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Evgeny I. Buchbinder

Publications and source records attributed to Evgeny I. Buchbinder.

At least 55 records · Page 3Linked to original sources
arXiv2009

The Fate of the Sound and Diffusion in Holographic Magnetic Field

It was shown in arXiv:0810.4094 [hep-th] that in the presence of the magnetic field the sound waves in (2+1) dimensional plasma disappear and are replaces by a diffusive mode. Similarly, the shear and charge diffusion fluctuations form a subdiffusive mode. However, since the limit of small magnetic field does not commute with the hydrodynamic limit it is not obvious whether or not these modes are stable under higher order corrections. Using AdS/CFT correspondence we show that in the case of M2-brane plasma these modes do exist as we find the corresponding supergravity solutions. This allowed us to compute the conductivity and the shear viscosity to all orders in magnetic field. We find that the viscosity to entropy ratio saturates the Kovtun-Son-Starinets bound. This extends the universality property of the shear viscosity to the case of the strongly coupled plasma in external magnetic field.

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arXiv2008

Sound Waves in (2+1) Dimensional Holographic Magnetic Fluids

We use the AdS/CFT correspondence to study propagation of sound waves in strongly coupled (2+1) dimensional conformal magnetic fluids. Our computation provides a nontrivial consistency check of the viscous magneto-hydrodynamics of Hartnoll-Kovtun-Muller-Sachdev to leading order in the external field. Depending on the behavior of the magnetic field in the hydrodynamic limit, we show that it can lead to further attenuation of sound waves in the (2+1) dimensional conformal plasma, or reduce the speed of sound. We present both field theory and dual supergravity descriptions of these phenomena. While to the leading order in momenta the dispersion of the sound waves obtained from the dual supergravity description agrees with the one predicted from field theory, we find a discrepancy at higher order. This suggests that further corrections to HKMS magneto-hydrodynamics are necessary.

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arXiv2008

Dynamically SUSY Breaking SQCD on F-Theory Seven-Branes

We study how dynamically breaking SQCD can be obtained on two intersecting seven-branes in F-theory. In the mechanism which we present in this paper one of the seven-branes is responsible for producing the low-energy gauge group and the other one is for generating vector bundle moduli. The fundamental matter charged under the gauge group is localized on the intersection. The mass of the matter fields is controlled by the vector bundle moduli. The analysis of under what conditions a sufficient number of the fundamental flavors becomes light turns out to be equivalent to the analysis of non-perturbative superpotentials for vector bundle moduli in Heterotic M-theory. We give an example in which we present an explicit equation in the moduli space whose zero locus corresponds to the fundamental fields becoming light. This allows us to provide a local F-theory realization of massive ${\cal N}=1$, $SU(N_c)$ SQCD in the free magnetic range which dynamically breaks supersymmetry.

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arXiv2008

Non-Gaussianities in New Ekpyrotic Cosmology

New Ekpyrotic Cosmology is an alternative scenario of the early universe which relies on a phase of slow contraction before the big bang. We calculate the 3-point and 4-point correlation functions of primordial density perturbations and find a generically large non-Gaussian signal, just below the current sensitivity level of CMB experiments. This is in contrast with slow-roll inflation, which predicts negligible non-Gaussianity. The model is also distinguishable from alternative inflationary scenarios that yield large non-Gaussianity, such as DBI inflation and the simplest curvaton-like models, through the shape dependence of the correlation functions. Non-Gaussianity therefore provides a distinguishing and testable prediction of New Ekpyrotic Cosmology.

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arXiv2007

Holographic Gauge Theories in Background Fields and Surface Operators

We construct a new class of supersymmetric surface operators in N=4 SYM and find the corresponding dual supergravity solutions. We show that the insertion of the surface operator - which is given by a WZW model supported on the surface - appears by integrating out the localized degrees of freedom along the surface which arise microscopically from a D3/D7 brane intersection. Consistency requires constructing N=4 SYM in the D7 supergravity background and not in flat space. This enlarges the class of holographic gauge theories dual to string theory backgrounds to gauge theories in non-trivial supergravity backgrounds. The dual Type IIB supergravity solutions we find reveal - among other features - that the holographic dual gauge theory does indeed live in the D7-brane background.

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arXiv2007

New Ekpyrotic Cosmology

In this paper, we present a new scenario of the early Universe that contains a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy condition without developing any ghost-like instabilities. Thus the contracting universe goes through a non-singular bounce and evolves smoothly into the expanding post big bang phase. The curvature perturbation acquires a scale-invariant spectrum well before the bounce in this scenario. It is sourced by the scale-invariant entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism recently proposed by Lehners et al. Since the background geometry is non-singular at all times, the curvature perturbation remains nearly constant on super horizon scales. It emerges from the bounce unscathed and imprints a scale-invariant spectrum of density fluctuations in the matter-radiation fluid at the onset of the hot big bang phase. The ekpyrotic potential can be chosen so that the spectrum has a ``red'' tilt, in accordance with the recent data from WMAP. As in the original Ekpyrotic scenario, the model predicts a negligible gravity wave signal on all observable scales. As such ``New Ekpyrotic Cosmology" provides a consistent and distinguishable alternative to inflation to account for the origin of the seeds of large scale structure.

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arXiv2007

On the Initial Conditions in New Ekpyrotic Cosmology

New Ekpyrotic Cosmology is an alternative scenario of early universe cosmology in which the universe existed before the big bang. The simplest model relies on two scalar fields, whose entropy perturbation leads to a scale-invariant spectrum of density fluctuations. The ekpyrotic solution has a tachyonic instability along the entropy field direction which, a priori, appears to require fine-tuning of the initial conditions. In this paper, we show that these can be achieved naturally by adding a small positive mass term for the tachyonic field and coupling to light fermions. Then, for a wide range of initial conditions, the tachyonic field gets stabilized with the appropriate values well before the onset of the ekpyrotic phase. Furthermore, we show that ekpyrotic theory is successful in solving the flatness, horizon and homogeneity problems of standard big bang cosmology. Motivated by the analysis of the tachyonic instability, we propose a simplification of the model in terms of new field variables. Instead of requiring two exponential potentials, one for each scalar field, it suffices to consider a single nearly exponential potential for one of the fields and a tachyonic mass term along the orthogonal direction in field space. All other terms in the potential are essentially arbitrary. This greatly widens the class of ekpyrotic potentials and allows substantial freedom in determining the spectral index and possible non-Gaussianity. We present a generalized expression for the spectral index which is easily consistent with the observed red tilt. We also argue that for a wide range of potentials non-Gaussianity can be substantial, within the reach of current observations.

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arXiv2007

Infrared Limit of Gluon Amplitudes at Strong Coupling

In this note, we propose that the infrared structure of gluon amplitudes at strong coupling can be fully extracted from a local consideration near cusps. This is consistent with field theory and correctly reproduces the infrared divergences of the four-gluon amplitude at strong coupling calculated recently by Alday and Maldacena.

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arXiv2006

Towards Realizing Dynamical SUSY Breaking in Heterotic Model Building

We study a new mechanism to dynamically break supersymmetry in the E8xE8 heterotic string. As discussed recently in the literature, a long-lived, meta-stable non-supersymmetric vacuum can be achieved in an N=1 SQCD whose spectrum contains a sufficient number of light fundamental flavors. In this paper, we present, within the context of the hidden sector of the weakly and strongly coupled heterotic string, a slope-stable, holomorphic vector bundle on a Calabi-Yau threefold for which all matter fields are in the fundamental representation and are massive at generic points in moduli space. It is shown, however, that near certain subvarieties in the moduli space a sufficient number of light matter fields can occur, providing an explicit heterotic model realizing dynamical SUSY breaking. This is demonstrated for the low-energy gauge group Spin(10). However, our methods immediately generalize to Spin(Nc), SU(Nc), and Sp(Nc), for a wide range of color index Nc. Moduli stabilization in vacua with a positive cosmological constant is briefly discussed.

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arXiv2006

Dynamical SUSY Breaking in Heterotic M-Theory

It is shown that four-dimensional N=1 supersymmetric QCD with massive flavors in the fundamental representation of the gauge group can be realized in the hidden sector of E8xE8 heterotic string vacua. The number of flavors can be chosen to lie in the range of validity of the free-magnetic dual, using which one can demonstrate the existence of long-lived meta-stable non-supersymmetric vacua. This is shown explicitly for the gauge group Spin(10), but the methods are applicable to Spin(Nc), SU(Nc) and Sp(Nc) for a wide range of color index Nc. Hidden sectors of this type can potentially be used as a mechanism to break supersymmetry within the context of heterotic M-theory.

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arXiv2006

Stable and Metastable Cosmic Strings in Heterotic M-Theory

We address the question of finding stable and metastable cosmic strings in quasi-realistic heterotic M-theory compactifications with stabilized moduli. According to Polchinski's conjecture, the only stable strings in the absence of massless fields are Aharonov-Bohm strings. Such strings could potentially be created in heterotic compactifications as bound states of open membranes, five-branes wrapped on four-cycles and solitonic strings. However, in generic compactifications, the process of moduli stabilization can conflict production of Aharanov-Bohm strings. In this case, heterotic cosmic strings will have to be unstable under breakage on monopoles. We estimate the monopole masses and find that they are big enough so that the strings can be metastable with a sufficiently long lifetime. On the other hand, if we allow one or more axions to remain massless at low energies, stable global strings can be produced.

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arXiv2005

The Dynamics of Small Instanton Phase Transitions

The small instanton transition of a five-brane colliding with one end of the S1/Z2 interval in heterotic M-theory is discussed, with emphasis on the transition moduli, their potential function and the associated non-perturbative superpotential. Using numerical methods, the equations of motion of these moduli coupled to an expanding Friedmann-Robertson-Walker spacetime are solved including non-perturbative interactions. It is shown that the five-brane collides with the end of the interval at a small instanton. However, the moduli then continue to evolve to an isolated minimum of the potential, where they are trapped by gravitational damping. The torsion free sheaf at the small instanton is ``smoothed out'' into a vector bundle at the isolated minimum, thus dynamically completing the small instanton phase transition. Radiative damping at the origin of moduli space is discussed and shown to be insufficient to trap the moduli at the small instanton point.

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arXiv2005

On Open Membranes, Cosmic Strings and Moduli Stabilization

We discuss how cosmic strings can be created in heterotic M-theory compactifications with stable moduli. We conclude that the only appropriate candidates seem to be fundamental open membranes with a small length. In four dimensions they will appear as strings with a small tension. We make an observation that, in the presence of the vector bundle moduli, it might be possible to stabilize a five-brane very close to the visible sector so that a macroscopic open membrane connecting this five-brane and the visible brane will have a sufficiently small length. We also discuss how to embed such cosmic strings in heterotic models with stable moduli and whether they can be created after inflation.

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arXiv2005

Two-Loop Amplitudes of Gluons and Octa-Cuts in N=4 Super Yang-Mills

After reduction techniques, two-loop amplitudes in N=4 super Yang-Mills theory can be written in a basis of integrals containing scalar double-box integrals with rational coefficients, though the complete basis is unknown. Generically, at two loops, the leading singular behavior of a scalar double box integral with seven propagators is captured by a hepta-cut. However, it turns out that a certain class of such integrals has an additional propagator-like singularity. One can then formally cut the new propagator to obtain an octa-cut which localizes the cut integral just as a quadruple cut does at one-loop. This immediately gives the coefficient of the scalar double box integral as a product of six tree-level amplitudes. We compute, as examples, several coefficients of the five- and six-gluon non-MHV two-loop amplitudes. We also discuss possible generalizations to higher loops.

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arXiv2005

Five-Brane Dynamics and Inflation in Heterotic M-Theory

Generic heterotic M-theory compactifications contain five-branes wrapping non-isolated genus zero or higher genus curves in a Calabi-Yau threefold. Non-perturbative superpotentials do not depend on moduli of such five-branes.We show that fluxes and non-perturbative effects can stabilize them in a non-supersymmetric AdS vacuum. We also show that these five-branes can be stabilized in a dS vacuum, if we modify the supergravity potential energy by Fayet-Iliopoulos terms. This allows us to stabilize all heterotic M-theory moduli in a dS vacuum in the most general compactification scenarios. In addition, we demonstrate that, by this modification, one can create an inflationary potential. The inflationary phase is represented by a five-brane approaching the visible brane. We give a qualitative argument how extra states becoming light, when the five-brane comes too close, can terminate inflation. Eventually, the five-brane hits the visible brane and disappears through a small instanton transition. The post-inflationary system of moduli has simpler stability properties. It can be stabilized in a dS vacuum with a small cosmological constant.

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arXiv2004

Instanton Moduli in String Theory

Expressions for the number of moduli of arbitrary SU(n) vector bundles constructed via Fourier-Mukai transforms of spectral data over Calabi- Yau threefolds are derived and presented. This is done within the context of simply connected, elliptic Calabi-Yau threefolds with base Fr, but the methods have wider applicability. The condition for a vector bundle to possess the minimal number of moduli for fixed r and n is discussed and an explicit formula for the minimal number of moduli is presented. In addition, transition moduli for small instanton phase transitions involving non-positive spectral covers are defined, enumerated and given a geometrical interpretation.

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arXiv2004

Raising Anti de Sitter Vacua to de Sitter Vacua in Heterotic M-Theory

We explore the possibility of obtaining de Sitter vacua in strongly coupled heterotic models by adding various corrections to the supergravity potential energy. We show that, in a generic compactification scenario, Fayet-Iliopoulos terms can generate a de Sitter vacuum. The cosmological constant in this vacuum can be fine tuned to be consistent with observation. We also study moduli potentials in non-supersymmetric compactifications of $E_8 \times E_8$ theory with anti five-branes and $E_8 \times \bar E_8$ theory. We argue that they can be used to create a de Sitter vacuum only if some of the Kahler structure moduli are stabilized at values much less than the Calabi-Yau scale.

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