Non-Abelian coset string backgrounds from asymptotic and initial data

We describe hierarchies of exact string backgrounds obtained as non-Abelian cosets of orthogonal groups and having a space--time realization in terms of gauged WZW models. For each member in these hierarchies, the target-space backgrounds are generated by the ``boundary'' backgrounds of the next member. We explicitly demonstrate that this property holds to all orders in $\alpha'$. It is a consequence of the existence of an integrable marginal operator build on, generically, non-Abelian parafermion bilinears. These are dressed with the dilaton supported by the extra radial dimension, whose asymptotic value defines the boundary. Depending on the hierarchy, this boundary can be time-like or space-like with, in the latter case, potential cosmological applications.Comment: 26 page

Gravity duals of N=2 SCFTs and asymptotic emergence of the electrostatic description

We built the first eleven-dimensional supergravity solutions with SO(2,4)xSO(3)xU(1)_R symmetry that exhibit the asymptotic emergence of an extra U(1) isometry. This enables us to make the connection with the usual electrostatics-quiver description. The solution is obtained via the Toda frame of Kahler surfaces with vanishing scalar curvature and SU(2) action.Comment: 1+15 pages, Latex, v2: few minor changes, JHEP versio

(Weak) G_2 Holonomy from Self-duality, Flux and Supersymmetry

The aim of this paper is two-fold. First, we provide a simple and pedagogical discussion of how compactifications of M-theory or supergravity preserving some four-dimensional supersymmetry naturally lead to reduced holonomy or its generalization, reduced weak holonomy. We relate the existence of a (conformal) Killing spinor to the existence of certain closed and co-closed p-forms, and to the metric being Ricci flat or Einstein. Then, for seven-dimensional manifolds, we show that octonionic self-duality conditions on the spin connection are equivalent to G_2 holonomy and certain generalized self-duality conditions to weak G_2 holonomy. The latter lift to self-duality conditions for cohomogeneity-one spin(7) metrics. To illustrate the power of this approach, we present several examples where the self-duality condition largely simplifies the derivation of a G_2 or weak G_2 metric.Comment: references added, 25 pages, no figures, Late

Ferromagnetic phase transitions in SU(N)SU(N)

We study the thermodynamics of a non-abelian ferromagnet consisting of "atoms" each carrying a fundamental representation of $SU(N)$, coupled with long-range two-body quadratic interactions. We uncover a rich structure of phase transitions from non-magnetized, global $SU(N)$-invariant states to magnetized ones breaking global invariance to $SU(N-1) \times U(1)$. Phases can coexist, one being stable and the other metastable, and the transition between states involves latent heat exchange, unlike in usual $SU(2)$ ferromagnets. Coupling the system to an external non-abelian magnetic field further enriches the phase structure, leading to additional phases. The system manifests hysteresis phenomena both in the magnetic field, as in usual ferromagnets, and in the temperature, in analogy to supercooled water. Potential applications are in fundamental situations or as a phenomenological model.Comment: 49 pages, 17 figures, 2 appendice

NS5-branes on an ellipsis and novel marginal deformations with parafermions

We consider NS5-branes distributed along the circumference of an ellipsis and explicitly construct the corresponding gravitational background. This provides a continuous line of deformations between the limiting cases, considered before, in which the ellipsis degenerates into a circle or into a bar. We show that a slight deformation of the background corresponding to a circle distribution into an ellipsoidal one is described by a novel non-factorizable marginal perturbation of bilinears of dressed parafermions. The latter are naturally defined for the circle case since, as it was shown in the past, the background corresponds to an orbifold of the exact conformal field theory coset model SU(2)/U(1) times SL(2,R)/U(1). We explore the possibility to define parafermionic objects at generic points of the ellipsoidal families of backgrounds away from the circle point. We also discuss a new limiting case in which the ellipsis degenerates into two infinitely stretched parallel bars and show that the background is related to the Eguchi-Hanson metric, via T-duality.Comment: 24 page

Supersymmetry and finite-temperature strings

We describe finite temperature N=4 superstrings in D=5 by an effective four-dimensional supergravity of the thermal winding modes that can become tachyonic and trigger the instabilities at the Hagedorn temperature. Using a domain-wall ansatz, exact solutions to special BPS-type first order equations are found. They preserve half of the supersymmetries, contrary to the standard perturbative superstring at finite temperature that breaks all supersymmetries. Our solutions show no indication of any tachyonic instability and provide evidence for a new BPS phase of finite temperature superstrings that is stable for all temperatures. This would have important consequences for a stringy description of the early universe.Comment: 6 pages, Latex, To appear in the proceedings of The Ninth Marcel Grossmann Meeting, Rome, July 2-8, 200

Solving field equations in non-isometric coset CFT backgrounds

The largest known class of gravitational backgrounds with an exact string theoretical description is based on coset G/H CFTs and the corresponding gauged WZW models. These backgrounds generically lack isometries and are quite complicated. Thus the corresponding field equations seem impossible to solve and their use in physical applications becomes problematic. We develop a systematic general method enabling us to overcome this problem using group theory. The method is inspired by observations made in some elementary geometric coset and coset CFTs, but its full power is apparent in non-abelian cases. We analyze exhaustively the coset SU(2)xSU(2)/SU(2) and explicitly solve the scalar wave equation of the corresponding gravitational background. We also examine the high spin limit and derive the effective geometry that consistently captures the corresponding sector in the theory.Comment: Version to appear in Nuclear physics B, minor typos correcte