arXiv subjects

H. Lu

Publications and source records attributed to H. Lu.

At least 415 records · Page 23Linked to original sources

arXiv1997

Interacting Intersections

Intersecting p-branes can be viewed as higher-dimensional interpretations of multi-charge extremal p-branes, where some of the individual p-branes undergo diagonal dimensional oxidation, while the others oxidise vertically. Although the naive vertical oxidation of a single p-brane gives a continuum of p-branes, a more natural description arises if one considers a periodic array of p-branes in the higher dimension, implying a dependence on the compactification coordinates. This still reduces to the single lower-dimensional p-brane when viewed at distances large compared with the period. Applying the same logic to the multi-charge solutions, we are led to consider more general classes of intersecting p-brane solutions, again depending on the compactification coordinates, which turn out to be described by interacting functions rather than independent harmonic functions. These new solutions also provide a more satisfactory interpretation for the lower-dimensional multi-charge p-branes, which otherwise appear to be nothing more than the improbable coincidence of charge-centres of individual constituents with zero binding energy.

hep-th↗

arXiv1997

Multiplet Structures of BPS Solitons

There exist simple single-charge and multi-charge BPS p-brane solutions in the D-dimensional maximal supergravities. From these, one can fill out orbits in the charge vector space by acting with the global symmetry groups. We give a classification of these orbits, and the associated cosets that parameterise them.

hep-th↗

arXiv1997

Spectrum-generating Symmetries for BPS Solitons

We show that there exist nonlinearly realised duality symmetries that are independent of the standard supergravity global symmetries, and which provide active spectrum-generating symmetries for the fundamental BPS solitons. The additional ingredient, in any spacetime dimension, is a single scaling transformation that allows one to map between BPS solitons with different masses. Without the inclusion of this additional transformation, which is a symmetry of the classical equations of motion, but not the action, it is not possible to find a spectrum-generating symmetry. The necessity of including this scaling transformation highlights the vulnerability of duality multiplets to quantum anomalies. We argue that fundamental BPS solitons may be immune to this threat.

hep-th↗

arXiv1997

Supersymmetry without Supersymmetry

We present four-dimensional M-theory vacua with N>0 supersymmetry which, from the perspective of perturbative Type IIA string theory, have N=0. Such vacua can appear when the compactifying 7-manifold is a U(1) fibration. The missing superpartners are Dirichlet 0-branes. Someone unable to detect Ramond-Ramond charge would thus conclude that these worlds have no unbroken supersymmetry. In particular, the gravitinos (and also some of the gauge bosons) are 0-branes not seen in perturbation theory but which curiously remain massless however weak the string coupling.

hep-th↗

arXiv1997

Cosmological solutions, p-branes and the Wheeler-DeWitt equation

The low energy effective actions which arise from string theory or M-theory are considered in the cosmological context, where the graviton, dilaton and antisymmetric tensor field strengths depend only on time. We show that previous results can be extended to include cosmological solutions that are related to the E_N Toda equations. The solutions of the Wheeler-DeWitt equation in minisuperspace are obtained for some of the simpler cosmological models by introducing intertwining operators that generate canonical transformations which map the theories into free theories. We study the cosmological properties of these solutions, and also briefly discuss generalised Brans-Dicke models in our framework. The cosmological models are closely related to p-brane solitons, which we discuss in the context of the E_N Toda equations. We give the explicit solutions for extremal multi-charge (D-3)-branes in the truncated system described by the D_4 =O(4,4) Toda equations.

hep-th↗

arXiv1997

Towards a Field Theory of F-theory

We make a proposal for a bosonic field theory in twelve dimensions that admits the bosonic sector of eleven-dimensional supergravity as a consistent truncation. It can also be consistently truncated to a ten-dimensional Lagrangian that contains all the BPS p-brane solitons of the type IIB theory. The mechanism allowing the consistent truncation in the latter case is unusual, in that additional fields with an off-diagonal kinetic term are non-vanishing and yet do not contribute to the dynamics of the ten-dimensional theory. They do, however, influence the oxidation of solutions back to twelve dimensions. We present a discussion of the oxidations of all the basic BPS solitons of M-theory and the type IIB string to D=12. In particular, the NS-NS and R-R strings of the type IIB theory arise as the wrappings of membranes in D=12 around one or other circle of the compactifying 2-torus.

hep-th↗

arXiv1997

T-duality and U-duality in toroidally-compactified strings

We address the issue of T-duality and U-duality symmetries in the toroidally-compactified type IIA string. It is customary to take as a starting point the dimensionally-reduced maximal supergravity theories, with certain field strengths dualised such that the classical theory exhibits a global $E_{n(n)}$ symmetry, where n=11-D in D dimensions. A discrete subgroup then becomes the conjectured U-duality group. In dimensions D\le 6, these necessary dualisations include NS-NS fields, whose potentials, rather than merely their field strengths, appear explicitly in the couplings to the string worldsheet. Thus the usually-stated U-duality symmetries act non-locally on the fundamental fields of perturbative string theory. At least at the perturbative level, it seems to be more appropriate to consider the symmetries of the versions of the lower-dimensional supergravities in which no dualisations of NS-NS fields are required, although dualisations of the R-R fields are permissible since these couple to the string through their field strengths. Taking this viewpoint, the usual T-duality groups survive unscathed, as one would hope since T-duality is a perturbative symmetry, but the U-duality groups are modified in D\le 6.

hep-th↗

arXiv1997

p-brane Taxonomy

We review an approach to the construction and classification of p-brane solitons in arbitrary dimensions, with an emphasis on those that arise in toroidally-compactified M-theory. Procedures for constructing the low-energy supergravity limits in arbitrary dimensions, and for studying the supersymmetry properties of the solitons are presented. Wide classes of p-brane solutions are obtained, and their properties and classification in terms of bound states and intersections of M-branes are described. (Based on lectures presented at the Summer School in High-Energy Physics and Cosmology, Trieste, Italy, 10 Jun - 26 Jul 1996.)

hep-th↗

arXiv1997

Harmonic superpositions of non-extremal p-branes

The plot of allowed p and D values for p-brane solitons in D-dimensional supergravity is the same whether the solitons are extremal or non-extremal. One of the useful tools for relating different points on the plot is vertical dimensional reduction, which is possible if periodic arrays of p-brane solitons can be constructed. This is straightforward for extremal p-branes, since the no-force condition allows arbitrary multi-centre solutions to be constructed in terms of a general harmonic function on the transverse space. This has also been shown to be possible in the special case of non-extremal black holes in D=4 arrayed along an axis. In this paper, we extend previous results to include multi-scalar black holes, and dyonic black holes. We also consider their oxidation to higher dimensions, and we discuss general procedures for constructing the solutions, and studying their symmetries.

hep-th↗

arXiv1996

From p-branes to Cosmology

We study the relationship between static p-brane solitons and cosmological solutions of string theory or M-theory. We discuss two different ways in which extremal p-branes can be generalised to non-extremal ones, and show how wide classes of recently discussed cosmological models can be mapped into non-extremal p-brane solutions of one of these two kinds. We also extend previous discussions of cosmological solutions to include some that make use of cosmological-type terms in the effective action that can arise from the generalised dimensional reduction of string theory or M-theory.

hep-th↗

arXiv1996

From Topology to Generalised Dimensional Reduction

In the usual procedure for toroidal Kaluza-Klein reduction, all the higher-dimensional fields are taken to be independent of the coordinates on the internal space. It has recently been observed that a generalisation of this procedure is possible, which gives rise to lower-dimensional ``massive'' supergravities. The generalised reduction involves allowing gauge potentials in the higher dimension to have an additional linear dependence on the toroidal coordinates. In this paper, we show that a much wider class of generalised reductions is possible, in which higher-dimensional potentials have additional terms involving differential forms on the internal manifold whose exterior derivatives yield representatives of certain of its cohomology classes. We consider various examples, including the generalised reduction of M-theory and type II strings on K3, Calabi-Yau and 7-dimensional Joyce manifolds. The resulting massive supergravities support domain-wall solutions that arise by the vertical dimensional reduction of higher-dimensional solitonic p-branes and intersecting p-branes.

hep-th↗

arXiv1996

Domain Walls from M-branes

We discuss the vertical dimensional reduction of M-branes to domain walls in D=7 and D=4, by dimensional reduction on Ricci-flat 4-manifolds and 7-manifolds. In order to interpret the vertically-reduced 5-brane as a domain wall solution of a dimensionally-reduced theory in D=7, it is necessary to generalise the usual Kaluza-Klein ansatz, so that the 3-form potential in D=11 has an additional term that can generate the necessary cosmological term in D=7. We show how this can be done for general 4-manifolds, extending previous results for toroidal compactifications. By contrast, no generalisation of the Kaluza-Klein ansatz is necessary for the compactification of M-theory to a D=4 theory that admits the domain wall solution coming from the membrane in D=11.

hep-th↗

arXiv1996

Domain Walls in Massive Supergravities

We show how toroidally-compactified eleven-dimensional supergravity can be consistently truncated to yield a variety of maximally-supersymmetric ``massive'' supergravities in spacetime dimensions $D\le 8$. The mass terms arise as a consequence of making a more general ansatz than that in usual Kaluza-Klein dimensional reduction, in which one or more axions are given an additional linear dependence on one of the compactification coordinates. The lower-dimensional theories are nevertheless consistent truncations of eleven-dimensional supergravity. Owing to the fact that the generalised reduction commutes neither with U-duality nor with ordinary dimensional reduction, many different massive theories can result. The simplest examples arise when just a single axion has the additional linear coordinate dependence. We find five inequivalent such theories in D=7, and 71 inequivalent ones in D=4. The massive theories admit no maximally-symmetric vacuum solution, but they do admit $(D-2)$-brane solutions, i.e. domain walls, which preserve half the supersymmetry. We present examples of these solutions, and their oxidations to D=11. Some of the latter are new solutions of D=11 supergravity.

hep-th↗

arXiv1996

Cosmological Solutions in String Theories

We obtain a large class of cosmological solutions in the toroidally-compactified low energy limits of string theories in $D$ dimensions. We consider solutions where a $p$-dimensional subset of the spatial coordinates, parameterising a flat space, a sphere, or an hyperboloid, describes the spatial sections of the physically-observed universe. The equations of motion reduce to Liouville or $SL(N+1,R)$ Toda equations, which are exactly solvable. We study some of the cases in detail, and find that under suitable conditions they can describe four-dimensional expanding universes. We discuss also how the solutions in $D$ dimensions behave upon oxidation back to the $D=10$ string theory or $D=11$ M-theory.

hep-th↗

arXiv1996

Domain Walls from Anti-de Sitter Spacetime

We examine $(D-2)$-brane solutions in supergravities, showing that they fall into four categories depending on the details of the dilaton coupling. In general they describe domain walls, although in one of the four categories the metric describes anti-de Sitter spacetime. We study this case, and its $S^1$ dimensional reduction to a more conventional domain wall in detail, focussing in particular on the manner in which the unbroken supersymmetry of the anti-de Sitter solution is partially broken by the dimensional reduction to the domain wall.

hep-th↗

arXiv1996

Loop-corrected Entropy of Near-extremal Dilatonic $p$-branes

It has recently been shown that for certain classical non-dilatonic $p$-branes, the entropy and temperature satisfy the ideal-gas relation $S\sim T^{p}$ in the near-extremal regime. We extend these results to cases where the dilaton is non-vanishing, but nevertheless remains finite on the horizon in the extremal limit, showing that the ideal-gas relation is again satisfied. At the classical level, however, this relation does break down if the dilaton diverges on the horizon. We argue that such a divergence indicates the breakdown of the validity of the classical approximation, and that by taking string and worldsheet loop corrections into account, the validity of the entropy/temperature relation may be extended to include these cases. This opens up the possibility of giving a microscopic interpretation of the entropy for all near-extremal $p$-branes.

hep-th↗

arXiv1996

Intersecting M-branes and bound states

In this paper, we construct multi-scalar, multi-center $p$-brane solutions in toroidally compactified M-theory. We use these solutions to show that all supersymmetric $p$-branes can be viewed as bound states of certain basic building blocks, namely $p$-branes that preserve $1/2$ of the supersymmetry. We also explore the M-theory interpretation of $p$-branes in lower dimensions. We show that all the supersymmetric $p$-branes can be viewed as intersections of M-branes or boosted M-branes in $D=11$.

hep-th↗

arXiv1996

Vertical versus Diagonal Dimensional Reduction for p-branes

In addition to the double-dimensional reduction procedure that employs world-volume Killing symmetries of $p$-brane supergravity solutions and acts diagonally on a plot of $p$ versus spacetime dimension $D$, there exists a second procedure of ``vertical'' reduction. This reduces the transverse-space dimension via an integral that superposes solutions to the underlying Laplace equation. We show that vertical reduction is also closely related to the recently-introduced notion of intersecting $p$-branes. We illustrate this with examples, and also construct a new $D=11$ solution describing four intersecting membranes, which preserves $1/16$ of the supersymmetry. Given the two reduction schemes plus duality transformations at special points of the scalar modulus space, one may relate most of the $p$-brane solutions of relevance to superstring theory. We argue that the maximum classifying duality symmetry for this purpose is the Weyl group of the corresponding Cremmer-Julia supergravity symmetry $E_{r(+r)}$. We also discuss a separate class of duality-invariant $p$-branes with $p=D-3$.

hep-th↗