137 research outputs found
General considerations of the cosmological constant and the stabilization of moduli in the brane-world picture
We argue that the brane-world picture with matter-fields confined to 4-d
domain walls and with gravitational interactions across the bulk disallows
adding an arbitrary constant to the low-energy, 4-d effective theory -- which
finesses the usual cosmological constant problem. The analysis also points to
difficulties in stabilizing moduli fields; as an alternative, we suggest
scenarios in which the moduli motion is heavily damped by various cosmological
mechanisms and varying ultra-slowly with time.Comment: 5 pages, no figure
Unification with Enlarged Kaluza-Klein Dimensions
In minimal theories with extra spatial dimensions at scales mu_0 much lower
than the conventional GUT scale, unification can give too-large predictions for
alpha_3(M_Z) given alpha_1(M_Z) and alpha_2(M_Z) as empirical input. We
systematically study the effects of adding extra states above the
compactification scale on running of the gauge couplings and find several
simple examples that give unification where all alpha_i(M_Z) are consistent
with low-energy data. We study both the supersymmetric and nonsupersymmetric
unification.Comment: 18 pages, LaTeX, analysis for the susy case takes into account that
extra hypermultiplets come in conjugate pairs, minor changes in text and
references, to be published in Phys. Lett.
On the Ubiquity of K3 Fibrations in String Duality
We consider the general case of N=2 dual pairs of type IIA/heterotic string
theories in four dimensions. We show that if the type IIA string in this pair
can be viewed as having been compactified on a Calabi-Yau manifold in the usual
way then this manifold must be of the form of a K3 fibration. We also see how
the bound on the rank of the gauge group of the perturbative heterotic string
has a natural interpretation on the type IIA side.Comment: LateX, 16 pages, 1 figure, minor corrections, reference adde
Brane-Universe in Six Dimensions
Growing non-singular solution of 6-dimensional Einstein equations for the
4-brane in infinite transversal 2-space is found. This solution provides
gravitational trapping of matter and 4-dimensional gravity on the brane without
extra delta-like source. The suitable solution exists in the case of the
(2+4)-space and not exists for the (1+5)-signature.Comment: Version accepted in Phys.Lett. B. Minor correction
Classification of Flat Directions in Perturbative Heterotic Superstring Vacua with Anomalous U(1)
We develop techniques to classify D- and F-flat directions for N=1
supersymmetric string vacua of the perturbative heterotic string theory, which
possess an anomalous U(1) gauge group at the tree level. Genus-one corrections
generate a Fayet-Iliopoulos term for the D-term of U(1)_A, which is canceled by
non-zero vacuum expectation values (VEVs) of certain massless multiplets in
such a way that the anomalous U(1) is broken, while maintaining the D- and
F-flatness of the effective field theory. A systematic analysis of flat
directions is given for non-zero VEVs of non-Abelian singlets, and the
techniques are illustrated for a specific model. The approach sets the stage to
classify the D- and F-flat directions for a large class of perturbative string
vacua. This classification is a prerequisite to address systematically the
phenomenological consequences of these models.Comment: 26 page
Probing the neutron star interior and the Equation of State of cold dense matter with the SKA
With an average density higher than the nuclear density, neutron stars (NS)
provide a unique test-ground for nuclear physics, quantum chromodynamics (QCD),
and nuclear superfluidity. Determination of the fundamental interactions that
govern matter under such extreme conditions is one of the major unsolved
problems of modern physics, and -- since it is impossible to replicate these
conditions on Earth -- a major scientific motivation for SKA. The most
stringent observational constraints come from measurements of NS bulk
properties: each model for the microscopic behaviour of matter predicts a
specific density-pressure relation (its `Equation of state', EOS). This
generates a unique mass-radius relation which predicts a characteristic radius
for a large range of masses and a maximum mass above which NS collapse to black
holes. It also uniquely predicts other bulk quantities, like maximum spin
frequency and moment of inertia. The SKA, in Phase 1 and particularly in Phase
2 will, thanks to the exquisite timing precision enabled by its raw
sensitivity, and surveys that dramatically increase the number of sources: 1)
Provide many more precise NS mass measurements (high mass NS measurements are
particularly important for ruling out EOS models); 2) Allow the measurement of
the NS moment of inertia in highly relativistic binaries such as the Double
Pulsar; 3) Greatly increase the number of fast-spinning NS, with the potential
discovery of spin frequencies above those allowed by some EOS models; 4)
Improve our knowledge of new classes of binary pulsars such as black widows and
redbacks (which may be massive as a class) through sensitive broad-band radio
observations; and 5) Improve our understanding of dense matter superfluidity
and the state of matter in the interior through the study of rotational
glitches, provided that an ad-hoc campaign is developed.Comment: 22 pages, 8 figures, to be published in: "Advancing Astrophysics with
the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)04
Single Electron Transistors
Contains description of one research project.Joint Services Electronics Program Contract DAAL03-89-C-0001Joint Services Electronics Program Contract DAAL03-92-C-0001National Science Foundation Grant ECS 88-1325
U(1)' Symmetry Breaking in Supersymmetric E6 Models
We study the electroweak and symmetry breaking patterns in models
with the particle content of supersymmetric , including standard model
singlets and exotic quarks . Motivated by free fermionic string
models, we do not require -type relations between Yukawa couplings. In
particular, we assume that baryon and lepton numbers are conserved, so that the
exotic quarks can be light. Gauge invariance allows Yukawa interactions between
and Higgs doublets, and between and the exotic quarks, allowing
radiative symmetry breaking and the generation of an effective
parameter at the electroweak scale. For both the and
models, universal soft supersymmetry breaking parameters and Yukawa
universality at the high (string) scale do not yield acceptable low energy
phenomenology. Relaxing universality, we find solutions with phenomenologically
acceptable values of and the mixing angle. In addition,
by varying the charge assignments due to the mixing of
and of , it is possible to have acceptable low energy
phenomenology with universal boundary conditions.Comment: 24 pages, 6 figures, 4 tables, LaTex; minor revision of the numerical
results, typos corrected, reference adde
Ten-Dimensional Supergravity Constraints from the Pure Spinor Formalism for the Superstring
It has recently been shown that the ten-dimensional superstring can be
quantized using the BRST operator where
is a pure spinor satisfying and
is the fermionic supersymmetric derivative. In this paper, the pure
spinor version of superstring theory is formulated in a curved supergravity
background and it is shown that nilpotency and holomorphicity of the pure
spinor BRST operator imply the on-shell superspace constraints of the
supergravity background. This is shown to lowest order in for the
heterotic and Type II superstrings, thus providing a compact pure spinor
version of the ten-dimensional superspace constraints for N=1, Type IIA and
Type IIB supergravities. Since quantization is straightforward using the pure
spinor version of the superstring, it is expected that these methods can also
be used to compute higher-order corrections to the ten-dimensional
superspace constraints.Comment: 30 pages late
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