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Tomas Ortin

Publications and source records attributed to Tomas Ortin.

At least 91 records · Page 5Linked to original sources
arXiv1999

The General, Duality-Invariant Family of Non-BPS Black-Hole Solutions of N=4, d=4 Supergravity

We present the most general family of stationary point-like solutions of pure N=4, d=4 Supergravity characterized by completely independent electric and magnetic charges, mass, angular momentum and NUT charge plus the asymptotic values of the scalars. It includes, for specific values of the charges all previously known BPS and non-BPS, extreme and non-extreme black holes and Taub-NUT solutions. As a family of solutions, it is manifestly invariant under T and S duality transformations and exhibits a structure related to the underlying special geometry structure of the theory. Finally, we study briefly the black-hole-type subfamily of metrics and give explicit expressions for their entropy and temperature.

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arXiv1999

The Super D9-Brane and its Truncations

We consider two inequivalent truncations of the super D9--brane: the ``Heterotic'' and the ``Type I'' truncation. Both of them lead to an N=1 nonlinear supersymmetrization of the D=10 cosmological constant. The propagating degrees of freedom in the Heterotic and Type I truncation are given by the components of a D=10 vector multiplet and a single Majorana-Weyl spinor, respectively. As a by-product we find that, after the Type I truncation, the Ramond-Ramond super ten-form provides an interesting reformulation of the Volkov-Akulov action. These results can be extended to all dimensions in which spacetime filling D-branes exist, i.e. D=3,4,6 and 10.

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arXiv1999

The D8-Brane Tied up: String and Brane Solutions in Massive Type IIA Supergravity

We present two new solutions of Romans' massive type IIA supergravity characterized by the two non-trivial massive potentials of Romans' theory: the NSNS 2-form and the RR 7-form. They can be interpreted respectively as the intersection of a fundamental string and a D8-brane over a D0-brane and the intersection of a D6-brane with a D8-brane over a NSNS5-brane. The D8-brane manifests itself through the mass parameter and in the massless limit one recovers the standard fundamental string and D6-brane solutions. Although these solutions do not have the usual form for BPS bound states at threshold and each of them involves 3 objects, both of them preserve 1/4 of the supersymmetries.

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arXiv1997

Kaluza-Klein Monopoles and Gauged Sigma-Models

We propose an effective action for the eleven-dimensional (bosonic) Kaluza-Klein monopole solution. The construction of the action requires that the background fields admit an Abelian isometry group. The corresponding sigma-model is gauged with respect to this isometry. The gauged sigma-model is the source for the monopole solution. A direct (double) dimensional reduction of the action leads to the effective action of a 10-dimensional D-6-brane (IIA Kaluza-Klein monopole). We also show that the effective action of the 10-dimensional heterotic Kaluza-Klein monopole (which is a truncation of the IIA monopole action) is T-dual to the effective action of the solitonic 5-brane. We briefly discuss the kappa-symmetric extension of our proposal and the possible role of gauged sigma-models in connection with the conjectured M-theory 9-brane.

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arXiv1997

A Note on the D-2-Brane of the Massive Type IIA Theory and Gauged Sigma Models

Gauging the M-2-brane effective action with respect to an Abelian isometry in such a way that the invariance under gauge transformations of the 3-form potential is maintained (slightly modified) we obtain a fully covariant action with 11-dimensional target space that gives the massive D-2-brane effective action upon dimensional reduction in the direction of the gauged isometry.

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arXiv1997

Non-Supersymmetric (but) Extreme Black Holes, Scalar Hair and Other Open Problems

We give a brief overview of black-hole solutions in four-dimensional supergravity theories and their extremal and supersymmetric limits. We also address problems like cosmic censorship and no-hair theorems in supergravity theories. While supergravity by itself seems not to be enough to enforce cosmic censorship and absence of primary scalar hair, superstring theory may be.

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arXiv1997

Transformation of Black-Hole Hair under Duality and Supersymmetry

We study the transformation under the String Theory duality group of the observable charges (mass, angular momentum, NUT charge, electric, magnetic and different scalar charges) of four dimensional point-like objects whose asymptotic behavior constitutes a subclass closed under duality. The charges fall into two complex four-dimensional representations of the duality group. T duality (including Buscher's) has an O(1,2) action on them and S duality a U(1) action. The generalized Bogomol'nyi bound is an U(2,2)-invariant built out of one representations while the other representation (which includes the angular momentum) never appears in it. The bound is manifestly duality-invariant. Consistency between T duality and supersymmetry requires that primary scalar hair is included in the Bogomol'nyi bound. Four-dimensional supersymmetric massless black holes are the T duals in time of massive supersymmetric black holes. Non-extreme massless ``black holes'' are the T duals of the non-extreme black holes and have primary scalar hair and naked singularities.

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arXiv1997

Extremality Versus Supersymmetry in Stringy Black Holes

We study general black-hole solutions of the low-energy string effective action in arbitrary dimensions using a general metric that can describe them all in a unified way both in the extreme and non-extreme cases. We calculate the mass, temperature and entropy and study which relations amongst the charges and the mass lead to extremality. We find that the temperature always vanishes in the extreme limit and we find that, for a set of n charges (no further reducible by duality) there are 2^{(n-1)} combinations of the charges that imply extremality. Not all of these combinations can be central charge eigenvalues and, thus, there are in general extreme black holes which are not supersymmetric (or ``BPS-saturated''). In the N=8 supergravity case we argue that the existence of roughly as many supersymmetric and non-supersymmetric extreme black holes suggests the existence of an underlying twelve-dimensional structure.

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arXiv1996

Massive and Massless Black Holes

We review some recent work on the existence and classification of extreme black-hole-type solutions in N=8 supergravity. For the black holes considered (those that are also solutions of N=4 supergravity and of the Einstein-Maxwell-dilaton theory with coupling a) a complete classification is achieved: the only possible values of a are \sqrt{3},1,1/\sqrt{3},0. Up to U duality transformations there is only one solution for each of those values. The exception is a=0 for which an additional extreme but non-supersymmetric Reissner-Nordström black hole solution exists. We also study the so-called massless black-hole solutions. We argue that they can be understood as composite objects. At least one of the components would have ``negative mass''. We also argue that these states, being annihilated by all the generators in the supersymmetry algebra, could also constitute alternative vacua of the supergravity theory.

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arXiv1996

The Eleven-Dimensional Five-Brane

We consider the action, in arbitrary curved background, of the eleven-dimensional five-brane to second order in the curvature of the worldvolume tensor field. We show that this action gives upon double dimensional reduction the action of the Dirichlet four-brane up to the same order. We use this result as a starting point to discuss the structure of the action including terms of higher order in the worldvolume curvature.

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arXiv1996

Massless Black Holes as Black Diholes and Quadruholes

Massless black holes can be understood as bound states of a (positive mass) extreme a=\sqrt{3} black hole and a singular object with opposite (i.e. negative) mass with vanishing ADM (total) mass but non-vanishing gravitational field. Supersymmetric balance of forces is crucial for the existence of this kind of bound states and explains why the system does not move at the speed of light. We also explain how supersymmetry allows for negative mass as long as it is never isolated but in bound states of total non-negative mass.

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arXiv1996

Solution--Generating Transformations and the String Effective Action

We study exhaustively the solution-generating transformations (dualities) that occur in the context of the low-energy effective action of superstring theory. We first consider target-space duality (``T duality'') transformations in absence of vector fields. We find that for one isometry the full duality group is (SO^{\uparrow}(1,1))^{3} x D_{4}, the discrete part (D_{4}) being non-Abelian. We, then, include non-Abelian Yang--Mills fields and find the corresponding generalization of the T duality transformations. We study the α^{\prime} corrections to these transformations and show that the T duality rules considerably simplify if the gauge group is embedded in the holonomy group. Next, in the case in which there are Abelian vector fields, we consider the duality group that includes the transformation introduced by Sen that rotates among themselves components of the metric, axion and vector field. Finally we list the duality symmetries of the Type II theories with one isometry.

hep-th↗
arXiv1996

Supersymmetric Black Holes in $N=8$ Supergravity

We study the embedding of extreme (multi-) dilaton black hole solutions for the values of the parameter $a=\sqrt{3},1,1/\sqrt{3},0$ in $N=4$ and $N=8$ four-dimensional supergravity. For each black hole solution we find different embeddings in $N=4$ supergravity which have different numbers of unbroken supersymmetries. When embedded in $N=8$ supergravity, all different embeddings of the same solution have the same number of unbroken supersymmetries. Thus, there is a relation between the value of the parameter $a$ and the number of unbroken supersymmetries in $N=8$ supergravity, but not in $N=4$, and the different embeddings must be related by dualities of the $N=8$ theory which are not dualities of the $N=4$ theory. The only exception in this scheme is a {\it dyonic} embedding of the $a=0$ black-hole solution which seems to break all supersymmetries both in the $N=4$ and in the $N=8$ theories.

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arXiv1995

FERMION ZERO MODES AND BLACK-HOLE HYPERMULTIPLETS WITH RIGID SUPERSYMMETRY

The gravitini zero modes riding on top of the extreme Reissner-Nordstrom black-hole solution of N=2 supergravity are shown to be normalizable. The gravitini and dilatini zero modes of axion-dilaton extreme black-hole solutions of N=4 supergravity are also given and found to have finite norms. These norms are duality invariant. The finiteness and positivity of the norms in both cases are found to be correlated with the Witten-Israel-Nester construction; however, we have replaced the Witten condition by the pure-spin-3/2 constraint on the gravitini. We compare our calculation of the norms with the calculations which provide the moduli space metric for extreme black holes. The action of the N=2 hypermultiplet with an off-shell central charge describes the solitons of N=2 supergravity. This action, in the Majumdar-Papapetrou multi-black-hole background, is shown to be N=2 rigidly supersymmetric.

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arXiv1994

Supersymmetry and Stationary Solutions in Dilaton-Axion Gravity

New stationary solutions of $4$-dimensional dilaton-axion gravity are presented, which correspond to the charged Taub-NUT and Israel-Wilson-Perjes (IWP) solutions of Einstein-Maxwell theory. The charged axion-dilaton Taub-NUT solutions are shown to have a number of interesting properties: i) manifest $SL(2,R)$ symmetry, ii) an infinite throat in an extremal limit, iii) the throat limit coincides with an exact CFT construction. The IWP solutions are shown to admit supersymmetric Killing spinors, when embedded in $d=4,N=4$ supergravity. This poses a problem for the interpretation of supersymmetric rotating solutions as physical ground states. In the context of $10$-dimensional geometry, we show that dimensionally lifted versions of the IWP solutions are dual to certain gravitational waves in string theory.

hep-th↗
arXiv1994

Black-Hole-Wave Duality in String Theory

Extreme 4-dimensional dilaton black holes embedded into 10-dimensional geometry are shown to be dual to the gravitational waves in string theory. The corresponding gravitational waves are the generalization of pp-fronted waves, called supersymmetric string waves. They are given by Brinkmann metric and the two-form field, without a dilaton. The non-diagonal part of the metric of the dual partner of the wave together with the two-form field correspond to the vector field in 4-dimensional geometry of the charged extreme black holes.

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arXiv1993

Killing Spinor Identities

We have found generic Killing spinor identities which bosonic equations of motion have to satisfy in supersymmetric theories if the solutions admit Killing spinors. Those identities constrain possible quantum corrections to bosonic solutions with unbroken supersymmetries. As an application we show that purely electric static extreme dilaton black holes may acquire specific quantum corrections, but the purely magnetic ones cannot.

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