233 research outputs found
Discrete symmetries from hidden sectors
We study the presence of abelian discrete symmetries in globally consistent
orientifold compactifications based on rational conformal field theory. We
extend previous work [1] by allowing the discrete symmetries to be a linear
combination of U(1) gauge factors of the visible as well as the hidden sector.
This more general ansatz significantly increases the probability of finding a
discrete symmetry in the low energy effective action. Applied to globally
consistent MSSM-like Gepner constructions we find multiple models that allow
for matter parity or Baryon triality.Comment: 20 page
Foramen magnum, occipital condyles and hypoglossal canals morphometry: anatomical study with clinical implications
Background: Current study examines morphometric alterations of the foramen magnum (FM), occipital condyles (OCs) and hypoglossal canals (HCs) and highlights all the morphometric parameters of the FM area that present side asymmetry, gender dimorphism and are affected by the ageing.
Materials and methods: One hundred and forty-one (73 male and 68 female) Greek adult dry skulls were examined.
Results: Short and long OCs were detected in 27.7% and 26.2%. A combination of short OCs and long HCs was presented in 27.5%. A complete septum was found in 23.6% of the HCs and osseous spurs in 12.9%. Side asymmetry was detected regarding the HCs length (p = 0.046), the maximum extracranial (p = 0.001) and minimum intracranial (p = 0.001) diameters. Mean FM anteroposterior and transverse diameters, FM perimeter and FM surface area were significantly larger in male than in female skulls (p = 0.001 for each parameter). Similarly, the OCs length (right, p = 0.004 and left, p = 0.024) and width (right, p = 0.008 and left, p = 0.006) the left distance HC-OC posterior border (p = 0.048), the anterior (p = 0.011) and posterior (p = 0.001) intercondylar distances and the HCs right length (p = 0.046) were significantly greater in males. A significant decrease was observed with ageing in FM anteroposterior diameter (p = 0.038), FM surface area (p = 0.05), anterior intercondylar distance (p = 0.014) and HC-OC posterior border (p = 0.013).
Conclusions: The study confirmed that only specific HC dimensions showed side asymmetry (HCs maximum extracranial and minimum intracranial diameters and HCs length), gender dimorphism (HCs right length and left distance HC-OC posterior border) and age influence (HC-OC posterior border and HC left extracranial minimum diameter) among young, adults and elderly individuals. FM and OCs dimensions presented gender dimorphism and the age influenced only FM anteroposterior diameter and surface area and the anterior intercondylar distance. The safe zone of OCs drilling in Greeks, calculated by the distance HC-OC posterior border represents the maximum HC depth and is among the lowest values reported in the literature. The significant decrease of this distance with ageing confirms the existence of a drilling safe zone for young, adults and elder individuals. Regarding OCs length, the same probability exists dealing with a short or a long OC during condylectomy. Before planning a transcondylar approach, the coexistence of short OCs and long HCs should be taken into account. These outcomes will be useful for a safe surgery in the craniocervical region in Greeks
Phenomenological analysis of D-brane Pati-Salam vacua
In the present work we perform a phenomenological analysis of the effective
low energy models with Pati-Salam (PS) gauge symmetry derived in the context of
D-branes. A main issue in these models arises from the fact that the
right-handed fermions and the PS-symmetry breaking Higgs field transform
identically under the PS symmetry, causing unnatural matter-Higgs mixing
effects. We argue that this problem could be solved in particular D-brane
setups where these fields arise in different intersections. We further observe
that whenever a large Higgs mass term is generated in a particular class of
mass spectra, a splitting mechanism -reminiscent of the doublet triplet
splitting- may protect the neutral Higgs components from a heavy mass term. We
analyze the implications of each individual representation which in principle
is available in these models in order to specify the minimal spectrum required
to build up a consistent PS model which reconciles the low energy data. A short
discussion is devoted on the effects of stringy instanton corrections,
particularly those generating missing Yukawa couplings and contributing to the
fermion mass textures. We discuss the correlations of the intersecting D-brane
spectra with those obtained from Gepner constructions and analyze the
superpotential, the resulting mass textures and the low energy implications of
some examples of the latter along the lines proposed above.Comment: 50 pages, 3 figures (v2 - Minor corrections
Excavated type of rhomboid fossa of the clavicle: a radiological study
The excavated type of rhomboid fossa of the clavicle is a relatively neglected
anatomical structure that can potentially cause diagnostic problems. Its unilateral
occurrence may be confused by the physician as avascular necrosis, osteomyelitis,
or even a tumour. We studied 80 routine chest radiographs and identified
the clavicles with excavated type of rhomboid fossa. The sex, sidedness,
and handedness were recorded. An excavated type of rhomboid fossa was
present in 43 clavicles (26.88%), appearing more frequently in males than in
females. In addition, the incidence of the excavated type of rhomboid fossa
was greater on the right side than on the left. That type of fossa was also
present more frequently on the right side in right-handed specimens and on
the left side in left-handed specimens. The high incidence of the excavated
type of rhomboid fossa on the dominant hand supports the mechanical theory
of fossa formation. Radiologists and physicians should be aware of this fossa, as
it may resemble a pathological condition
D-brane Instantons on the T^6/Z_3 orientifold
We give a detailed microscopic derivation of gauge and stringy instanton
generated superpotentials for gauge theories living on D3-branes at
Z_3-orientifold singularities. Gauge instantons are generated by D(-1)-branes
and lead to Affleck, Dine and Seiberg (ADS) like superpotentials in the
effective N=1 gauge theories with three generations of bifundamental and
anti/symmetric matter. Stringy instanton effects are generated by Euclidean
ED3-branes wrapping four-cycles on T^6/\Z_3. They give rise to Majorana masses
in one case and non-renormalizable superpotentials for the other cases. Finally
we determine the conditions under which ADS like superpotentials are generated
in N=1 gauge theories with adjoints, fundamentals, symmetric and antisymmetric
chiral matter.Comment: 31 pages, no figure
Simplifying one-loop amplitudes in superstring theory
We show that 4-point vector boson one-loop amplitudes, computed in ref.[1] in
the RNS formalism, around vacuum configurations with open unoriented strings,
preserving at least N=1 SUSY in D=4, satisfy the correct supersymmetry Ward
identities, in that they vanish for non MHV configurations (++++) and (-+++).
In the MHV case (--++) we drastically simplify their expressions. We then study
factorisation and the limiting IR and UV behaviour and find some unexpected
results. In particular no massless poles are exposed at generic values of the
modular parameter. Relying on the supersymmetric properties of our bosonic
amplitudes, we extend them to manifestly supersymmetric super-amplitudes and
compare our results with those obtained in the D=4 hybrid formalism, pointing
out difficulties in reconciling the two approaches for contributions from N=1,2
sectors.Comment: 38 pages plus appendice
SU(5) D-brane realizations, Yukawa couplings and proton stability
We discuss SU(5) Grand Unified Theories in the context of orientifold
compactifications. Specifically, we investigate two and three D-brane stack
realizations of the Georgi-Glashow and the flipped SU(5) model and analyze them
with respect to their Yukawa couplings. As pointed out in arXiv:0909.0271 the
most economical Georgi-Glashow realization based on two stacks generically
suffers from a disastrous large proton decay rate. We show that allowing for an
additional U(1) D-brane stack this as well as other phenomenological problems
can be resolved. We exemplify with globally consistent Georgi-Glashow models
based on RCFT that these D-brane quivers can be indeed embedded in a global
setting. These globally consistent realizations admit rigid O(1) instantons
inducing the perturbatively missing coupling 10105^H. Finally we show that
flipped SU(5) D-brane realizations even with multiple U(1) D-brane stacks are
plagued by severe phenomenological drawbacks which generically cannot be
overcome.Comment: 34 pages v2 minor correction
Gauge Threshold Corrections for Local String Models
We study gauge threshold corrections for local brane models embedded in a
large compact space. A large bulk volume gives important contributions to the
Konishi and super-Weyl anomalies and the effective field theory analysis
implies the unification scale should be enhanced in a model-independent way
from M_s to R M_s. For local D3/D3 models this result is supported by the
explicit string computations. In this case the scale R M_s comes from the
necessity of global cancellation of RR tadpoles sourced by the local model. We
also study D3/D7 models and discuss discrepancies with the effective field
theory analysis. We comment on phenomenological implications for gauge coupling
unification and for the GUT scale.Comment: 30 pages; v2: references added, minor typos correcte
Symplectic structure of N=1 supergravity with anomalies and Chern-Simons terms
The general actions of matter-coupled N=1 supergravity have Peccei-Quinn
terms that may violate gauge and supersymmetry invariance. In addition, N=1
supergravity with vector multiplets may also contain generalized Chern-Simons
terms. These have often been neglected in the literature despite their
importance for gauge and supersymmetry invariance. We clarify the interplay of
Peccei-Quinn terms, generalized Chern-Simons terms and quantum anomalies in the
context of N=1 supergravity and exhibit conditions that have to be satisfied
for their mutual consistency. This extension of the previously known N=1
matter-coupled supergravity actions follows naturally from the embedding of the
gauge group into the group of symplectic duality transformations. Our results
regarding this extension provide the supersymmetric framework for studies of
string compactifications with axionic shift symmetries, generalized
Chern-Simons terms and quantum anomalies.Comment: 27 pages; v2: typos corrected; version to be published in
Class.Quantum Gra
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