2,125 research outputs found
D-brane Instantons in Type II String Theory
We review recent progress in determining the effects of D-brane instantons in
N=1 supersymmetric compactifications of Type II string theory to four
dimensions. We describe the abstract D-brane instanton calculus for holomorphic
couplings such as the superpotential, the gauge kinetic function and higher
fermionic F-terms. This includes a discussion of multi-instanton effects and
the implications of background fluxes for the instanton sector. Our
presentation also highlights, but is not restricted to the computation of
D-brane instanton effects in quiver gauge theories on D-branes at
singularities. We then summarize the concrete consequences of stringy D-brane
instantons for the construction of semi-realistic models of particle physics or
SUSY-breaking in compact and non-compact geometries.Comment: Invited review to appear in Annu.Rev.Nuc.Part.Sci 2009 59; 69 pages,
8 figures, 5 tables; v2: 1 reference adde
New higher-order transition in causal dynamical triangulations
We reinvestigate the recently discovered bifurcation phase transition in
Causal Dynamical Triangulations (CDT) and provide further evidence that it is a
higher order transition. We also investigate the impact of introducing matter
in the form of massless scalar fields to CDT. We discuss the impact of scalar
fields on the measured spatial volumes and fluctuation profiles in addition to
analysing how the scalar fields influence the position of the bifurcation
transition.Comment: 15 pages, 11 figures. Conforms with version accepted for publication
in Phys. Rev.
Simulation of some quantum gates, with decoherence
Methods and results for numerical simulations of one and two interacting
rf-Squid systems suitable for adiabatic quantum gates are presented. These are
based on high accuracy numerical solutions to the static and time dependent
Schroedinger equation for the full Squid Hamiltonian in one and two variables.
Among the points examined in the static analysis is the range of validity of
the effective two-state or ``spin 1/2'' picture. A range of parameters is
determined where the picture holds to good accuracy as the energy levels
undergo gate manipulations. Some general points are presented concerning the
relations between device parameters and ``good'' quantum mechanical state
spaces.
The time dependent simulations allow the examination of suitable conditions
for adiabatic behavior, and permits the introduction of a random noise to
simulate the effects of decoherence. A formula is derived and tested relating
the random noise to the decoherence rate. Sensitivity to device and operating
parameters for the logical gates NOT and CNOT are examined, with particular
attention to values of the tunnel parameter beta slightly above one. It appears
that with values of beta close to one, a quantum CNOT gate is possible even
with rather short decoherence times.
Many of the methods and results will apply to coupled double-potential well
systems in general.Comment: 26 pages, 15 figures, Some clarification added on decoherence
treatment, many small errors corrected, symbols on some figures enlarged,
refs added. No change in conten
Planckian Birth of the Quantum de Sitter Universe
We show that the quantum universe emerging from a nonperturbative, Lorentzian
sum-over-geometries can be described with high accuracy by a four-dimensional
de Sitter spacetime. By a scaling analysis involving Newton's constant, we
establish that the linear size of the quantum universes under study is in
between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum
fluctuations around the de Sitter universe in this regime are to good
approximation still describable semiclassically. The numerical evidence
presented comes from a regularization of quantum gravity in terms of causal
dynamical triangulations.Comment: Article unchanged. Line added in acknowledgmen
Toward Realistic Intersecting D-Brane Models
We provide a pedagogical introduction to a recently studied class of
phenomenologically interesting string models, known as Intersecting D-Brane
Models. The gauge fields of the Standard-Model are localized on D-branes
wrapping certain compact cycles on an underlying geometry, whose intersections
can give rise to chiral fermions. We address the basic issues and also provide
an overview of the recent activity in this field. This article is intended to
serve non-experts with explanations of the fundamental aspects, and also to
provide some orientation for both experts and non-experts in this active field
of string phenomenology.Comment: 85 pages, 8 figures, Latex, Bibtex, v2: refs added, typos correcte
Moduli Stabilization in Toroidal Type IIB Orientifolds
We discuss the first step in the moduli stabilization program a la KKLT for a
general class of resolved toroidal type IIB orientifolds. In particular, we
discuss their geometry, the topology of the divisors relevant for the D3-brane
instantons which can contribute to the superpotential, and some non--trivial
aspects of the orientifold action.Comment: 15 pages, 3 figures, contribution to the proceedings of the RTN
workshop "Constituents, Fundamental Forces and Symmetries of the Universe",
Corfu, Greece, 20-26 September 200
The Cleo Rich Detector
We describe the design, construction and performance of a Ring Imaging
Cherenkov Detector (RICH) constructed to identify charged particles in the CLEO
experiment. Cherenkov radiation occurs in LiF crystals, both planar and ones
with a novel ``sawtooth''-shaped exit surface. Photons in the wavelength
interval 135--165 nm are detected using multi-wire chambers filled with a
mixture of methane gas and triethylamine vapor. Excellent pion/kaon separation
is demonstrated.Comment: 75 pages, 57 figures, (updated July 26, 2005 to reflect reviewers
comments), to be published in NIM
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