2,334 research outputs found
Brane Supersymmetry Breaking
We show how to construct chiral tachyon-free perturbative orientifold models,
where supersymmetry is broken at the string scale on a collection of branes
while, to lowest order, the bulk and the other branes are supersymmetric. In
higher orders, supersymmetry breaking is mediated to the remaining sectors, but
is suppressed by the size of the transverse space or by the distance from the
brane where supersymmetry breaking primarily occurred. This setting is of
interest for orbifold models with discrete torsion, and is of direct relevance
for low-scale string models. It can guarantee the stability of the gauge
hierarchy against gravitational radiative corrections, allowing an almost exact
supergravity a millimeter away from a non-supersymmetric world.Comment: 15 pages, LaTe
Axion alternatives
If recent results of the PVLAS collaboration proved to be correct, some
alternative to the traditional axion models are needed. We present one of the
simplest possible modifications of axion paradigm, which explains the results
of PVLAS experiment, while avoiding all the astrophysical and cosmological
restrictions. We also mention other possible models that possess similar
effects.Comment: 12 pages, 3 figure
Magnetic fluxes and moduli stabilization
Stabilization of closed string moduli in toroidal orientifold
compactifications of type IIB string theory are studied using constant internal
magnetic fields on D-branes and 3-form fluxes that preserve N=1 supersymmetry
in four dimensions. Our analysis corrects and extends previous work by us, and
indicates that charged scalar VEV's need to be turned on, in addition to the
fluxes, in order to construct a consistent supersymmetric model. As an explicit
example, we first show the stabilization of all Kahler class and complex
structure moduli by turning on magnetic fluxes on different sets of D9-branes
that wrap the internal space T^6 in a compactified type I string theory, when a
charged scalar on one of these branes acquires a non-zero VEV. The latter can
also be determined by adding extra magnetized branes, as we demonstrate in a
subsequent example. In a different model with magnetized D7-branes, in a IIB
orientifold on T^6/Z_2, we show the stabilization of all the closed string
moduli, including the axion-dilaton at weak string coupling g_s, by turning on
appropriate closed string 3-form fluxes.Comment: v2: minor changes, added discussio
Supersymmetry breaking, open strings and M-theory
We study supersymmetry breaking by Scherk-Schwarz compactifications in type I
string theory. While in the gravitational sector all mass splittings are
proportional to a (large) compactification radius, supersymmetry remains
unbroken for the massless excitations of D-branes orthogonal to the large
dimension. In this sector, supersymmetry breaking can then be mediated by
gravitational interactions alone, that are expected to be suppressed by powers
of the Planck mass. The mechanism is non perturbative from the heterotic
viewpoint and requires a compactification radius at intermediate energies of
order 10^{12}-10^{14} GeV. This can also explain the value of Newton's constant
if the string scale is close to the unification scale, of order 10^{16} GeV.Comment: 47 pages, LaTeX, typos correcte
Moduli stabilization with open and closed string fluxes
We study the stabilization of all closed string moduli in the T^6/Z_2
orientifold, using constant internal magnetic fields and 3-form fluxes that
preserve N=1 supersymmetry in four dimensions. We first analyze the
stabilization of Kahler class and complex structure moduli by turning on
magnetic fluxes on different sets of D9 branes that wrap the internal space
T^6/Z_2. We present explicit consistent string constructions, satisfying in
particular tadpole cancellation, where the radii can take arbitrarily large
values by tuning the winding numbers appropriately. We then show that the
dilaton-axion modulus can also be fixed by turning on closed string constant
3-form fluxes, consistently with the supersymmetry preserved by the magnetic
fields, providing at the same time perturbative values for the string coupling.
Finally, several models are presented combining open string magnetic fields
that fix part of Kahler class and complex structure moduli, with closed string
3-form fluxes that stabilize the remaining ones together with the dilaton.Comment: 49 pages, a new model added, as well as improvements and reference
Palatini inflation in models with an term
The Starobinsky model, considered in the framework of the Palatini formalism,
in contrast to the metric formulation, does not provide us with a model for
inflation, due to the absence of a propagating scalar degree of freedom that
can play the role of the inflaton. In the present article we study the Palatini
formulation of the Starobinsky model coupled, in general nonminimally, to
scalar fields and analyze its inflationary behavior. We consider scalars,
minimally or nonminimally coupled to the Starobinsky model, such as a quadratic
model, the induced gravity model or the standard Higgs-like inflation model and
analyze the corresponding modifications favorable to inflation. In addition we
examine the case of a classically scale-invariant model driven by the
Coleman-Weinberg mechanism. In the slow-roll approximation, we analyze the
inflationary predictions of these models and compare them to the latest
constraints from the Planck collaboration. In all cases, we find that the
effect of the term is to lower the value of the tensor-to-scalar ratio.Comment: 22 pages, 8 figures, JCAP accepted versio
Making Ends Meet: String Unification and Low-Energy Data
A long-standing problem in string phenomenology has been the fact that the
string unification scale disagrees with the GUT scale obtained by extrapolating
low-energy data within the framework of the minimal supersymmetric standard
model (MSSM). In this paper we examine several effects that may modify the
minimal string predictions and thereby bring string-scale unification into
agreement with low-energy data. These include heavy string threshold
corrections, non-standard hypercharge normalizations, light SUSY thresholds,
intermediate gauge structure, and thresholds arising from extra matter beyond
the MSSM. We explicitly evaluate these contributions within a variety of
realistic free-fermionic string models, including the flipped SU(5), SO(6) x
SO(4), and various SU(3) x SU(2) x U(1) models, and find that most of these
sources do not substantially alter the minimal string predictions. Indeed, we
find that the only way to reconcile string unification with low-energy data is
through certain types of extra matter. Remarkably, however, many of the
realistic string models contain precisely this required matter in their
low-energy spectra.Comment: 10 pages, standard LaTeX, 1 figure (Encapsulated PostScript), version
published in Phys. Rev. Lett. 75 (1995) 264
Physical States of the Quantum Conformal Factor
The conformal factor of the spacetime metric becomes dynamical due to the
trace anomaly of matter fields. Its dynamics is described by an effective
action which we quantize by canonical methods on the Einstein universe . We find an infinite tower of discrete states which satisfy the
constraints of quantum diffeomorphism invariance. These physical states are in
one-to-one correspondence with operators constructed by integrating integer
powers of the Ricci scalar.Comment: PlainTeX File, 34 page
Brane to bulk supersymmetry breaking and radion force at micron distances
We study mediation of supersymmetry breaking in the bulk, in models with
primordial supersymmetry breaking on D-branes at the string scale, in the TeV
region. We compute the gravitino and scalar masses up to one-loop level, as
well as the radion coupling to matter. We find that the latter mediates a model
independent force at submillimeter distances that can be tested in
micro-gravity experiments for any dimensionality of the bulk. In the case of
two large dimensions, our type I string framework provides an example which
allows to stabilize the radion potential and determine the desired hierarchy
between the string and Planck scales.Comment: One equation and one figure corrected. Two references added. Version
to be published in Nuclear Physics
A New Gauge Mediation Theory
We propose a class of models with gauge mediation of supersymmetry breaking,
inspired by simple brane constructions, where R-symmetry is very weakly broken.
The gauge sector has an extended N=2 supersymmetry and the two electroweak
Higgses form an N=2 hypermultiplet, while quarks and leptons remain in N=1
chiral multiplets. Supersymmetry is broken via the D-term expectation value of
a secluded U(1) and it is transmitted to the Standard Model via gauge
interactions of messengers in N=2 hypermultiplets: gauginos thus receive Dirac
masses. The model has several distinct experimental signatures with respect to
ordinary models of gauge or gravity mediation realizations of the Minimal
Supersymmetric Standard Model (MSSM). First, it predicts extra states as a
third chargino that can be observed at collider experiments. Second, the
absence of a D-flat direction in the Higgs sector implies a lightest Higgs
behaving exactly as the Standard Model one and thus a reduction of the `little'
fine-tuning in the low tan(beta) region. This breaking of supersymmetry can be
easily implemented in string theory modelsComment: 23 pages, 3 figures, uses axodraw.sty. v2: a mistake in the radiative
generation of the scalar masses is corrected. The main conclusions are
unchange
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