1,789 research outputs found
Weierstrass meets Enriques
We study in detail the degeneration of K3 to T^4/Z_2. We obtain an explicit
embedding of the lattice of collapsed cycles of T^4/Z_2 into the lattice of
integral cycles of K3 in two different ways. Our first method exploits the
duality to the heterotic string on T^3. This allows us to describe the
degeneration in terms of Wilson lines. Our second method is based on the
blow-up of T^4/Z_2. From this blow-up, we directly construct the full lattice
of integral cycles of K3. Finally, we use our results to describe the action of
the Enriques involution on elliptic K3 surfaces, finding that a Weierstrass
model description is consistent with the Enriques involution only in the
F-theory limit.Comment: 35 pages, 9 figure
Tate Form and Weak Coupling Limits in F-theory
We consider the weak coupling limit of F-theory in the presence of
non-Abelian gauge groups implemented using the traditional ansatz coming from
Tate's algorithm. We classify the types of singularities that could appear in
the weak coupling limit and explain their resolution. In particular, the weak
coupling limit of SU(n) gauge groups leads to an orientifold theory which
suffers from conifold singulaties that do not admit a crepant resolution
compatible with the orientifold involution. We present a simple resolution to
this problem by introducing a new weak coupling regime that admits
singularities compatible with both a crepant resolution and an orientifold
symmetry. We also comment on possible applications of the new limit to model
building. We finally discuss other unexpected phenomena as for example the
existence of several non-equivalent directions to flow from strong to weak
coupling leading to different gauge groups.Comment: 34 page
Human Development and Tourism Specialization. Evidence from a Panel of Developed and Developing Countries
In the present study we analyze the relationship between tourism and human development for a selection of 63 countries from 1996 to 2008. Findings confirm that, on average, tourism is positively associated with human development. By decomposing the effect of tourism on each human development indicator, we find that literacy rate appears to be the most affected. This result suggests that the impact of tourism in the host country is much broader respect to the purely economic effect. Furthermore, it suggests the need of further investigating the relationship between human development and tourism
Fluxes and Warping for Gauge Couplings in F-theory
We compute flux-dependent corrections in the four-dimensional F-theory
effective action using the M-theory dual description. In M-theory the 7-brane
fluxes are encoded by four-form flux and modify the background geometry and
Kaluza-Klein reduction ansatz. In particular, the flux sources a warp factor
which also depends on the torus directions of the compactification fourfold.
This dependence is crucial in the derivation of the four-dimensional action,
although the torus fiber is auxiliary in F-theory. In M-theory the 7-branes are
described by an infinite array of Taub-NUT spaces. We use the explicit metric
on this geometry to derive the locally corrected warp factor and M-theory
three-from as closed expressions. We focus on contributions to the 7-brane
gauge coupling function from this M-theory back-reaction and show that terms
quadratic in the internal seven-brane flux are induced. The real part of the
gauge coupling function is modified by the M-theory warp factor while the
imaginary part is corrected due to a modified M-theory three-form potential.
The obtained contributions match the known weak string coupling result, but
also yield additional terms suppressed at weak coupling. This shows that the
completion of the M-theory reduction opens the way to compute various
corrections in a genuine F-theory setting away from the weak string coupling
limit.Comment: 46 page
New F-theory lifts II: Permutation orientifolds and enhanced singularities
In this paper, a procedure is developed to construct compact F-theory
fourfolds corresponding to perturbative IIB O7/O3 models on CICY threefolds
with permutation involutions. The method is explained in generality, and then
applied to specific examples where the involution permutes two Del Pezzo
surfaces. The fourfold construction is successfully tested by comparing the D3
charges predicted by F-theory and IIB string theory. The constructed smooth
fourfolds are then taken to the locus in moduli space where they have enhanced
SU(5) singularities. A general, intuitive method is developed for engineering
the desired singularities in Weierstrass models for complicated D7-brane
setups.Comment: 24 pages, 8 tables, typos corrected, minor errors corrected, refs.
adde
On Instanton Effects in F-theory
We revisit the issue of M5-brane instanton corrections to the superpotential
in F-theory compactifications on elliptically fibered Calabi-Yau fourfolds.
Elaborating on concrete geometries, we compare the instanton zero modes for
non-perturbative F-theory models with the zero modes in their perturbative Sen
limit. The fermionic matter zero modes localized on the intersection of the
instanton with the space-time filling D7-branes show up in a geometric way in
F-theory. Methods for their computation are developed and, not surprisingly,
exceptional gauge group structures do appear. Finally, quite intriguing
geometrical aspects of the one-loop determinant are discussed.Comment: 52 pages, 8 figures, 13 tables; v2: extended discussion of matter
zero modes, refs added; v3: sections 3.3 + 4.1 restructure
A new CY elliptic fibration and tadpole cancellation
Tadpole cancellation in Sen limits in F-theory was recently studied by Aluffi
and Esole. We extend their results, generalizing the elliptic fibrations they
used and obtaining a new case of universal tadpole cancellation, at least
numerically. We could not find an actual Sen limit having the correct brane
content, and we argue that such a limit may not exist. We also give a uniform
description of the fibration used by Aluffi and Esole as well as a new, simple,
fibration which has non-Kodaira type fibers.Comment: 18 pages; added references and minor change
Effects of pressure-controlled and volume-controlled ventilation on respiratory mechanics and systemic stress response during laparoscopic cholecystectomy
Superconformal Yang-Mills quantum mechanics and Calogero model with OSp(N|2,R) symmetry
In spacetime dimension two, pure Yang-Mills possesses no physical degrees of
freedom, and consequently it admits a supersymmetric extension to couple to an
arbitrary number, N say, of Majorana-Weyl gauginos. This results in (N,0) super
Yang-Mills. Further, its dimensional reduction to mechanics doubles the number
of supersymmetries, from N to N+N, to include conformal supercharges, and leads
to a superconformal Yang-Mills quantum mechanics with symmetry group
OSp(N|2,R). We comment on its connection to AdS_2 \times S^{N-1} and reduction
to a supersymmetric Calogero model.Comment: 1+28 pages, no figure; Refs added. To appear in JHE
Abelian Gauge Fluxes and Local Models in F-Theory
We analyze the Abelian gauge fluxes in local F-theory models with G_S=SU(6)
and SO(10). For the case of G_S=SO(10), there is a no-go theorem which states
that for an exotic-free spectrum, there are no solutions for U(1)^2 gauge
fluxes. We explicitly construct the U(1)^2 gauge fluxes with an exotic-free
bulk spectrum for the case of G_S=SU(6). We also analyze the conditions for the
curves supporting the given field content and discuss non-minimal spectra of
the MSSM with doublet-triplet splitting.Comment: 43 pages, 15 tables; typos corrected, reference adde
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