix model. This allows us to solve for the expectation values of the chiral operators as functions of a finite number of ``integration constants.'' From this, we can derive the Dijkgraaf-Vafa relation of the effective superpotential to a matrix model. Some of our results are applicable to more general theories. For example, we determine the classical relations and quantum deformations of the chiral ring of $\N=1$ super Yang-Mills theory with SU(N) gauge group, showing, as one consequence, that all supersymmetric vacua of this theory have a nonzero chiral condensate.

hep-th↗
arXiv2002

Strings in Time-Dependent Orbifolds

We continue and extend our earlier investigation ``Strings in a Time-Dependent Orbifold'' (hep-th/0204168). We formulate conditions for an orbifold to be amenable to perturbative string analysis and classify the low dimensional orbifolds satisfying these conditions. We analyze the tree and torus amplitudes of some of these orbifolds. The tree amplitudes exhibit a new kind of infrared divergences which are a result of some ultraviolet effects. These UV enhanced IR divergences can be interpreted as due to back reaction of the geometry. We argue that for this reason the three dimensional parabolic orbifold is not amenable to perturbation theory. Similarly, the smooth four dimensional null-brane tensored with sufficiently few noncompact dimensions also appears problematic. However, when the number of noncompact dimensions is sufficiently large perturbation theory in these time dependent backgrounds seems consistent.

hep-th↗
arXiv2002

Strings in a Time-Dependent Orbifold

We consider string theory in a time dependent orbifold with a null singularity. The singularity separates a contracting universe from an expanding universe, thus constituting a big crunch followed by a big bang. We quantize the theory both in light-cone gauge and covariantly. We also compute some tree and one loop amplitudes which exhibit interesting behavior near the singularity. Our results are compatible with the possibility that strings can pass through the singularity from the contracting to the expanding universe, but they also indicate the need for further study of certain divergent scattering amplitudes.

hep-th↗
arXiv2002

From Big Crunch to Big Bang

We consider conditions under which a universe contracting towards a big crunch can make a transition to an expanding big bang universe. A promising example is 11-dimensional M-theory in which the eleventh dimension collapses, bounces, and re-expands. At the bounce, the model can reduce to a weakly coupled heterotic string theory and, we conjecture, it may be possible to follow the transition from contraction to expansion. The possibility opens the door to new classes of cosmological models. For example, we discuss how it suggests a major simplification and modification of the recently proposed ekpyrotic scenario.

hep-th↗
arXiv2001

D-brane Charges in Five-brane backgrounds

We discuss the discrete Z_k D-brane charges (twisted K-theory charges) in five-brane backgrounds from several different points of view. In particular, we interpret it as a result of a standard Higgs mechanism. We show that certain degrees of freedom (singletons) on the boundary of space can extend the corresponding Z_k symmetry to U(1). Related ideas clarify the role of AdS singletons in the AdS/CFT correspondence.

hep-th↗
arXiv2001

D-Brane Instantons and K-Theory Charges

We discuss some physical issues related to the K-theoretic classification of D-brane charges, putting an emphasis on the role of D-brane instantons. The relation to D-instantons provides a physical interpretation to the mathematical algorithm for computing K-theory known as the ``Atiyah-Hirzebruch spectral sequence.'' Conjecturally, a formulation in terms of D-instantons leads to a computationally useful formulation of K-homology in general. As an application and illustration of this viewpoint we discuss some issues connected with D-brane charges associated with branes in WZW models. We discuss the case of SU(3) in detail, and comment on the general picture of branes in SU(N), based on a recent result of M. Hopkins.

hep-th↗
arXiv2001

Geometrical interpretation of D-branes in gauged WZW models

We show that one can construct D-branes in parafermionic and WZW theories (and their orbifolds) which have very natural geometrical interpretations, and yet are not automatically included in the standard Cardy construction of D-branes in rational conformal field theory. The relation between these theories and their T-dual description leads to an analogy between these D-branes and the familiar A-branes and B-branes of N=2 theories.

hep-th↗
arXiv2000

A Note on Background Independence in Noncommutative Gauge Theories, Matrix Model and Tachyon Condensation

Using the construction of D-branes with nonzero $B$ field in the matrix model we give a physical interpretation of the known background independence in gauge theories on a noncommutative space. The background independent variables are identified as the degrees of freedom of the underlying matrix model. This clarifies and extends some recent results about the end point of tachyon condensation in D-branes with a $B$ field. We also explain the freedom in the description which is parametrized by a two form $Φ$ from the points of view of the noncommutative geometry on the worldvolume of the branes, and of the first quantized string theory.

hep-th↗
arXiv2000

OM Theory in Diverse Dimensions

Open string theories can be decoupled from closed strings and gravity by scaling to the critical electric field. We propose dual descriptions for the strong coupling limit of these NCOS (Non-Commutative Open String) theories in six or fewer spacetime dimensions. In particular, we conjecture that the five-dimensionsal NCOS theory at strong coupling, is a theory of light Open Membranes (OM), decoupled from gravity, on an M5-brane with a near-critical three-form field strength. The relation of OM theory to NCOS theories resembles that of M theory to Type II closed string theories. In two dimensions we conjecture that supersymmetric U($n$) gauge theory with a unit of electric flux is dual to the NCOS theory with string coupling $1 \over n$. A construction based on NS5-branes leads to new theories in six dimensions generalising the little string theory. A web of dualities relates all the above theories when they are compactified on tori.

hep-th↗
arXiv2000

Comments on Noncommutative Perturbative Dynamics

We analyze further the IR singularities that appear in noncommutative field theories on R^d. We argue that all IR singularities in nonplanar one loop diagrams may be interpreted as arising from the tree level exchanges of new light degrees of freedom, one coupling to each relevant operator. These exchanges are reminiscent of closed string exchanges in the double twist diagrams in open string theory. Some of these degrees of freedom are required to have propagators that are inverse linear or logarithmic. We suggest that these can be interpreted as free propagators in one or two extra dimensions respectively. We also calculate some of the IR singular terms appearing at two loops in noncommutative scalar field theories and find a complicated momentum dependence which is more difficult to interpret.

hep-th↗