Existence of Majorana fermions for M-branes wrapped in space and time

We show that it is possible to define Majorana (s)pinor fields on M-branes which have been identified under the action of the antipodal map on the adS factor of the throat geometry, or which have been wrapped on two-cycles of arbitrary genus. This is an important consistency check, since it means that one may still take the generators of supertranslations in superspace to transform as Majorana fermions under the adjoint action of $Spin(10,1)$, even though the antipodally identified M2-brane is {\it not} space-orientable. We point out that similar conclusions hold for any p-branes which have the generic (adS)$~{\times}~$(Sphere) throat geometry.Comment: 10 pages REVTe

Puncture of gravitating domain walls

We investigate the semi-classical instability of vacuum domain walls to processes where the domain walls decay by the formation of closed string loop boundaries on their worldvolumes. Intuitively, a wall which is initially spherical may `pop', so that a hole corresponding to a string boundary component on the wall, may form. We find instantons, and calculate the rates, for such processes. We show that after puncture, the hole grows exponentially at the same rate that the wall expands. It follows that the wall is never completely thermalized by a single expanding hole; at arbitrarily late times there is still a large, thin shell of matter which may drive an exponential expansion of the universe. We also study the situation where the wall is subjected to multiple punctures. We find that in order to completely annihilate the wall by this process, at least four string loops must be nucleated. We argue that this process may be relevant in certain brane-world scenarios, where the universe itself is a domain wall.Comment: 13 pages REVTeX, 3 .ps figures, added some references - version to appear in Physics Letters

Some applications of differential topology in general relativity

Recently, there have been several applications of differential and algebraic topology to problems concerned with the global structure of spacetimes. In this paper, we derive obstructions to the existence of spin-Lorentz and pin-Lorentz cobordisms and we show that for compact spacetimes with non-empty boundary there is no relationship between the homotopy type of the Lorentz metric and the causal structure. We also point out that spin-Lorentz and tetrad cobordism are equivalent. Furthermore, because the original work [7] on metric homotopy and causality may not be known to a wide audience, we present an overview of the results here.Comment: 24 pages LaTeX, 8 xfig figures available from A. Chamblin at [email protected], published in Jour. of Geometry and Physics, 13, pages 357-377 (1994

Thermal Equilibration of Brane-Worlds

We analyze the thermodynamical properties of brane-worlds, with a focus on the second model of Randall and Sundrum. We point out that during an inflationary phase on the brane, black holes will tend to be thermally nucleated in the bulk. This leads us to ask the question: Can the black hole - brane-world system evolve towards a configuration of thermal equilibrium? To answer this, we generalize the second Randall-Sundrum scenario to allow for non-static bulk regions on each side of the brane-world. Explicitly, we take the bulk to be a {\it Vaidya-AdS} metric, which describes the gravitational collapse of a spherically symmetric null dust fluid in Anti-de Sitter spacetime. Using the background subtraction technique to calculate the Euclidean action, we argue that at late times a sufficiently large black hole will relax to a point of thermal equilibrium with the brane-world environment. These results have interesting implications for early-universe cosmology.Comment: 4 pages REVTeX. The basic argument is simplified. Version to appear in Physics Letters

Top Quark Production from Black Holes at the CERN LHC

LHC is expected to be a top quark factory. If the fundamental Planck scale is near a TeV, then we also expect the top quarks to be produced from black holes via Hawking radiation. In this paper we calculate the cross sections for top quark production from black holes at the LHC and compare it with the direct top quark cross section via parton fusion processes at next-to-next-to-leading order (NNLO). We find that the top quark production from black holes can be larger or smaller than the pQCD predictions at NNLO depending upon the Planck mass and black hole mass. Hence the observation of very high rates for massive particle production (top quarks, higgs or supersymmetry) at the LHC may be an useful signature for black hole production.Comment: 13 pages latex, 3 figures; Accepted for publication in Physics Letters