Publications (95)414.96 Total impact
 [Show abstract] [Hide abstract] ABSTRACT: A very famous result of gauge/gravity duality is the universality of the ratio of shear viscosity to entropy density in every field theory holographically dual to classical, twoderivative (Einstein) gravity. We present a way to obtain deviation form this universality by breaking the rotational symmetry spontaneously. In anisotropic fluids additional shear modes exist and their corresponding shear viscosities may be nonuniversal. We confirm this by explicitly calculating the shear viscosities in a transversely isotropic background, a pwave superfluid, and study its critical behavior. This is a first decisive step towards further applications of gauge/gravity duality to physical systems.
 [Show abstract] [Hide abstract] ABSTRACT: We use gaugegravity duality to compute spectral functions of fermionic operators in a stronglycoupled defect field theory in pwave superfluid states. The field theory is (3+1)dimensional N = 4 \mathcal{N} = 4 supersymmetric SU(N c ) YangMills theory, in the ’t Hooft limit and with large coupling, coupled to two massless flavors of (2+1)dimensional N = 4 \mathcal{N} = 4 supersymmetric matter. We show that a sufficiently large chemical potential for a U(1) subgroup of the global SU(2) isospin symmetry triggers a phase transition to a pwave superfluid state, and in that state we compute spectral functions for the fermionic superpartners of mesons valued in the adjoint of SU(2) isospin. In the spectral functions we see the breaking of rotational symmetry and the emergence of a Fermi surface comprised of isolated points as we cool the system through the superfluid phase transition. The dual gravitational description is two coincident probe D5branes in AdS 5 × S 5 with nontrivial worldvolume SU(2) gauge fields. We extract spectral functions from solutions of the linearized equations of motion for the D5branes’ worldvolume fermions, which couple to one another through the worldvolume gauge field. We develop an efficient method to compute retarded Green’s functions from a system of coupled bulk fermions. We also perform the holographic renormalization of free bulk fermions in any asymptotically Euclidean AdS space. KeywordsGaugegravity correspondenceAdSCFT CorrespondenceBrane Dynamics in Gauge Theories
 [Show abstract] [Hide abstract] ABSTRACT: We numerically construct asymptotically Antide Sitter charged black hole solutions of (4+1)dimensional SU(2) Einstein–Yang–Mills theory that, for sufficiently low temperature, develop vector hair. Via gauge–gravity duality, these solutions describe a stronglycoupled conformal field theory at finite temperature and density that undergoes a phase transition to a superfluid state with spontaneously broken rotational symmetry (a pwave superfluid state). The bulk theory has a single free parameter, the ratio of the fivedimensional gravitational constant to the Yang–Mills coupling, which we denote as α. Previous analyses have shown that in the socalled probe limit, where α goes to zero and hence the gauge fields are ignored in Einstein's equation, the transition to the superfluid state is second order. We construct fully backreacted solutions, where α is finite and the gauge fields are included in Einstein's equation, and find that for values of α above a critical value αc=0.365±0.001 in units of the AdS radius, the transition becomes first order.
 [Show abstract] [Hide abstract] ABSTRACT: We present an analysis and classiffication of vector and scalar fluctuations in a D3/D7brane setup at finite temperature and baryon density. The system is dual to an \( \mathcal{N} = 2 \) supersymmetric YangMills theory with SU(N c ) gauge group and N f hypermultiplets in the fundamental representation in the quenched approximation. We improve significantly over previous results on the quasinormal mode spectrum of D7branes and stress their novel physical interpretation. Amongst our findings is a new purely imaginary scalar mode that becomes tachyonic at sufficiently low temperature and baryon density. We establish the existence of a critical density above which the scalar mode stays in the stable regime for all temperatures. In the vector sector we study the crossover from the hydrodynamic to the quasiparticle regime and find that it moves to shorter wavelengths for lower temperatures. At zero baryon density the quasinormal modes move toward distinct discrete attractor frequencies that depend on the momentum as we increase the temperature. At finite baryon density, however, the trajectories show a turning behavior such that for low temperature the quasinormal mode spectrum approaches the spectrum of the supersymmetric zero temperature normal modes. We interpret this as resolution of the singular quasinormal mode spectrum that appears at the limiting D7brane embedding at vanishing baryon density.

Article: Adding flavor to AdS 4 / CFT 3
[Show abstract] [Hide abstract] ABSTRACT: Aharony, Bergman, Jafferis, and Maldacena have proposed that the lowenergy description of multiple M2branes at C4/Z(k) singularity is a (2+1)dimensional N = 6 supersymmetric U(Nc) x U(Nc) ChernSimons matter theory, the ABJM theory. In the largeNc limit, its holographic dual is supergravity in AdS(4) x S7/Z(k). We study various ways to add fields that transform in the fundamental representation of the gauge groups, i.e. flavor fields, to the ABJM theory. We work in a probe limit and perform analyses in both the supergravity and field theory descriptions. In the supergravity description we find a large class of supersymmetric embeddings of probe flavor branes. In the field theory description, we present a general method to determine the couplings of the flavor fields to the fields of the ABJM theory. We then study four examples in detail: codimensionzero N = 3 supersymmetric flavor, described in supergravity by KaluzaKlein monopoles or D6branes; codimensionone N = (0, 6) supersymmetric chiral flavor, described by D8branes; codimensionone N = (3, 3) supersymmetric nonchiral flavor, described by M5/D4branes; codimensiontwo N = 4 supersymmetric flavor, described by M2/D2branes. Finally we discuss special physical equivalences between brane embeddings in Mtheory, and their interpretation in the field theory description.  [Show abstract] [Hide abstract] ABSTRACT: Cosmology and string theory are two areas of fundamental physics which have progressed significantly over the last 25 years. Joining both areas together provides the possibility of finding microscopic explanations for the history of the early uni verse on the one hand, and of deriving observational tests for string theory on the other. In the subsequent seven chapters, different aspects of string cosmology are introduced and discussed.
 [Show abstract] [Hide abstract] ABSTRACT: We give a detailed account and extensions of a holographic flavor superconductivity model which we have proposed recently. The model has an explicit field theory realization as strongly coupled N=2 Super YangMills theory with fundamental matter at finite temperature and finite isospin chemical potential. Using gauge/gravity duality, i.e. a probe of two flavor D7branes in the AdS black hole background, we show that the system undergoes a second order phase transition with critical exponent 1/2. The new ground state may be interpreted as a rho meson superfluid. It shows signatures known from superconductivity, such as an infinite dc conductivity and a gap in the frequencydependent conductivity. We present a stringy picture of the condensation mechanism in terms of a recombination of strings. We give a detailed account of the evaluation of the nonAbelian DiracBornInfeld action involved using two different methods. Finally we also consider the case of massive flavors and discuss the holographic MeissnerOchsenfeld effect in our scenario. Comment: 23 pages, 22 figures. v2: small corrections and section about dynamical mass generation revised
 [Show abstract] [Hide abstract] ABSTRACT: In this talk we present applications of the AdS/CFT correspondence to strongly coupled systems at finite temperature and particle density. The model we investigate contains adjoint matter described by the gauge multiplet of N=4, as well as fundamental matter given by the hypermultiplet of N=2 Super YangMills theory. In some aspects these systems can be thought of as models for the quarkgluon plasma. In the first part we review some properties of meson spectra obtained from these holographic models. We discuss the implications of finite temperature and particle density in these stringtheory motivated setups. In particular, we find a broadening of the vector meson peaks in the relevant spectral function at finite density. However, we do not observe a movement of the resonances to lower frequencies. Moreover, we analyze the effects of strong coupling on heavy meson diffusion in medium. To do so, we make use of an effective model with dipole interaction, which is valid for heavy quarks at arbitrary coupling strength. We calculate the momentum broadening  normalized to the inmedium mass shift  and compare the large 't Hooft coupling AdS/CFT result with a perturbative result for weak coupling. We find that the momentum broadening is reduced at large 't Hooft coupling, leading to increased relaxation time. Comment: 6 pages, 3 figures, prepared for the proceedings of the 8th Conference "Quark Confinement and the Hadron Spectrum", September 1st6th 2008, Mainz, Germany; to appear in Proceedings of Science, based on a talk given by Felix Rust

Article: Adding Flavor to AdS4/CFT3
[Show abstract] [Hide abstract] ABSTRACT: Aharony, Bergman, Jafferis, and Maldacena have proposed that the lowenergy description of multiple M2branes at a C4/Zk singularity is a (2+1)dimensional N=6 supersymmetric U(Nc) x U(Nc) ChernSimons matter theory, the ABJM theory. In the largeNc limit, its holographic dual is supergravity in AdS4 x S7/Zk. We study various ways to add fields that transform in the fundamental representation of the gauge groups, i.e. flavor fields, to the ABJM theory. We work in a probe limit and perform analyses in both the supergravity and field theory descriptions. In the supergravity description we find a large class of supersymmetric embeddings of probe flavor branes. In the field theory description, we present a general method to determine the couplings of the flavor fields to the fields of the ABJM theory. We then study four examples in detail: codimensionzero N=3 supersymmetric flavor, described in supergravity by KaluzaKlein monopoles or D6branes; codimensionone N=(0,6) supersymmetric chiral flavor, described by D8branes; codimensionone N=(3,3) supersymmetric nonchiral flavor, described by M5/D4branes; codimensiontwo N=4 supersymmetric flavor, described by M2/D2branes. Finally we discuss special physical equivalences between brane embeddings in Mtheory, and their interpretation in the field theory description.  [Show abstract] [Hide abstract] ABSTRACT: We investigate the thermodynamics of a thermal field theory in presence of both a baryon and an isospin chemical potential. For this we consider a probe of several D7branes embedded in the AdSSchwarzschild black hole background. We determine the structure of the phase diagram and calculate the relevant thermodynamical quantities both in the canonical and in the grand canonical ensemble. We discuss how accidental symmetries present reflect themselves in the phase diagram: In the case of two flavors, for small densities , there is a rotational symmetry in the plane spanned by the baryon and isospin density which breaks down at large densities, leaving a Z 4 symmetry. Finally, we calculate vector mode spectral functions and determine their dependence on either the baryon or the isospin density. For large densities, a new excitation forms with energy below the known supersymmetric spectrum. Increasing the density further, this excitation becomes unstable. We speculate that this instability indicates a new phase of condensed mesons.
 [Show abstract] [Hide abstract] ABSTRACT: We consider thermal stronglycoupled N=2 SYM theory with fundamental matter at finite isospin chemical potential. Using gauge/gravity duality, i.e. a probe of two flavor D7branes embedded in the AdS black hole background, we find a critical temperature at which the system undergoes a second order phase transition. The critical exponent of this transition is one half and coincides with the result from mean field theory. In the thermodynamically favored phase, a flavor current acquires a vev and breaks an Abelian symmetry spontaneously. This new phase shows signatures known from superconductivity, such as an infinite dc conductivity and a gap in the frequencydependent conductivity. The gravity setup allows for an explicit identification of the degrees of freedom in the dual field theory, as well as for a dual string picture of the condensation process.
 [Show abstract] [Hide abstract] ABSTRACT: We construct electrically charged AdS_5 black hole solutions whose charge, mass and boostparameters vary slowly with the spacetime coordinates. From the perspective of the dual theory, these are equivalent to hydrodynamic configurations with varying chemical potential, temperature and velocity fields. We compute the boundary theory transport coefficients associated with a derivative expansion of the energy momentum tensor and Rcharge current up to second order. In particular, we find a first order transport coefficient associated with the axial component of the current. Comment: 31 pages, v2: published version; added some references, discussion of the chargecurrent changed, results unchanged
 [Show abstract] [Hide abstract] ABSTRACT: We consider a heavy meson moving slowly through high temperature nonAbelian plasmas. Using a simple dipole effective Lagrangian, we calculate the inmedium mass shift and the drag coefficient of the meson in = 4 Super Yang Mills theory at weak and strong coupling. As anticipated, in the large N limit the mass shift is finite while the drag is suppressed by 1/N2 . After comparing results to perturbative QCD estimates (which are also calculated), we reach the conclusion that relative to weak coupling expectations the effect of strong coupling is to reduce the momentum diffusion rate and to increase the relaxation time by up to a factor of four.
 [Show abstract] [Hide abstract] ABSTRACT: We construct the gravity dual of a field theory with flavour in which there are FayetIliopoulos (FI) terms present. For this purpose we turn on a constant KalbRamond B field in four internal space directions of AdS5 × S5 together with a D7 brane probe wrapping AdS5 × S3. The B field induces noncommutativity on the four internal directions of the D7 brane probe perpendicular to the AdS boundary. We argue that the moduli space of the Higgs part of mixed CoulombHiggs states in the dual field theory is described by the ADHM equations for noncommutative instantons on the four internal directions of the D7 brane probe. In particular, the global symmetries match. The FI term arises as the holographic dual of an antiselfdual B field. We discuss possible applications for this construction.
 [Show abstract] [Hide abstract] ABSTRACT: Gauge/gravity duals with flavour such as D7 probe branes added to deformed AdS_5 × S^5 backgrounds provide dual descriptions of spontaneous chiral symmetry breaking and light mesons, as well as of a first order phase transition for the fundamental degrees of freedom at finite temperature in the deconfined phase. The quasiparticle degrees of freedom are studied using the appropriate spectral function at both finite temperature and density.

Article: Mesons in gauge/gravity duals
[Show abstract] [Hide abstract] ABSTRACT: We review recent progress in studying mesons within gauge/gravity duality, in the context of adding flavour degrees of freedom to generalizations of the AdS/CFT correspondence. Our main focus is on the “topdown approach” of considering models constructed within string theory. We explain the stringtheoretical constructions in detail, aiming at nonspecialists. These give rise to a new way of describing strongly coupled confining largeN gauge theories similar to largeN QCD. In particular, we consider gravity dual descriptions of spontaneous chiral symmetry breaking, and compare with lattice results. A further topic covered is the behaviour of flavour bound states in finitetemperature field theories dual to a gravity background involving a black hole. We also describe the “bottom up” phenomenological approach to mesons within AdS/QCD. Some previously unpublished results are also included.  [Show abstract] [Hide abstract] ABSTRACT: We give a brief overview over AdS/CFT and discuss generalizations to field theories with running gauge coupling, partially or fully broken supersymmetry and with added flavor degrees of freedom. Moreover we discuss applications such as chiral symmetry breaking and meson spectra. © Copyright owned by the author(s) under the terms of the Creative Commons AttributionNonCommercial ShareAlike Licence.
 [Show abstract] [Hide abstract] ABSTRACT: We consider gauge/gravity duality with flavor for the finitetemperature field theory dual of the AdSSchwarzschild black hole background with embedded D7brane probes. In particular, we investigate spectral functions at finite baryon density in the black hole phase. We determine the resonance frequencies corresponding to mesonmass peaks as function of the quark mass over temperature ratio. We find that these frequencies have a minimum for a finite value of the quark mass. If the quotient of quark mass and temperature is increased further, the peaks move to larger frequencies. At the same time the peaks narrow, in agreement with the formation of nearly stable vector meson states which exactly reproduce the meson mass spectrum found at zero temperature. We also calculate the diffusion coefficient, which has finite value for all quark mass to temperature ratios, and exhibits a firstorder phase transition. Finally we consider an isospin chemical potential and find that the spectral functions display a resonance peak splitting, similar to the isospin meson mass splitting observed in effective QCD models. Comment: 15 pages, REVTeX, 12 figures, Typos corrected, extended discussion of the dependence of the spectra on changes in the quark mass
 [Show abstract] [Hide abstract] ABSTRACT: We investigate gauge/gravity duals with flavour for which puregauge KalbRamond B fields are turned on in the background, into which a D7 brane probe is embedded. First we consider the case of a magnetic field in two of the spatial boundary directions. We show that at finite temperature, i.e. in the AdSSchwarzschild background, the B field has a stabilizing effect on the mesons and chiral symmetry breaking occurs for a sufficiently large value of the B field. Then we turn to the electric case of a B field in the temporal direction and one spatial boundary direction. In this case, there is a singular region in which it is necessary to turn on a gauge field on the brane in order to ensure reality of the brane action. We find that the brane embeddings are attracted towards this region. Far away from this region, in the weak field case at zero temperature, we investigate the meson spectrum and find a mass shift similar to the Stark effect. Comment: 34 pages, 18 figures, v2: added references and comments on mode decoupling, on thermodynamics and holographic renormalisation, JHEP style, v3: Final published version
 [Show abstract] [Hide abstract] ABSTRACT: We study the gauge/gravity dual of a finite temperature field theory at finite isospin chemical potential by considering a probe of two coincident D7branes embedded in the AdSSchwarzschild black hole background. The isospin chemical potential is obtained by giving a vacuum expectation value to the time component of the nonAbelian gauge field on the brane. The fluctuations of the nonAbelian gauge field on the brane are dual to the SU(2) flavor current in the field theory. For the embedding corresponding to vanishing quark mass, we calculate all Green functions corresponding to the components of the flavor current correlator. We discuss the physical properties of these Green functions, which go beyond linear response theory. In particular, we show that the isospin chemical potential leads to a frequencydependent isospin diffusion coefficient.
Publication Stats
3k  Citations  
414.96  Total Impact Points  
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Institutions

20072014

Max Planck Institute of Physics
München, Bavaria, Germany


2006

LudwigMaximiliansUniversity of Munich
München, Bavaria, Germany


20022005

Humboldt University of Berlin
 Department of Physics
Berlín, Berlin, Germany


19992001

Massachusetts Institute of Technology
 Center for Theoretical Physics
Cambridge, MA, United States


19971999

University of Leipzig
 Institute of Theoretical Physics
Leipzig, Saxony, Germany
