91 research outputs found
The Apparent Universe
We exploit the parallel between dynamical black holes and cosmological
spacetimes to describe the evolution of Friedmann-Lema\^itre-Robertson-Walker
universes from the point of view of an observer in terms of the dynamics of the
apparent horizon. Using the Hayward-Kodama formalism of dynamical black holes,
we clarify the role of the Clausius relation to derive the Friedmann equations
for a universe, in the spirit of Jacobson's work on the thermodynamics of
spacetime. We also show how dynamics at the horizon naturally leads to the
quantum-mechanical process of Hawking radiation. We comment on the connection
of this work with recent ideas to consider our observable Universe as a
Bose-Einstein condensate and on the corresponding role of vacuum energy.Comment: 16 pages, 1 figur
Modular Invariant Formulation of Multi-Gaugino and Matter Condensation
Using the linear multiplet formulation for the dilaton superfield, we
construct an effective lagrangian for hidden-sector gaugino condensation in
string effective field theories with arbitrary gauge groups and matter.
Nonperturbative string corrections to the K\"ahler potential are invoked to
stabilize the dilaton at a supersymmetry breaking minimum of the potential.
When the cosmological constant is tuned to zero the moduli are stabilized at
their self-dual points, and the vev's of their F-component superpartners
vanish. Numerical analyses of one- and two-condensate examples with massless
chiral matter show considerable enhancement of the gauge hierarchy with respect
to the E_8 case. The nonperturbative string effects required for dilaton
stabilization may have implications for gauge coupling unification. As a
comparison, we also consider a parallel approach based on the commonly used
chiral formulation.Comment: 37 pages, LaTeX, 3 postscript figures. Footnote of p.26 and ref.19
are corrected. Ref.16 is update
Pseudo-anomalous U(1) symmetry in the strong coupling limit of the heterotic string
We discuss, in the context of the strongly coupled E_8 \times E_8 heterotic
string proposed by Horava and Witten, the appearance of anomalous U(1)_X
symmetries of a nonperturbative origin, related to the presence, after
compactification, of five-branes in the five-dimensional bulk of the theory. We
compute the gauge anomalies and the induced Fayet-Iliopoulos terms on each
boundary, which we find to be lower than the universal one induced in the
weakly coupled case.Comment: 15 pages, 2 figures, late
Supersymmetry breaking and weakly vs. strongly coupled string theory
In the context of the field theory limit of superstrings, we consider an
almost realistic model of supersymmetry breaking by gaugino condensation which
includes, through nonperturbative corrections to the K\"ahler potential,
dilaton stabilization at a value compatible with a weak coupling regime.
Invariance under modular transformations is ensured through a Green-Schwarz
term and string threshold corrections, which lead to moduli stabilization at
the self-dual point. We are thus in a position to discuss several issues of
physical relevance: gravitino, dilaton and moduli masses, axion, soft
supersymmetry breaking parameters and gauge coupling unification.Comment: LATEX, 15 page
One Loop Soft Supersymmetry Breaking Terms in Superstring Effective Theories
We perform a systematic analysis of soft supersymmetry breaking terms at the
one loop level in a large class of string effective field theories. This
includes the so-called anomaly mediated contributions. We illustrate our
results for several classes of orbifold models. In particular, we discuss a
class of models where soft supersymmetry breaking terms are determined by quasi
model independent anomaly mediated contributions, with possibly non-vanishing
scalar masses at the one loop level. We show that the latter contribution
depends on the detailed prescription of the regularization process which is
assumed to represent the Planck scale physics of the underlying fundamental
theory. The usual anomaly mediation case with vanishing scalar masses at one
loop is not found to be generic. However gaugino masses and A-terms always
vanish at tree level if supersymmetry breaking is moduli dominated with the
moduli stabilized at self-dual points, whereas the vanishing of the B-term
depends on the origin of the mu-term in the underlying theory. We also discuss
the supersymmetric spectrum of O-I and O-II models, as well as a model of
gaugino condensation. For reference, explicit spectra corresponding to a Higgs
mass of 114 GeV are given. Finally, we address general strategies for
distinguishing among these models.Comment: 49 pages: includes three tables and nine figure
The radion in brane cosmology
We consider the homogeneous cosmological radion, which we define as the interbrane distance in a two brane and symmetrical configuration. In a coordinate system where one of the brane is at rest, the junction conditions for the second (moving) brane give directly the (non-linear) equations of motion for the radion. We analyse the radion fluctuations and solve the non-linear dynamics in some simple cases of interest
Fermion mass hierarchies in low energy supergravity and superstring models
We investigate the problem of the fermion mass hierarchy in supergravity
models with flat directions of the scalar potential associated with some gauge
singlet moduli fields. The low-energy Yukawa couplings are nontrivial
homogeneous functions of the moduli and a geometric constraint between them
plays, in a large class of models, a crucial role in generating hierarchies.
Explicit examples are given for no-scale type supergravity models. The Yukawa
couplings are dynamical variables at low energy, to be determined by a
minimization process which amounts to fixing ratios of the moduli fields. The
Minimal Supersymmetric Standard Model (MSSM) is studied and the constraints
needed on the parameters in order to have a top quark much heavier than the
other fermions are worked out. The bottom mass is explicitly computed and shown
to be compatible with the experimental data for a large region of the parameter
space.Comment: 28 pages, Late
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