arXiv subjects
Publications and source records attributed to Steven S. Gubser.
Using the gauge-string duality, I compute two-point functions of the force acting on an external quark moving through a finite temperature bath of N=4 super-Yang-Mills theory. I comment on the possible relevance of the string theory calculations to heavy quarks propagating through a quark-gluon plasma.
Computations of the drag force on a heavy quark moving through a thermal state of strongly coupled N=4 super-Yang-Mills theory have appeared recently in hep-th/0605158, hep-ph/0605199, and hep-th/0605182. I compare the strength of this effect between N=4 gauge theory and QCD, using the static force between external quarks to normalize the 't Hooft coupling. Comparing N=4 and QCD at fixed energy density then leads to a relaxation time of roughly 2 fm/c for charm quarks moving through a quark-gluon plasma at T=250 MeV. This estimate should be regarded as preliminary because of the difficulties of comparing two such different theories.
We investigate the stability against small deformations of strings dangling into AdS_5-Schwarzschild from a moving heavy quark-anti-quark pair. We speculate that emission of massive string states may be an important part of the evolution of certain unstable configurations.
Scalar self interactions are known to weaken considerably the current constraints on scalar-mediated fifth forces. We consider a scalar field with a quartic self interaction and gravitation-strength Yukawa couplings to matter particles. After discussing the phenomenology of this scalar field, we assess the ability of ongoing and planned experiments to detect the fifth force mediated by such a field. Assuming that the quartic and matter couplings are of order unity, the current-generation Eot-Wash experiment at the University of Washington will be able to explore an interesting subset of parameter space. The next-generation Eot-Wash experiment is expected to be able to detect, or to rule out, the fifth force due to such a scalar with unit quartic and matter couplings at the 3 sigma confidence level.
Using AdS/CFT, we compute the Fourier space profile of generated by a heavy quark moving through a thermal plasma of strongly coupled N=4 super-Yang-Mills theory. We find evidence of a wake whose description includes gauge fields with large momenta. We comment on the possible relevance of our results to relativistic heavy ion collisions.
We develop the linear equations that describe graviton perturbations of AdS_5-Schwarzschild generated by a string trailing behind an external quark moving with constant velocity. Solving these equations allows us to evaluate the stress tensor in the boundary gauge theory. Components of the stress tensor exhibit directional structures in Fourier space at both large and small momentum. We comment on the possible relevance of our results to relativistic heavy ion collisions.
We present evidence that there is a non-trivial fixed point for the AdS_{D+1} non-linear sigma model in two dimensions, without any matter fields or additional couplings beyond the standard quadratic action subject to a quadratic constraint. A zero of the beta function, both in the bosonic and supersymmetric cases, appears to arise from competition between one-loop and higher loop effects. A string vacuum based on such a fixed point would have string scale curvature. The evidence presented is based on fixed-order calculations carried to four loops (corresponding to O(α'^3) in the spacetime effective action) and on large D calculations carried to O(D^{-2}) (but to all orders in α'). We discuss ways in which the evidence might be misleading, and we discuss some features of the putative fixed point, including the central charge and an operator of negative dimension. We speculate that an approximately AdS_5 version of this construction may provide a holographic dual for pure Yang-Mills theory, and that quotients of an AdS_3 version might stand in for Calabi-Yau manifolds in compactifications to four dimensions.
The AdS/CFT correspondence and a classical test string approximation are used to calculate the drag force on an external quark moving in a thermal plasma of N=4 super-Yang-Mills theory. This computation is motivated by the phenomenon of jet-quenching in relativistic heavy ion collisions.
We calculate the beta function of non-linear sigma models with S^{D+1} and AdS_{D+1} target spaces in a 1/D expansion up to order 1/D^2 and to all orders in α'. This beta function encodes partial information about the spacetime effective action for the heterotic string to all orders in α'. We argue that a zero of the beta function, corresponding to a worldsheet CFT with AdS_{D+1} target space, arises from competition between the one-loop and higher-loop terms, similarly to the bosonic and supersymmetric cases studied previously in hep-th/0512355. Various critical exponents of the non-linear sigma model are calculated, and checks of the calculation are presented.
We investigate the Gregory-Laflamme instability for bound states of branes in type II string theory and in M-theory. We examine systems with two different constituent branes: for instance, D3-F1 or D4-D0. For the cases in which the Gregory-Laflamme instability can occur, we describe the boundary of thermodynamic stability. We also present an argument for the validity of the Correlated Stability Conjecture, generalizing earlier work by Reall. We discuss the implications for OM theory and NCOS theory, finding that in both cases, there exists some critical temperature above which the system becomes unstable to clumping of the open strings/membranes.
We demonstrate explicit counter-examples to the Correlated Stability Conjecture (CSC), which claims that the horizon of a black brane is unstable precisely if that horizon has a thermodynamic instability, meaning that its matrix of susceptibilities has a negative eigenvalue. These examples involve phase transitions near the horizon. Ways to restrict or revise the CSC are suggested. One of our examples shows that N=1* gauge theory has a second order chiral symmetry breaking phase transition at a temperature well above the confinement scale.
The Reissner-Nordstrom black hole in four dimensions can be made unstable without violating the dominant energy condition by introducing a real massive scalar with non-renormalizable interactions with the gauge field. New stable black hole solutions then exist with greater entropy for fixed mass and charge than the Reissner-Nordstrom solution. In these new solutions, the scalar condenses to a non-zero value near the horizon. Various generalizations of these hairy black holes are discussed, and an attempt is made to characterize when black hole hair can occur.
The D2-D0 bound state exhibits a Gregory-Laflamme instability when it is sufficiently non-extremal. If there are no D0-branes, the requisite non-extremality is finite. When most of the extremal mass comes from D0-branes, the requisite non-extremality is very small. The location of the threshhold for the instability is determined using a local thermodynamic analysis which is then checked against a numerical analysis of the linearized equations of motion. The thermodynamic analysis reveals an instability of non-commutative field theory at finite temperature, which may occur only at very long wavelengths as the decoupling limit is approached.
It is well known that the thermodynamics of certain near-extremal black holes in asymptotically flat space can be lifted to an effective string description created from the intersection of D-branes. In this paper we present evidence that the semiclassical thermodynamics of near-extremal R-charged black holes in AdS(5)xS(5) is described in a similar manner by effective strings created from the intersection of giant gravitons on the S(5). We also present a free fermion description of the supersymmetric limit of the one-charge black hole, and we give a crude catalog of the microstates of the two and three-charge black holes in terms of operators in the dual conformal field theory.
Certain Hamiltonians based on two coupled quantum mechanical spins exhibit degenerate eigenvalues despite having no obvious non-abelian symmetries. Operators acting to permute the degenerate states do not have a simple form when expressed as polynomials of the generators of rotations for the respective spins. As observed in [1], one such Hamiltonian helps explain resonances in the spin relaxation rate of optically pumped Rb_2, as a function of applied magnetic field. We give an explanation of why the degeneracies exist, based on properties of the commutator and anti-commutator of the Hamiltonian and its image under magnetic field reversal.
We interpret D-strings at the bottom of the warped deformed conifold as axionic strings in the dual cascading SU(N+M) x SU(N) gauge theory. The axion is a massless pseudo-scalar glueball which we find in the supergravity fluctuation spectrum and interpret as the Goldstone boson of spontaneously broken U(1) baryon number symmetry. The existence of this massless glueball, anticipated in hep-th/0101013, supports the idea that the cascading gauge theory is on the baryonic branch, i.e. the U(1)_B global symmetry is broken by expectation values of baryonic operators. We also find a massless scalar glueball, which is a superpartner of the pseudo-scalar. This scalar mode is a mixture of an NS-NS 2-form and a metric perturbation of the warped deformed conifold of a type first considered in hep-th/0012034.
The warped deformed conifold background of type IIB theory is dual to the cascading $SU(M(p+1))\times SU(Mp)$ gauge theory. We show that this background realizes the (super-)Goldstone mechanism where the U(1) baryon number symmetry is broken by expectation values of baryonic operators. The resulting massless pseudo-scalar and scalar glueballs are identified in the supergravity spectrum. A D-string is then dual to a global string in the gauge theory. Upon compactification, the Goldstone mechanism turns into the Higgs mechanism, and the global strings turn into ANO strings.
The mass of a scalar field mediating a fifth force is tightly constrained by experiments. We show, however, that adding a quartic self-interaction for such a scalar makes most tests much less constraining: the non-linear equation of motion masks the coupling of the scalar to matter through the chameleon mechanism. We discuss consequences for fifth force experiments. In particular, we find that, with quartic coupling of order unity, a gravitational strength interaction with matter is allowed by current constraints. We show that our chameleon scalar field results in experimental signatures that could be detected through modest improvements of current laboratory set-ups.