1,117 research outputs found
Multi-instanton and string loop corrections in toroidal orbifold models
We analyze N=2 (perturbative and non-perturbative) corrections to the
effective theory in type I orbifold models where a dual heterotic description
is available. These corrections may play an important role in phenomenological
scenarios. More precisely, we consider two particular compactifications: the
Bianchi-Sagnotti-Gimon-Polchinski orbifold and a freely-acting Z_2 x Z_2
orbifold with N=1 supersymmetry and gauge group SO(q) x SO(32-q). By exploiting
perturbative calculations of the physical gauge couplings on the heterotic
side, we obtain multi-instanton and one-loop string corrections to the K\"ahler
potential and the gauge kinetic function for these models. The non-perturbative
corrections appear as sums over relevant Hecke operators, whereas the one-loop
correction to the K\"ahler potential matches the expression proposed in [1,2].
We argue that these corrections are universal in a given class of models where
target-space modular invariance (or a subgroup of it) holds.Comment: 37 pages, 3 figure
Metastable Vacua in Brane Worlds
We analyze vacuum decay in brane world setups, where a free scalar field in
five dimensions has a localized potential admitting metastable vacua. We study
in particular the bounce solution and its properties in flat and warped spaces.
In the latter case, placing into a deeply warped region the term in the
potential that lifts the vacuum degeneracy, can increase indefinitely the
lifetime of the false vacuum. We discuss the application to metastable vacua in
supersymmetric brane-world constructions.Comment: LaTeX, 24 page
On the origin of constrained superfields
In this work we analyze constrained superfields in supersymmetry and
supergravity. We propose a constraint that, in combination with the constrained
goldstino multiplet, consistently removes any selected component from a generic
superfield. We also describe its origin, providing the operators whose
equations of motion lead to the decoupling of such components. We illustrate
our proposal by means of various examples and show how known constraints can be
reproduced by our method.Comment: 20 pages, to be published in JHE
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
Classical running of neutrino masses from six dimensions
We discuss a six dimensional mass generation for the neutrinos. Active
neutrinos live on a three-brane and interact via a brane localized mass term
with a bulk six-dimensional standard model singlet (sterile) Weyl fermion, the
two dimensions being transverse to the three-brane.
We derive the physical neutrino mass spectrum and show that the active
neutrino mass and Kaluza-Klein masses have a logarithmic cutoff divergence
related to the zero-size limit of the three-brane in the transverse space. This
translates into a renormalisation group running of the neutrino masses above
the Kaluza-Klein compactification scale coming from classical effects, without
any new non-singlet particles in the spectrum. For compact radii in the eV--MeV
range, relevant for neutrino physics, this scenario predicts running neutrino
masses which could affect, in particular, neutrinoless double beta decay
experiments.Comment: 23 pages, 2 figure
Testing extra dimensions with boundaries using Newton's law modifications
Extra dimensions with boundaries are often used in the literature, to provide
phenomenological models that mimic the standard model. In this context, we
explore possible modifications to Newton's law due to the existence of an
extra-dimensional space, at the boundary of which the gravitational field obeys
Dirichlet, Neumann or mixed boundary conditions. We focus on two types of extra
space, namely, the disk and the interval. As we prove, in order to have a
consistent Newton's law modification (i.e., of the Yukawa-type), some of the
extra-dimensional spaces that have been used in the literature, must be ruled
out.Comment: Published version, title changed, 6 figure
Gaugino condensation and the anomalous
We study gaugino condensation in presence of an anomalous gauge group
and find that global supersymmetry is dynamically broken. An example of
particular interest is provided by effective string models with 4-dimensional
Green-Schwarz anomaly cancellation mechanism. The structure of the hidden
sector is constrained by the anomaly cancellation conditions and the scale of
gaugino condensation is shifted compared with the usual case. We explicitly
compute the resulting soft supersymmetry breaking terms.Comment: 13 page
On tadpoles and vacuum redefinitions in String Theory
Tadpoles accompany, in one form or another, all attempts to realize
supersymmetry breaking in String Theory, making the present constructions at
best incomplete. Whereas these tadpoles are typically large, a closer look at
the problem from a perturbative viewpoint has the potential of illuminating at
least some of its qualitative features in String Theory. A possible scheme to
this effect was proposed long ago by Fischler and Susskind, but incorporating
background redefinitions in string amplitudes in a systematic fashion has long
proved very difficult. In the first part of this paper, drawing from field
theory examples, we thus begin to explore what one can learn by working
perturbatively in a ``wrong'' vacuum. While unnatural in Field Theory, this
procedure presents evident advantages in String Theory, whose definition in
curved backgrounds is mostly beyond reach at the present time. At the field
theory level, we also identify and characterize some special choices of vacua
where tadpole resummations terminate after a few contributions. In the second
part we present a notable example where vacuum redefinitions can be dealt with
to some extent at the full string level, providing some evidence for a new link
between IIB and 0B orientifolds. We finally show that NS-NS tadpoles do not
manifest themselves to lowest order in certain classes of string constructions
with broken supersymmetry and parallel branes, including brane-antibrane pairs
and brane supersymmetry breaking models, that therefore have UV finite
threshold corrections at one loop.Comment: 51 pages, LaTeX, 7 eps figures. Typos corrected, refs added. Final
version to appear in Nucl. Phys. B. Thanks to W. Mueck for very interesting
correspondence. v3 was accidentally in draft forma
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