730 research outputs found
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
Effective description of brane terms in extra dimensions
We study how theories defined in (extra-dimensional) spaces with localized
defects can be described perturbatively by effective field theories in which
the width of the defects vanishes. These effective theories must incorporate a
``classical'' renormalization, and we propose a renormalization prescription a
la dimensional regularization for codimension 1, which can be easily used in
phenomenological applications. As a check of the validity of this setting, we
compare some general predictions of the renormalized effective theory with
those obtained in a particular ultraviolet completion based on deconstruction.Comment: 28 page
Beyond MFV in family symmetry theories of fermion masses
Minimal Flavour Violation (MFV) postulates that the only source of flavour
changing neutral currents and CP violation, as in the Standard Model, is the
CKM matrix. However it does not address the origin of fermion masses and mixing
and models that do usually have a structure that goes well beyond the MFV
framework. In this paper we compare the MFV predictions with those obtained in
models based on spontaneously broken (horizontal) family symmetries, both
Abelian and non-Abelian. The generic suppression of flavour changing processes
in these models turns out to be weaker than in the MFV hypothesis. Despite
this, in the supersymmetric case, the suppression may still be consistent with
a solution to the hierarchy problem, with masses of superpartners below 1 TeV.
A comparison of FCNC and CP violation in processes involving a variety of
different family quantum numbers should be able to distinguish between various
family symmetry models and models satisfying the MFV hypothesis.Comment: 34 pages, no figure
The vacuum structure in a supersymmetric gauged Nambu-Jona-Lasinio model
The dynamical breakdown of the symmetry triggered by a
top-antitop condensate is studied in a supersymmetric version of the gauged
Nambu-Jona-Lasinio model. An effective potential approach is used to
investigate the vacuum structure and the equivalence with the minimal
supersymmetric standard model. The role of the soft supersymmetry breaking
terms is analyzed in detail in a version of the model where the electroweak
gauge interactions are turned off.Comment: 32 pages (+2 figures not included), Latex, LPTHE 93/0
Flavour in supersymmetry: horizontal symmetries or wave function renormalisation
We compare theoretical and experimental predictions of two main classes of
models addressing fermion mass hierarchies and flavour changing neutral
currents (FCNC) effects in supersymmetry: Froggatt-Nielsen (FN) U(1) gauged
flavour models and Nelson-Strassler/extra dimensional models with hierarchical
wave functions for the families. We show that whereas the two lead to identical
predictions in the fermion mass matrices, the second class generates a stronger
suppression of FCNC effects. We prove that, whereas at first sight the FN setup
is more constrained due to anomaly cancelation conditions, imposing unification
of gauge couplings in the second setup generates conditions which precisely
match the mixed anomaly constraints in the FN setup. Finally, we provide an
economical extra dimensional realisation of the hierarchical wave functions
scenario in which the leptonic FCNC can be efficiently suppressed due to the
strong coupling (CFT) origin of the electron mass.Comment: 23 page
A Rationale for Long-lived Quarks and Leptons at the LHC: Low Energy Flavour Theory
In the framework of gauged flavour symmetries, new fermions in parity
symmetric representations of the standard model are generically needed for the
compensation of mixed anomalies. The key point is that their masses are also
protected by flavour symmetries and some of them are expected to lie way below
the flavour symmetry breaking scale(s), which has to occur many orders of
magnitude above the electroweak scale to be compatible with the available data
from flavour changing neutral currents and CP violation experiments. We argue
that, actually, some of these fermions would plausibly get masses within the
LHC range. If they are taken to be heavy quarks and leptons, in
(bi)-fundamental representations of the standard model symmetries, their
mixings with the light ones are strongly constrained to be very small by
electroweak precision data. The alternative chosen here is to exactly forbid
such mixings by breaking of flavour symmetries into an exact discrete symmetry,
the so-called proton-hexality, primarily suggested to avoid proton decay. As a
consequence of the large value needed for the flavour breaking scale, those
heavy particles are long-lived and rather appropriate for the current and
future searches at the LHC for quasi-stable hadrons and leptons. In fact, the
LHC experiments have already started to look for them.Comment: 10 pages, 1 figur
Gaugino Condensation in M-theory on S^1/Z_2
In the low energy limit of for M-theory on S^1/Z_2, we calculate the gaugino
condensate potential in four dimensions using the background solutions due to
Horava. We show that this potential is free of delta-function singularities and
has the same form as the potential in the weakly coupled heterotic string. A
general flux quantization rule for the three-form field of M-theory on S^1/Z_2
is given and checked in certain limiting cases. This rule is used to fix the
free parameter in the potential originating from a zero mode of the form field.
Finally, we calculate soft supersymmetry breaking terms. We find that
corrections to the Kahler potential and the gauge kinetic function, which can
be large in the strongly coupled region, contribute significantly to certain
soft terms. In particular, for supersymmetry breaking in the T-modulus
direction, the small values of gaugino masses and trilinear couplings that
occur in the weakly coupled, large radius regime are enhanced to order m_3/2 in
M-theory. The scalar soft masses remain small even, in the strong coupling
M-theory limit.Comment: 20 pages, LATE
CMB Imprints of a Pre-Inflationary Climbing Phase
We discuss the implications for cosmic microwave background (CMB)
observables, of a class of pre-inflationary dynamics suggested by string models
where SUSY is broken due to the presence of D-branes and orientifolds
preserving incompatible portions of it. In these models the would-be inflaton
is forced to emerge from the initial singularity climbing up a mild exponential
potential, until it bounces against a steep exponential potential of "brane
SUSY breaking" scenarios, and as a result the ensuing descent gives rise to an
inflationary epoch that begins when the system is still well off its eventual
attractor. If a pre-inflationary climbing phase of this type had occurred
within 6-7 e-folds of the horizon exit for the largest observable wavelengths,
displacement off the attractor and initial-state effects would conspire to
suppress power in the primordial scalar spectrum, enhancing it in the tensor
spectrum and typically superposing oscillations on both. We investigate these
imprints on CMB observables over a range of parameters, examine their
statistical significance, and provide a semi-analytic rationale for our
results. It is tempting to ascribe at least part of the large-angle anomalies
in the CMB to pre-inflationary dynamics of this type.Comment: 38 pages, LaTeX, 11 eps figures, references added, matches version to
appear in JCA
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