arXiv subjects

Andrei Parnachev

Publications and source records attributed to Andrei Parnachev.

At least 55 records · Page 3Linked to original sources

arXiv2010

Holographic Lovelock Gravities and Black Holes

We study holographic implications of Lovelock gravities in AdS spacetimes. For a generic Lovelock gravity in arbitrary spacetime dimensions we formulate the existence condition for asymptotically AdS black holes. We consider small fluctuations around these black holes and determine the constraint on Lovelock parameters by demanding causality of the boundary theory. For the case of cubic Lovelock gravity in seven spacetime dimensions we compute the holographic Weyl anomaly and determine the three point functions of the stress energy tensor in the boundary CFT. Remarkably, these correlators happen to satisfy the same relation as the one imposed by supersymmetry. We then compute the energy flux; requiring it to be positive is shown to be completely equivalent to requiring causality of the finite temperature CFT dual to the black hole. These constraints are not stringent enough to place any positive lower bound on the value of viscosity. Finally, we conjecture an expression for the energy flux valid for any Lovelock theory in arbitrary dimensions.

hep-th↗

arXiv2010

AdS_7/CFT_6, Gauss-Bonnet Gravity, and Viscosity Bound

We study the relation between the causality and the positivity of energy bounds in Gauss-Bonnet gravity in AdS_7 background and find a precise agreement. Requiring the group velocity of metastable states to be bounded by the speed of light places a bound on the value of Gauss-Bonnet coupling. To find the positivity of energy constraints we compute the parameters which determine the angular distribution of the energy flux in terms of three independent coefficients specifying the three-point function of the stress-energy tensor. We then relate the latter to the Weyl anomaly of the six-dimensional CFT and compute the anomaly holographically. The resulting upper bound on the Gauss-Bonnet coupling coincides with that from causality and results in a new bound on viscosity/entropy ratio.

hep-th↗

arXiv2009

Notes on Properties of Holographic Matter

Probe branes with finite worldvolume electric flux in the background created by a stack of Dp branes describe holographically strongly interacting fundamental matter at finite density. We identify two quantities whose leading low temperature behavior is independent of the dimensionality of the probe branes: specific heat and DC conductivity. This behavior can be inferred from the dynamics of the fundamental strings which provide a good description of the probe branes in the regime of low temperatures and finite densities. We also comment on the speed of sound on the branes and the temperature dependence of DC conductivity at vanishing charge density.

hep-th↗

arXiv2009

Comments on Bounds on Central Charges in N=1 Superconformal Theories

The ratio of central charges in four-dimensional CFTs has been suggested by Hofman and Maldacena to lie within an interval whose boundaries are fixed by the number of supersymmetries. We compute this ratio for a set of interacting N=1 superconformal field theories which arise as RG fixed points of supersymmetric Yang-Mills theories with adjoint and fundamental matter. We do not find violations of the proposed bounds, which appear to be saturated by free field theories.

hep-th↗

arXiv2008

Holographic Responses of Fermion Matter

We consider the $D4-D8-\bar{D8}$ brane system which serves as ultraviolet completion of the Nambu-Jona-Lasinio model, where the only degrees of freedom carrying baryon charge are fermions. By turning on chemical potential for this charge one may expect the formation of the Fermi liquid ground state. At strong coupling we use the dual holographic description to investigate the responses of the system to small perturbations. In the chirally symmetric phase we find that the density dependent part of the heat capacity vanishes linearly with temperature. We also observe a zero sound excitation in the collisionless regime, whose speed is equal to that of normal sound in the hydrodynamic regime. Both the linear dependence of the heat capacity and the existence of zero sound are properties of the Fermi liquid ground state. We also compute the two-point function of the currents at vanishing frequency but do not find any singularities at finite values of the momentum.

hep-th↗

arXiv2008

Phase Transitions, theta Behavior and Instantons in QCD and its Holographic Model

In the holographic model of QCD, theta dependence sharply changes at the point of confinement deconfinement phase transition. In large N QCD such a change in theta behavior can be related to the breakdown of the instanton expansion at some critical temperature T_c. Associating this temperature with confinement-deconfinement phase transition leads to the description of the latter in terms of dissociation of instantons into the fractionally charged instanton quarks. To elucidate this picture, we introduce the nonvanishing chiral condensate in the deconfining phase and assume a specific lagrangian for the eta' field in the confining phase. In the resulting picture the high temperature phase of the theory consists of the dilute gas of instantons, while the low temperature phase is described in terms of freely moving fractional instanton quarks.

hep-ph↗

arXiv2008

Comments on Fermi Liquid from Holography

We investigate the signatures of Fermi liquid formation in the N=4 super Yang-Mills theory coupled to fundamental hypermultiplet at nonvanishing chemical potential for the global U(1) vector symmetry. At strong 't Hooft coupling the system can be analyzed in terms of the D7 brane dynamics in AdS_5 x S^5 background. The phases with vanishing and finite charge density are separated at zero temperature by a quantum phase transition. In case of vanishing hypermultiplet mass, Karch, Son and Starinets discovered a gapless excitation whose speed equals the speed of sound. We find that this zero sound mode persists to all values of the hypermultiplet mass, and its speed vanishes at the point of phase transition. The value of critical exponent and the ratio of the velocities of zero and first sounds are consistent with the predictions of Landau Fermi liquid theory at strong coupling.

hep-th↗

arXiv2008

Topological Entanglement Entropy and Holography

We study the entanglement entropy in confining theories with gravity duals using the holographic prescription of Ryu and Takayanagi. The entanglement entropy between a region and its complement is proportional to the minimal area of a bulk hypersurface ending on their border. We consider a disk in 2+1 dimensions and a ball in 3+1 dimensions and find in both cases two types of bulk hypersurfaces with different topology, similar to the case of the slab geometry considered by Klebanov, Kutasov and Murugan. Depending on the value of the radius, one or the other type of hypersurfaces dominates the calculation of entanglement entropy. In 2+1 dimensions a useful measure of topological order of the ground state is the topological entanglement entropy, which is defined to be the constant term in the entanglement entropy of a disk in the limit of large radius. We compute this quantity and find that it vanishes for confining gauge theory, in accord with our expectations. In 3+1 dimensions the analogous quantity is shown to be generically nonzero and cutoff-dependent.

hep-th↗

arXiv2008

Holographic QCD with Isospin Chemical Potential

We consider configurations of D4-D8-\bar D8 branes which correspond to large N QCD with non-vanishing temperature and chemical potential for baryon number and isospin. We study the holographic dual of this model and find a rich phase structure. The phases, distinguished by the values of quark condensates, are separated by the surfaces of first order phase transitions. The picture is in many respects similar to the expected phase structure of QCD in the chiral limit.

hep-th↗

arXiv2007

Photoemission with Chemical Potential from QCD Gravity Dual

We consider a $D4-D8-\bar D8$ brane construction which gives rise to a large N QCD at sufficiently small energies. Using the gravity dual of this system, we study chiral phase transition at finite chemical potential and temperature and find a line of first order phase transitions in the phase plane. We compute the spectral function and the photon emission rate. The trace of the spectral function is monotonic at vanishing chemical potential, but develops some interesting features as the value of the chemical potential is increased.

hep-th↗

arXiv2006

Chiral Phase Transition from String Theory

The low energy dynamics of a certain D-brane configuration in string theory is described at weak t'Hooft coupling by a non-local version of the Nambu-Jona-Lasinio model. We study this system at finite temperature and strong t'Hooft coupling, using the string theory dual. We show that for sufficiently low temperatures chiral symmetry is broken, while for temperatures larger then the critical value, it gets restored. We compute the latent heat and observe that the phase transition is of the first order.

hep-th↗

arXiv2005

On Non-Critical Superstring/Black Hole Transition

An interesting case of string/black hole transition occurs in two-dimensional non-critical string theory dressed with a compact CFT. In these models the high energy densities of states of perturbative strings and black holes have the same leading behavior when the Hawking temperature of the black hole is equal to the Hagedorn temperature of perturbative strings. We compare the first subleading terms in the black hole and closed string entropies in this setting and argue that the entropy interpolates between these expressions as the energy is varied. We compute the subleading correction to the black hole entropy for a specific simple model.

hep-th↗

arXiv2005

The Silence of the Little Strings

We study the hydrodynamics of the high-energy phase of Little String Theory. The poles of the retarded two-point function of the stress energy tensor contain information about the speed of sound and the kinetic coefficients, such as shear and bulk viscosity. We compute this two-point function in the dual string theory and analytically continue it to Lorentzian signature. We perform an independent check of our results by the Lorentzian supergravity calculation in the background of non-extremal NS5-branes. The speed of sound vanishes at the Hagedorn temperature. The ratio of shear viscosity to entropy density is equal to the universal value 1/4πand does not receive α' corrections. The ratio of bulk viscosity to entropy density equals 1/10π. We also compute the R-charge diffusion constant. In addition to the hydrodynamic singularities, the correlators have an infinite series of finite-gap poles, and a massless pole with zero attenuation.

hep-th↗

arXiv2005

The Ground Ring of N=2 Minimal String Theory

We study the $\NN=2$ string theory or the $\NN=4$ topological string on the deformed CHS background. That is, we consider the $\NN=2$ minimal model coupled to the $\NN=2$ Liouville theory. This model describes holographically the topological sector of Little String Theory. We use degenerate vectors of the respective $\NN=2$ Verma modules to find the set of BRST cohomologies at ghost number zero--the ground ring, and exhibit its structure. Physical operators at ghost number one constitute a module of the ground ring, so the latter can be used to constrain the S-matrix of the theory. We also comment on the inequivalence of BRST cohomologies of the $\NN=2$ string theory in different pictures.

hep-th↗

arXiv2005

Profiling the Brane Drain in a Nonsupersymmetric Orbifold

We study D-branes in a nonsupersymmetric orbifold of type C^2/Γ, perturbed by a tachyon condensate, using a gauged linear sigma model. The RG flow has both higgs and coulomb branches, and each branch supports different branes. The coulomb branch branes account for the ``brane drain'' from the higgs branch, but their precise relation to fractional branes has hitherto been unknown. Building on the results of hep-th/0403016 we construct, in detail, the map between fractional branes and the coulomb/higgs branch branes for two examples in the type 0 theory. This map depends on the phase of the tachyon condensate in a surprising and intricate way. In the mirror Landau-Ginzburg picture the dependence on the tachyon phase is manifested by discontinuous changes in the shape of the D-brane.

hep-th↗

arXiv2004

Localized Tachyons and the Quantum McKay Correspondence

The condensation of closed string tachyons localized at the fixed point of a C^d/Γorbifold can be studied in the framework of renormalization group flow in a gauged linear sigma model. The evolution of the Higgs branch along the flow describes a resolution of singularities via the process of tachyon condensation. The study of the fate of D-branes in this process has lead to a notion of a ``quantum McKay correspondence.'' This is a hypothetical correspondence between fractional branes in an orbifold singularity in the ultraviolet with the Coulomb and Higgs branch branes in the infrared. In this paper we present some nontrivial evidence for this correspondence in the case C^2/Z_n by relating the intersection form of fractional branes to that of ``Higgs branch branes,'' the latter being branes which wrap nontrivial cycles in the resolved space.

hep-th↗

arXiv2003

Central Charges and $U(1)_R$ Symmetries in ${\cal N}=1$ Super Yang-Mills

We use recent results of Intriligator and Wecht [hep-th/0304128] to study the phase structure of $\NN=1$ super Yang-Mills theory with gauge group $SU(N_c)$, a chiral superfield in the adjoint, and $N_f$ chiral superfields in the fundamental representation of the gauge group. Our discussion sheds new light on [hep-th/0304128] and supports the conjecture that the central charge $a$ decreases under RG flows and is non-negative in unitary four dimensional conformal field theories.

hep-th↗

arXiv2002

Strings in the near plane wave background and AdS/CFT

We study the AdS/CFT correspondence for string states which flow into plane wave states in the Penrose limit. Leading finite radius corrections to the string spectrum are compared with scaling dimensions of finite R-charge BMN-like operators. We find agreement between string and gauge theory results.

hep-th↗