260 research outputs found
Who is willing to help Ukrainian refugees and why? The role of individual prosocial dispositions and superordinate European identity
info:eu-repo/semantics/publishe
String Form Factors
We compute the cross section for scattering of light string probes by
randomly excited closed strings. For high energy probes, the cross section
factorizes and can be used to define effective form factors for the excited
targets. These form factors are well defined without the need for infinite
subtractions and contain information about the shape and size of typical
strings. For highly excited strings the elastic form factor can be written in
terms of the `plasma dispersion function', which describes charge screening in
high temperature plasmas.Comment: 18 pages, 3 figures. Typos corrected, 1 footnote (in Section 4) and 1
reference adde
Decay of long-lived massive closed superstring states: Exact results
We find a one-parameter family of long-lived physical string states in type
II superstring theory. We compute the decay rate by an exact numerical
evaluation of the imaginary part of the one-loop propagator. Remarkably, the
lifetime rapidly increases with the mass. We find a power-law dependence of the
form , where the value of depends on
the parameter characterizing the state. For the most stable state in this
family, one has . The dominant decay channel of these massive
string states is by emission of soft massless particles. The quantum states can
be viewed semiclassically as closed strings which cannot break during the
classical evolution.Comment: Latex, 5 figures, 35 pages (= 23 pages + appendices). Minor
correction
Exactly solvable model of superstring in Ramond-Ramond plane wave background
We describe in detail the solution of type IIB superstring theory in the
maximally supersymmetric plane-wave background with constant null Ramond-Ramond
5-form field strength. The corresponding light-cone Green-Schwarz action found
in hep-th/0112044 is quadratic in both bosonic and fermionic coordinates. We
find the spectrum of the light-cone Hamiltonian and the string representation
of the supersymmetry algebra. The superstring Hamiltonian has a
``harmonic-oscillator'' form in both the string-oscillator and the zero-mode
parts and thus has discrete spectrum in all 8 transverse directions. We analyze
the structure of the zero-mode sector of the theory, establishing the precise
correspondence between the lowest-lying ``massless'' string states and the type
IIB supergravity fluctuation modes in the plane-wave background. The zero-mode
spectrum has certain similarity to the supergravity spectrum in AdS_5 x S^5 of
which the plane-wave background is a special limit. We also compare the
plane-wave string spectrum with expected form of the light-cone gauge spectrum
of superstring in AdS_5 x S^5.Comment: 33 pages, latex. v4: minor sign corrections in (1.5) and (3.62), to
appear in PR
Wavy Strings: Black or Bright?
Recent developments in string theory have brought forth a considerable
interest in time-dependent hair on extended objects. This novel new hair is
typically characterized by a wave profile along the horizon and angular
momentum quantum numbers in the transverse space. In this work, we
present an extensive treatment of such oscillating black objects, focusing on
their geometric properties. We first give a theorem of purely geometric nature,
stating that such wavy hair cannot be detected by any scalar invariant built
out of the curvature and/or matter fields. However, we show that the tidal
forces detected by an infalling observer diverge at the `horizon' of a black
string superposed with a vibration in any mode with . The same
argument applied to longitudinal () waves detects only finite tidal
forces. We also provide an example with a manifestly smooth metric, proving
that at least a certain class of these longitudinal waves have regular
horizons.Comment: 45 pages, latex, no figure
The Enhancon, Black Holes, and the Second Law
We revisit the physics of five-dimensional black holes constructed from D5-
and D1-branes and momentum modes in type IIB string theory compactified on K3.
Since these black holes incorporate D5-branes wrapped on K3, an enhancon locus
appears in the spacetime geometry. With a `small' number of D1-branes, the
entropy of a black hole is maximised by including precisely half as many
D5-branes as there are D1-branes in the black hole. Any attempts to introduce
more D5-branes, and so reduce the entropy, are thwarted by the appearance of
the enhancon locus above the horizon, which then prevents their approach. The
enhancon mechanism thereby acts to uphold the Second Law of Thermodynamics.
This result generalises: For each type of bound state object which can be made
of both types of brane, we show that a new type of enhancon exists at
successively smaller radii in the geometry, again acting to prevent any
reduction of the entropy just when needed. We briefly explore the appearance of
the enhancon in the black hole interior.Comment: 22 pages, 2 figures, latex, epsfig (v2: Fixed trivial typos.
Superheavy Dark Matter with Discrete Gauge Symmetries
We show that there are discrete gauge symmetries protect naturally heavy X
particles from decaying into the ordinary light particles in the supersymmetric
standard model. This makes the proposal very attractive that the superheavy X
particles constitute a part of the dark matter in the present universe. It is
more interesting that there are a class of discrete gauge symmetries which
naturally accommodate a long-lived unstable X particle. We find that in some
discrete Z_{10} models, for example, a superheavy X particle has lifetime
\tau_X \simeq 10^{11}-10^{26} years for its mass M_X \simeq 10^{13}-10^{14}
GeV. This long lifetime is guaranteed by the absence of lower dimensional
operators (of light particles) couple to the X. We briefly discuss a possible
explanation for the recently observed ultra-high-energy cosmic ray events by
the decay of this unstable X particle.Comment: 9 pages, Late
On Large N Gauge Theories from Orientifolds
We consider four dimensional supersymmetric gauge theories
obtained via orientifolds of Type IIB on Abelian C^3/G orbifolds. We construct
all such theories that have well defined world-sheet expansion. The number of
such orientifolds is rather limited. We explain this fact in the context of
recent developments in four dimensional Type IIB orientifolds. In particular,
we elaborate these issues in some examples of theories where world-sheet
description is inadequate due to non-perturbative (from the orientifold
viewpoint) states arising from D-branes wrapping (collapsed) 2-cycles in the
orbifold. We find complete agreement with the corresponding statements recently
discussed in the context of Type I compactifications on toroidal orbifolds.
This provides a non-trivial check for correctness of the corresponding
conclusions. We also find non-trivial agreement with various field theory
expectations, and point out their origin in string language. The orientifold
gauge theories that do possess well defined world-sheet description have the
property that in the large N limit computation of any M-point correlation
function in these theories reduces to the corresponding computation in the
parent oriented theory.Comment: 21 pages, revtex, minor errors and misprints corrected (to appear in
Phys. Rev. D
Introduction to M Theory and AdS/CFT Duality
An introductory survey of some of the developments that have taken place in
superstring theory in the past few years is presented. The main focus is on
three particular dualities. The first one is the appearance of an 11th
dimension in the strong coupling limit of the type IIA theory, which give rise
to M theory. The second one is the duality between the type IIB theory
compactified on a circle and M theory on a two-torus. The final topic is an
introduction to the recently proposed duality between superstring theory or M
theory on certain anti de Sitter space backgrounds and conformally invariant
quantum field theories.Comment: 26 pages; To be published in the Proceedings of a conference held in
Corfu, Greece in September 1998. v2: reference adde
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