arXiv subjects

Alex Buchel

Publications and source records attributed to Alex Buchel.

140 records · Page 8Linked to original sources

arXiv2002

Does the Tachyon Matter?

We study time-dependent solutions of Einstein-Maxwell gravity in four dimensions coupled to tachyon matter--the Dirac-Born-Infeld Lagrangian that provides an effective description of a decaying tachyon on an unstable D-brane in string theory. Asymptotically, the solutions are similar to the recently studied space-like brane solutions and carry S-brane charge. They do not break the Lorentzian R-symmetry. We study the tachyon matter as a probe in such a background and analyze its backreaction. For early/late times, the tachyon field has a constant energy density and vanishing pressure as in flat space. On the other hand, at intermediate times, the energy density of the tachyon diverges and produces a space-like curvature singularity.

hep-th↗

arXiv2002

On Time-dependent Backgrounds in Supergravity and String Theory

Time-dependent solutions of supergravity and string theory are studied. The examples are obtained from de Sitter deformation of gauge/gravity dualities, analytical continuation of static solutions, and ``exactly solvable'' worldsheet models. Among other things, it is shown that turning on a Hubble parameter in the background of a confining gauge theory in four dimensions can restore chiral symmetry. Some of the solutions obtained from analytical continuation have the interpretation of a universe with a bounce separating a big bang from a big crunch singularity. In the worldsheet context, it is argued why string propagation close to a Milne-type cosmological singularity might be physically non-singular.

hep-th↗

arXiv2002

Gauge/gravity correspondence in accelerating universe

We discuss time-dependent backgrounds of type IIB supergravity realizing gravitation duals of gauge theories formulated in de Sitter space-time as a tool of embedding de Sitter in a supergravity. We show that only the gravitational duals to non-conformal gauge theories are sensitive to a specific value of a Hubble parameter. We consider two nontrivial solutions of this type: a gravity dual to six-dimensional (1,1) little string theory, and to a four-dimensional cascading SU(N+M)xSU(N) supersymmetric gauge theory (related to fractional D3-branes on a singular conifold according to Klebanov et al), in accelerating universe. In both cases we argue that the IR singularity of the geometry is regulated by the expansion of the gauge theory background space-time.

hep-th↗

arXiv2001

On the thermodynamic instability of LST

The high energy thermodynamics of Little String Theory (LST) is known to be unstable. An unresolved question is whether the corresponding instability in LST holographic dual is of stringy or supergravity origin. We study UV thermodynamics of a large metric deformation of the LST dual realized (in the extremal case) by type IIB fivebranes wrapping a two-sphere of a resolved conifold, and demonstrate that the resulting black hole has negative specific heat. This explicitly shows that the high energy thermodynamic instability of the LST holographic dual is of the supergravity origin.

hep-th↗

arXiv2001

Comments on fractional instantons in N=2 gauge theories

N=1^* gauge theories are believed to have fractional instanton contributions in the confining vacua. D3 brane probe computations in gravitation dual of large-N N=2^* gauge theories point to the absence of such contributions in the low energy gauge dynamics. We study fractional instantons in N=2 SU(2) Yang-Mills theory from the field theoretical perspective. We present new solutions to the Seiberg-Witten SU(2) monodromy problem with the same perturbative asymptotic, a massless monopole and a dyon singularity on the moduli space, and fractional instanton corrections to N=2 prepotential in the semi-classical region of the moduli space. We show that fractional instantons lead to infinite monopole (dyon) condensate in mass deformed N=2 gauge theories.

hep-th↗

arXiv2001

Comments on supergravity dual of pure N=1 Super Yang Mills theory with unbroken chiral symmetry

Maldacena and Nunez [hep-th/0008001] identified a gravity solution describing pure N=1 Yang-Mills (YM) in the IR. Their (smooth) supergravity solution exhibits confinement and the U(1)_R chiral symmetry breaking of the dual YM theory, while the singular solution corresponds to the gauge theory phase with unbroken U(1)_R chiral symmetry. In this paper we discuss supersymmetric type IIB compactifications on resolved conifolds with torsion. We rederive singular background of [hep-th/0008001] directly from the supersymmetry conditions. This solution is the relevant starting point to study non-BPS backgrounds dual to the high temperature phase of pure YM. We construct the simplest black hole solution in this background. We argue that it has a regular Schwarzschild horizon and provides a resolution of the IR singularity of the chirally symmetric extremal solution as suggested in [hep-th/0011146].

hep-th↗

arXiv2001

Finite temperature resolution of the Klebanov-Tseytlin singularity

Naked singularities in the gravitational backgrounds dual to gauge theories can be hidden behind the black hole horizon. We present an exact black hole solution in the Klebanov-Tseytlin geometry [hep-th/0002159]. Our solution realizes Maldacena dual of the finite temperature N=1 duality cascade of [hep-th/0007191] above the temperature of the chiral symmetry breaking. We compare the Bekenstein-Hawking entropy of the black hole with the entropy of the SU(N+M)xSU(N) gauge theory.

hep-th↗

arXiv2000

Gauge Dual and Noncommutative Extension of an N=2 Supergravity Solution

We investigate some properties of a recent supergravity solution of Pilch and Warner, which is dual to the N=4 gauge theory softly broken to N=2. We verify that a D3-brane probe has the expected moduli space and its effective action can be brought to N=2 form. The kinetic term for the probe vanishes on an enhancon locus, as in earlier work on large-n N=2 theories, though for the Pilch-Warner solution this locus is a line rather than a ring. On the gauge theory side we find that the probe metric can be obtained from a perturbative one-loop calculation; this principle may be useful in obtaining the supergravity dual at more general points in the N=2 gauge theory moduli space. We then turn on a B-field, following earlier work on the N=4 theory, to obtain the supergravity dual to the noncommutative N=2 theory.

hep-th↗

arXiv1999

New S-dualities in N=2 supersymmetric SU(2) x SU(2) gauge theory

New S-dualities in a scale invariant N=2 gauge theory with SU(2) x SU(2) gauge group are derived from embeddings of the theory in two different larger asymptotically free theories. The true coupling space of the scale invariant theory is a 20-fold identification of the coupling space found in the M- and string-theory derivations of the low energy effective action, implying a larger S-duality group. Also, this coupling space is different from the naively expected direct product of two SL(2,Z) fundamental domains, as it contains a different topology of fixed points.

hep-th↗

arXiv1999

Anomaly Cancelations in Orientifolds with Quantized B Flux

We consider anomaly cancelations in Type IIB orientifolds on T^4/Z_N with quantized NS-NS sector background B-flux. For a rank b B-flux on T^4 (b is always even) and when N is even, the cancelation requires a 2^{b/2} multiplicity of states in the 59-open string sector. We identify the twisted sector R-R scalars and tensor multiplets which are involved in the Green-Schwarz mechanism. We give more details of the construction of these models and argue that consistency with the 2^{b/2} multiplicity of 59-sector states requires a modification of the relation between the open string 1-loop channel modulus and the closed string tree channel modulus in the 59-cylinder amplitudes.

hep-th↗

arXiv1998

The Nonperturbative Gauge Coupling of N=2 Supersymmetric Theories

We argue that the topology of the quantum coupling space and the low energy effective action on the Coulomb branch of scale invariant N=2 SU(n) gauge theories pick out a preferred nonperturbative definition of the gauge coupling up to non-singular holomorphic reparametrizations.

hep-th↗

arXiv1998

Deriving N=2 S-dualities from Scaling for Product Gauge Groups

S-dualities in scale invariant N=2 supersymmetric field theories with product gauge groups are derived by embedding those theories in asymptotically free theories with higher rank gauge groups. S-duality transformations on the couplings of the scale invariant theory follow from the geometry of the embedding of the scale invariant theory in the Coulomb branch of the asymptotically free theory.

hep-th↗

arXiv1996

Statistical Mechanics of Cracks: Thermodynamic Limit, Fluctuations, Breakdown, and Asymptotics of Elastic Theory

We study a class of models for brittle fracture: elastic theory models which allow for cracks but not for plastic flow. We show that these models exhibit, at all finite temperatures, a transition to fracture under applied load similar to that at a first order liquid-gas transition. We study this transition at low temperature for small tension. We discuss the appropriate thermodynamic limit of these theories: a large class of boundary conditions is identified for which the energy release for a crack becomes independent of the macroscopic shape of the material. Using the complex variable method in a two-dimensional elastic theory, we prove that the energy release in an isotropically stretched material due to the creation of an arbitrary curvy cut is the same to cubic order as the energy release for the straight cut with the same end points. We find the normal modes and the energy spectrum for crack shape fluctuations and for crack surface phonons, under a uniform isotropic tension. For small uniform isotropic tension in two dimensions we calculate the essential singularity associated with fracturing the material in a saddle point approximation including quadratic fluctuations. This singularity determines the lifetime of the material (half-life for fracture), and also determines the asymptotic divergence of the high-order corrections to the zero temperature elastic coefficients. We calculate the asymptotic ratio of the high-order elastic coefficients of the inverse bulk modulus and argue that the result is unchanged by nonlinearities --- the ratio of the high-order nonlinear terms are determined solely by the linear theory.

cond-mat.mtrl-sci↗

arXiv1996

Elastic Theory Has Zero Radius of Convergence

Nonlinear elastic theory studies the elastic constants of a material (such as Young's modulus or bulk modulus) as a power series in the applied load. The inverse bulk modulus K, for example depends on the compression P: $ {1/ K(P)} = c_0 + c_1 P + c_2 P^2 \cdots + c_n P^n + \cdots $. Elastic materials that allow cracks are unstable at finite temperature with respect to fracture under a stretching load; as a result, the above power series has zero radius of convergence and thus can at best be an asymptotic series. Considering thermal nucleation of cracks in a two-dimensional isotropic, linear--elastic material at finite temperature we compute the asymptotic form $ { c_{n+1}/ c_n}\to C n^{1/2}$ as $n \rightarrow \infty$. We present an explicit formula for $C$ as a function of temperature and material properties.

cond-mat↗