149 research outputs found
Black diholes in five dimensions
Using a generalized Weyl formalism, we show how stationary, axisymmetric
solutions of the four-dimensional vacuum Einstein equation can be turned into
static, axisymmetric solutions of five-dimensional dilaton gravity coupled to a
two-form gauge field. This procedure is then used to obtain new solutions of
the latter theory describing pairs of extremal magnetic black holes with
opposite charges, known as black diholes. These diholes are kept in static
equilibrium by membrane-like conical singularities stretching along two
different directions. We also present solutions describing diholes suspended in
a background magnetic field, and with unbalanced charges.Comment: 21 pages, 2 figures; reference adde
Rotating Circular Strings, and Infinite Non-Uniqueness of Black Rings
We present new self-gravitating solutions in five dimensions that describe
circular strings, i.e., rings, electrically coupled to a two-form potential (as
e.g., fundamental strings do), or to a dual magnetic one-form. The rings are
prevented from collapsing by rotation, and they create a field analogous to a
dipole, with no net charge measured at infinity. They can have a regular
horizon, and we show that this implies the existence of an infinite number of
black rings, labeled by a continuous parameter, with the same mass and angular
momentum as neutral black rings and black holes. We also discuss the solution
for a rotating loop of fundamental string. We show how more general rings arise
from intersections of branes with a regular horizon (even at extremality),
closely related to the configurations that yield the four-dimensional black
hole with four charges. We reproduce the Bekenstein-Hawking entropy of a large
extremal ring through a microscopic calculation. Finally, we discuss some
qualitative ideas for a microscopic understanding of neutral and dipole black
rings.Comment: 31 pages, 7 figures. v2: minor changes, added reference. v3:
erroneous values of T_{ww} (eq.(3.39)) and n_p (eq.(5.20)) corrected, and
accompanying discussion amended. In the journal version these corrections
appear as an appended erratum. No major changes involve
Classification of Higher Dimensional Spacetimes
We algebraically classify some higher dimensional spacetimes, including a
number of vacuum solutions of the Einstein field equations which can represent
higher dimensional black holes. We discuss some consequences of this work.Comment: 16 pages, 1 Tabl
Caged Black Holes: Black Holes in Compactified Spacetimes I -- Theory
In backgrounds with compact dimensions there may exist several phases of
black objects including the black-hole and the black-string. The phase
transition between them raises puzzles and touches fundamental issues such as
topology change, uniqueness and Cosmic Censorship. No analytic solution is
known for the black hole, and moreover, one can expect approximate solutions
only for very small black holes, while the phase transition physics happens
when the black hole is large. Hence we turn to numerical solutions. Here some
theoretical background to the numerical analysis is given, while the results
will appear in a forthcoming paper. Goals for a numerical analysis are set. The
scalar charge and tension along the compact dimension are defined and used as
improved order parameters which put both the black hole and the black string at
finite values on the phase diagram. Predictions for small black holes are
presented. The differential and the integrated forms of the first law are
derived, and the latter (Smarr's formula) can be used to estimate the ``overall
numerical error''. Field asymptotics and expressions for physical quantities in
terms of the numerical ones are supplied. Techniques include ``method of
equivalent charges'', free energy, dimensional reduction, and analytic
perturbation for small black holes.Comment: 23 pages. v3: version to be published in PRD, 3 references adde
Spatial infinity in higher dimensional spacetimes
Motivated by recent studies on the uniqueness or non-uniqueness of higher
dimensional black hole spacetime, we investigate the asymptotic structure of
spatial infinity in n-dimensional spacetimes(). It turns out that the
geometry of spatial infinity does not have maximal symmetry due to the
non-trivial Weyl tensor {}^{(n-1)}C_{abcd} in general. We also address static
spacetime and its multipole moments P_{a_1 a_2 ... a_s}. Contrasting with four
dimensions, we stress that the local structure of spacetimes cannot be unique
under fixed a multipole moments in static vacuum spacetimes. For example, we
will consider the generalized Schwarzschild spacetimes which are deformed black
hole spacetimes with the same multipole moments as spherical Schwarzschild
black holes. To specify the local structure of static vacuum solution we need
some additional information, at least, the Weyl tensor {}^{(n-2)}C_{abcd} at
spatial infinity.Comment: 6 pages, accepted for publication in Physical Review D, published
versio
Relativistic Calculations for Photonuclear Reactions (III): A Consistent Relativistic Analysis of the (e,e'p) and (gamma,p) Reactions
Relativistic calculations for the quasifree electron scattering process (e,
e'p) and the direct knockout contribution to (gamma, p) reactions are
presented. The spectroscopic factors determined from the former reaction are
used to fix the magnitude of the knockout contribution to the (gamma, p)
reaction at 60 MeV. The results obtained for several nuclei indicate that the
knockout contributions are much larger in magnitude and hence closer to the
data than predicted in an earlier comparison based on non-relativistic
calculations. We discuss the sensitivity of the results to the choice of
parameters for the binding and final state interactions. We find these
uncertainties to be more pronounced at the larger missing momenta explored by
the (gamma, p) reaction. The implications of the present results for the size
of contributions due to meson exchange currents are discussed.Comment: LaTeX, 21 pages including 5 figures, submitted to Nuc. Phys.
AdS/CFT, Multitrace Deformations and New Instabilities of Nonlocal String Theories
We study "multitrace" deformations of large N master fields in models with a
mass gap. In particular, we determine the conditions for the multitrace
couplings to drive tachyonic instabilities. These tachyons represent new local
instabilities of the associated nonlocal string theories. In the particular
case of Dp-branes at finite temperature, we consider topology-changing phase
transitions and the effect of multitrace perturbations on the corresponding
phase diagrams.Comment: harvmac. 28 pages. 9 eps figure
Relativistic versus Nonrelativistic Optical Potentials in A(e,e'p)B Reactions
We investigate the role of relativistic and nonrelativistic optical
potentials used in the analysis of () data. We find that the
relativistic calculations produce smaller () cross sections even in the
case in which both relativistic and nonrelativistic optical potentials fit
equally well the elastic proton--nucleus scattering data. Compared to the
nonrelativistic impulse approximation, this effect is due to a depletion in the
nuclear interior of the relativistic nucleon current, which should be taken
into account in the nonrelativistic treatment by a proper redefinition of the
effective current operator.Comment: Added one new figure, the formalism section has been enlarged and the
list of references updated. Added one appendix. This version will appear in
Phys. Rev. C. Revtex 3.0, 6 figures (not included). Full postscript version
of the file and figures available at
http://www.nikhefk.nikhef.nl/projects/Theory/preprints
Sequences of Bubbles and Holes: New Phases of Kaluza-Klein Black Holes
We construct and analyze a large class of exact five- and six-dimensional
regular and static solutions of the vacuum Einstein equations. These solutions
describe sequences of Kaluza-Klein bubbles and black holes, placed alternately
so that the black holes are held apart by the bubbles. Asymptotically the
solutions are Minkowski-space times a circle, i.e. Kaluza-Klein space, so they
are part of the (\mu,n) phase diagram introduced in hep-th/0309116. In
particular, they occupy a hitherto unexplored region of the phase diagram,
since their relative tension exceeds that of the uniform black string. The
solutions contain bubbles and black holes of various topologies, including
six-dimensional black holes with ring topology S^3 x S^1 and tuboid topology
S^2 x S^1 x S^1. The bubbles support the S^1's of the horizons against
gravitational collapse. We find two maps between solutions, one that relates
five- and six-dimensional solutions, and another that relates solutions in the
same dimension by interchanging bubbles and black holes. To illustrate the
richness of the phase structure and the non-uniqueness in the (\mu,n) phase
diagram, we consider in detail particular examples of the general class of
solutions.Comment: 71 pages, 22 figures, v2: Typos fixed, comment added in sec. 5.
Nuclear Medium Effects in the Relativistic Treatment of Quasifree Electron Scattering
Non-relativistic reduction of the S-matrix for the quasifree electron
scattering process is studied in order to
understand the source of differences between non-relativistic and relativistic
models. We perform an effective Pauli reduction on the relativistic expression
for the S-matrix in the one-photon exchange approximation. The reduction is
applied to the nucleon current only; the electrons are treated fully
relativistically. An expansion of the amplitude results in a power series in
the nuclear potentials. The series is found to converge rapidly only if the
nuclear potentials are included in the nuclear current operator. The results
can be cast in a form which reproduces the non-relativistic amplitudes in the
limit that the potentials are removed from the nuclear current operator. Large
differences can be found between calculations which do and do not include the
nuclear potentials in the different orders of the nuclear current operator. In
the high missing momentum region we find that the non-relativistic calculations
with potentials included in the nuclear current up to second order give results
which are close to those of the fully relativistic calculation. This behavior
is an indication of the importance of the medium modifications of the nuclear
currents in this model, which are naturally built into the relativistic
treatment of the reaction.Comment: Latex, 26 pages including 5 uuencoded postscript figures. accepted
for publication in Phys. Rev. C
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