12 research outputs found
Intersecting Brane Models of Particle Physics and the Higgs Mechanism
We analyze a recently constructed class of D-brane theories with the fermion
spectrum of the SM at the intersection of D6-branes wrapping a compact toroidal
space. We show how the SM Higgs mechanism appears as a brane recombination
effect in which the branes giving rise to U(2)_L \times U(1) recombine into a
single brane related to U(1)_{em}. We also show how one can construct D6-brane
models which respect some supersymmetry at every intersection. These are
quasi-supersymmetric models of the type introduced in hep-th/0201205 which may
be depicted in terms of SUSY-quivers and may stabilize the hierarchy between
the weak scale and a fundamental scale of order 10-100 TeV present in low
string scale models. Several explicit D6-brane models with three generations of
quarks and leptons and different SUSY-quiver structure are presented. One can
prove on general grounds that if one wants to build a (factorizable) D6-brane
configuration with the SM gauge group and N = 1 SUSY (or quasi-SUSY), also a
massless (B-L) generator must be initially present in any model. If in addition
we insist on lef- and right-handed fermions respecting the same N=1 SUSY, the
brane configurations are forced to have intersections giving rise to Higgs
multiplets, providing for a rationale for the very existence of the SM Higgs
sector.Comment: 58 pages, using JHEP3.cls, 13 figures. Minor corrections and
references update
TeV-Scale Z' Bosons from D-branes
Generic D-brane string models of particle physics predict the existence of
extra U(1) gauge symmetries beyond hypercharge. These symmetries are not of the
E_6 class but rather include the gauging of Baryon and Lepton numbers as well
as certain Peccei-Quinn-like symmetries. Some of the U(1)'s have triangle
anomalies, but they are cancelled by a Green-Schwarz mechanism. The
corresponding gauge bosons typically acquire a mass of order the string scale
M_S by combining with two-index antisymmetric fields coming from the closed
string sector of the theory. We argue that in string models with a low string
scale M_S proportional to 1-10 TeV, the presence of these generic U(1)'s may be
amenable to experimental test. Present constraints from electroweak precision
data already set important bounds on the mass of these extra gauge bosons. In
particular, for large classes of models, rho-parameter constraints imply M_S >=
1.5 TeV. In the present scheme some fraction of the experimentally measured Z^0
mass would be due not to the Higgs mechanism, but rather to the mixing with
these closed string fields. We give explicit formulae for recently constructed
classes of intersecting D6- and D5-brane models yielding the Standard Model
(SM) fermion spectrum.Comment: 46 pages, LaTeX, JHEP.cls, 21 Figures. minor correction
Fermion Masses and Mixing in Intersecting Branes Scenarios
We study the structure of Yukawa couplings in intersecting D6-branes wrapping
a factorizable 6-torus compact space T^6. Models with MSSM-like spectrum are
analyzed and found to fail in predicting the quark mass spectrum because of the
way in which the family structure for the left-handed, right-handed quarks and,
eventually, the Higgses is `factorized' among the different tori. In order to
circumvent this, we present a model with three supersymmetric Higgs doublets
which satisfies the anomaly cancellation condition in a more natural way than
the previous models, where quarks were not treated universally regarding their
branes assignments, or some particular branes were singled out being invariant
under orientifold projection. In our model, the family structures for the left,
right quarks, left leptons and the Higgses arise from one of the tori and can
naturally lead to universal strength Yukawa couplings which accommodate the
quark mass hierarchy and the mixing angles.Comment: 21 pages, latex, matches the Phys. Rev. D versio
SUSY Quivers, Intersecting Branes and the Modest Hierarchy Problem
We present a class of chiral non-supersymmetric D=4 field theories in which
quadratic divergences appear only at two loops. They may be depicted as ``SUSY
quivers'' in which the nodes represent a gauge group with extended e.g., N=4
SUSY whereas links represent bifundamental matter fields which transform as
chiral multiplets with respect to different N=1 subgroups. One can obtain this
type of field theories from simple D6-brane configurations on Type IIA string
theory compactified on a six-torus. We discuss the conditions under which this
kind of structure is obtained from D6-brane intersections. We also discuss some
aspects of the effective low-energy field theory. In particular we compute
gauge couplings and Fayet-Iliopoulos terms from the Born-Infeld action and show
how they match the field theory results. This class of theories may be of
phenomenological interest in order to understand the modest hierarchy problem
i.e., the stability of the hierarchy between the weak scale and a fundamental
scale of order 10-100 TeV which appears e.g. in low string scale models.
Specific D-brane models with the spectrum of the SUSY Standard Model and three
generations are presented.Comment: 36 pages, using JHEP3.cls, 8 figures. References update
D6-branes and torsion
The D6-brane spectrum of type IIA vacua based on twisted tori and RR
background fluxes is analyzed. In particular, we compute the torsion factors of
the (co)homology groups H_n and describe the effect that they have on D6-brane
physics. For instance, the fact that H_3 contains Z_N subgroups explains why RR
tadpole conditions are affected by geometric fluxes. In addition, the presence
of torsional (co)homology shows why some D6-brane moduli are lifted, and it
suggests how the D-brane discretum appears in type IIA flux compactifications.
Finally, we give a clear, geometrical understanding of the Freed-Witten anomaly
in the present type IIA setup, and discuss its consequences for the
construction of semi-realistic flux vacua.Comment: 35 pages, 1 figure. One reference adde
Yukawa hierarchies at the point of in F-theory
We analyse the structure of Yukawa couplings in local SU(5) F-theory models
with enhancement. In this setting the symmetry is broken down to
SU(5) by a 7-brane configuration described by T-branes, all the Yukawa
couplings are generated in the vicinity of a point and only one family of
quarks and leptons is massive at tree-level. The other two families obtain
their masses when non-perturbative effects are taken into account, being
hierarchically lighter than the third family. However, and contrary to previous
results, we find that this hierarchy of fermion masses is not always
appropriate to reproduce measured data. We find instead that different T-brane
configurations breaking to SU(5) give rise to distinct hierarchical
patterns for the holomorphic Yukawa couplings. Only some of these patterns
allow to fit the observed fermion masses with reasonable local model parameter
values, adding further constraints to the construction of F-theory GUTs. We
consider an model where such appropriate hierarchy is realised and
compute its physical Yukawas, showing that realistic charged fermions masses
can indeed be obtained in this case.Comment: 46 pages + appendices, 5 figures. v2, added references and typos
corrected, version accepted on JHEP. v3, typos correcte
Yukawa couplings in intersecting D-brane models
We compute the Yukawa couplings among chiral fields in toroidal Type II
compactifications with wrapping D6-branes intersecting at angles. Those models
can yield realistic standard model spectrum living at the intersections. The
Yukawa couplings depend both on the Kahler and open string moduli but not on
the complex structure. They arise from worldsheet instanton corrections and are
found to be given by products of complex Jacobi theta functions with
characteristics. The Yukawa couplings for a particular intersecting brane
configuration yielding the chiral spectrum of the MSSM are computed as an
example. We also show how our methods can be extended to compute Yukawa
couplings on certain classes of elliptically fibered CY manifolds which are
mirror to complex cones over del Pezzo surfaces. We find that the Yukawa
couplings in intersecting D6-brane models have a mathematical interpretation in
the context of homological mirror symmetry. In particular, the computation of
such Yukawa couplings is related to the construction of Fukaya's category in a
generic symplectic manifold.Comment: 47 pages, using JHEP3.cls, 11 figures. Typos and other minor
corrections. References adde
Assessing strategies of coypu hunting and use in the Salado River Depression (Buenos Aires province)
Coypu (Myocastor coypus) was intensively exploited during the Late Holocene in different areas of the Pampean Region and the Northeast of Argentina. This species was one of the most important faunal resources for the hunter-gatherer-fishers that inhabited the Salado River Depression (Buenos Aires Province). In this area, coypu was exploited for meat and marrow consumption, and its fur was also well appreciated. As a resource available in lagoons near the base camps, organizing special hunting parties to obtain this prey would not have been necessary.In this contribution, we analyze the capture and hunting strategies, handling, and consumption of coypu, using data from zooarchaeological studies, experimental archaeology, ethnoarchaeology, lithic weapons, and the pottery employed for its processing. Even though the capture of coypu would have been possible using only a stick, and without other sophisticated tools, evidence from the Salado Depression suggests past hunters would have also used arrows and/or bola stones.Fil: GonzĂĄlez, Marta Isabel. Universidad de Buenos Aires. Facultad de FilosofĂa y Letras. Instituto de ArqueologĂa; ArgentinaFil: Escosteguy, Paula Daniela. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad de Buenos Aires. Facultad de FilosofĂa y Letras. Instituto de ArqueologĂa; ArgentinaFil: Salemme, Monica Cira. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro Austral de Investigaciones CientĂficas; Argentina. Universidad Nacional de Tierra del Fuego; ArgentinaFil: Frere, Maria Magdalena. Universidad de Buenos Aires. Facultad de FilosofĂa y Letras. Instituto de ArqueologĂa; ArgentinaFil: Weitzel, MarĂa Celeste. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Provincia de Buenos Aires. Municipalidad de Necochea. Museo de Ciencias Naturales de Necochea. Area de ArqueologĂa y AntropologĂa; ArgentinaFil: Vecchi, Rordrigo. Universidad Nacional del Sur. Departamento de Humanidades; Argentin