235 research outputs found
On Susy Standard-like models from orbifolds of D=6 Gepner orientifolds
As a further elaboration of the proposal of Ref. [1] we address the
construction of Standard-like models from configurations of stacks of
orientifold planes and D-branes on an internal space with the structure
. As a first step, the construction of
D=6 Type II B orientifolds on Gepner points, in the diagonal invariant case and
for both, odd and even, affine levels is discussed. We build up the explicit
expressions for B-type boundary states and crosscaps and obtain the amplitudes
among them. From such amplitudes we read the corresponding spectra and the
tadpole cancellation equations. Further compactification on a T^2 torus, by
simultaneously orbifolding the Gepner and the torus internal sectors, is
performed. The embedding of the orbifold action in the brane sector breaks the
original gauge groups and leads to N=1 supersymmetric chiral spectra. Whenever
even orbifold action on the torus is considered, new branes, with worldvolume
transverse to torus coordinates, must be included. The detailed rules for
obtaining the D=4 model spectra and tadpole equations are shown. As an
illustration we present a 3 generations Left-Right symmetric model that can be
further broken to a MSSM model.Comment: 40 pages, 2 figures, added references, table 3 correcte
Intersecting D5-brane models with massive vector-like leptons
We construct eight-stack intersecting D5-brane models, with an orbifold
transverse space, that yield the (non-supersymmetric) standard model up to
vector-like leptons. The matter includes right-chiral neutrinos and the models
have the renormalisable Yukawa couplings to tachyonic Higgs doublets needed to
generate mass terms for {\it all} matter, including the vector-like leptons.
The models are constrained by the requirement that twisted tadpoles cancel,
that the gauge boson coupled to the weak hypercharge does not get a
string-scale mass via a generalised Green-Schwarz mechanism, and that there are
no surviving, unwanted gauged U(1) symmetries coupled to matter.
Gauge coupling constant ratios close to those measured are easily obtained
for reasonable values of the parameters, consistently with having the string
scale close to the electroweak scale, as required to avoid the hierarchy
problem. Unwanted (colour-triplet, charged-singlet, and neutral-singlet) scalar
tachyons can be removed by a suitable choice of the parameters.Comment: 15 pages, LaTeX fil
Kahler Potentials of Chiral Matter Fields for Calabi-Yau String Compactifications
The Kahler potential is the least understood part of effective N=1
supersymmetric theories derived from string compactifications. Even at
tree-level, the Kahler potential for the physical matter fields, as a function
of the moduli fields, is unknown for generic Calabi-Yau compactifications and
has only been computed for simple toroidal orientifolds. In this paper we
describe how the modular dependence of matter metrics may be extracted in a
perturbative expansion in the Kahler moduli. Scaling arguments, locality and
knowledge of the structure of the physical Yukawa couplings are sufficient to
find the relevant Kahler potential. Using these techniques we compute the
`modular weights' for bifundamental matter on wrapped D7 branes for
large-volume IIB Calabi-Yau flux compactifications. We also apply our
techniques to the case of toroidal compactifications, obtaining results
consistent with those present in the literature. Our techniques do not provide
the complex structure moduli dependence of the Kahler potential, but are
sufficient to extract relevant information about the canonically normalised
matter fields and the soft supersymmetry breaking terms in gravity mediated
scenarios.Comment: JHEP style, 24 pages, 4 figures. v2: New section and reference adde
Low-Energy Supersymmetry Breaking from String Flux Compactifications: Benchmark Scenarios
Soft supersymmetry breaking terms were recently derived for type IIB string
flux compactifications with all moduli stabilised. Depending on the choice of
the discrete input parameters of the compactification such as fluxes and ranks
of hidden gauge groups, the string scale was found to have any value between
the TeV and GUT scales. We study the phenomenological implications of these
compactifications at low energy. Three realistic scenarios can be identified
depending on whether the Standard Model lies on D3 or D7 branes and on the
value of the string scale. For the MSSM on D7 branes and the string scale
between 10^12 GeV and 10^17 GeV we find that the LSP is a neutralino, while for
lower scales it is the stop. At the GUT scale the results of the fluxed MSSM
are reproduced, but now with all moduli stabilised. For the MSSM on D3 branes
we identify two realistic scenarios. The first one corresponds to an
intermediate string scale version of split supersymmetry. The second is a
stringy mSUGRA scenario. This requires tuning of the flux parameters to obtain
the GUT scale. Phenomenological constraints from dark matter, (g-2)_mu and
BR(b->s gamma) are considered for the three scenarios. We provide benchmark
points with the MSSM spectrum, making the models suitable for a detailed
phenomenological analysis.Comment: 29 pages, 12 figures, reference adde
Reconstrucción del paisaje arqueológico por muestreo bayesiano
In this paper we analyze a wetland section of the Samborombon Bay coast, occupied by hunter-gatherers-fishermen during the late Holocene. In an area of approximately 5 km2 , characterized by the presence of channels of estuarine regime, tala forest vegetation and grasslands, we conducted a sampling design using Bayesian resolution. The sampling aimed to find all possible occupations in the area. Incorporating information prior to analysis allowed defining the distance between sampling points with greater efficiency, minimizing the work required to cover a large area, as well as the risk of loss of potential findings. We summarize the methodology used, provide the results of this sampling methodology and discuss the intensity of human space use. The applied methodology in a sector with low archaeological visibility allowed us to have information about human occupation of the area, the characteristics of the archaeological record and the probable spatial distribution of different activities, in a reasonable time and low cost.En este trabajo se analiza un sector del Humedal costero de la Bahía de Samborombón que fue ocupado por cazadores-recolectores-pescadores durante el Holoceno tardío. En una superficie aproximada de 5 km2 , en un ambiente caracterizado por la presencia de canales de régimen estuarial, montes de tala y pastizal pampeano, se realizó un diseño de muestreo de carácter regional utilizando estadística bayesiana. El muestreo tuvo por objetivo localizar la totalidad de las posibles ocupaciones en los montes de tala dentro del área. La incorporación de información previa al análisis posibilitó la determinación de la distancia entre puntos de muestreo con una mayor eficacia, minimizando el trabajo requerido para cubrir un área de grandes dimensiones, como así también el riesgo de pérdida de probables hallazgos. La realización de un muestreo probabilístico en un sector con baja visibilidad arqueológica, ha permitido en un tiempo razonable y a bajo costo contar con información sobre la ocupación del área y las características del registro arqueológico en relación a las tecnofacturas, y la posible distribución espacial de diferentes actividades
On the Duality between Perturbative Heterotic Orbifolds and M-Theory on T^4/Z_N
The heterotic string compactified on an orbifold T^4/\IZ_N
has gauge group with (massless) states in its twisted sectors
which are charged under both gauge group factors. In the dual M-theory on
(T^4/\IZ_N)\otimes(S^1/\IZ_2) the two group factors are separated in the
eleventh direction and the G and G' gauge fields are confined to the two
boundary planes, respectively. We present a scenario which allows for a
resolution of this apparent paradox and assigns all massless matter multiplets
locally to the different six-dimensional boundary fixed planes. The resolution
consists of diagonal mixing between the gauge groups which live on the
connecting seven-planes (6d and the eleventh dimension) and one of the gauge
group factors. We present evidence supporting this mixing by considering gauge
couplings and verify local anomaly cancellation. We also discuss open problems
which arise in the presence of U_1 factors.Comment: 45 pages, one figur
A bound on 6D N=1 supergravities
We prove that there are only finitely many distinct semi-simple gauge groups
and matter representations possible in consistent 6D chiral (1,0) supergravity
theories with one tensor multiplet. The proof relies only on features of the
low-energy theory; the consistency conditions we impose are that anomalies
should be cancelled by the Green-Schwarz mechanism, and that the kinetic terms
for all fields should be positive in some region of moduli space. This result
does not apply to the case of the non-chiral (1,1) supergravities, which are
not constrained by anomaly cancellation.Comment: 23 pages, no figures; two paragraphs added to the proof in Appendix A
covering the SU(2) and SU(3) case, other minor correction
Towards Realistic String Vacua From Branes At Singularities
We report on progress towards constructing string models incorporating both
realistic D-brane matter content and moduli stabilisation with dynamical
low-scale supersymmetry breaking. The general framework is that of local
D-brane models embedded into the LARGE volume approach to moduli stabilisation.
We review quiver theories on del Pezzo () singularities including
both D3 and D7 branes. We provide supersymmetric examples with three
quark/lepton families and the gauge symmetries of the Standard, Left-Right
Symmetric, Pati-Salam and Trinification models, without unwanted chiral
exotics. We describe how the singularity structure leads to family symmetries
governing the Yukawa couplings which may give mass hierarchies among the
different generations. We outline how these models can be embedded into compact
Calabi-Yau compactifications with LARGE volume moduli stabilisation, and state
the minimal conditions for this to be possible. We study the general structure
of soft supersymmetry breaking. At the singularity all leading order
contributions to the soft terms (both gravity- and anomaly-mediation) vanish.
We enumerate subleading contributions and estimate their magnitude. We also
describe model-independent physical implications of this scenario. These
include the masses of anomalous and non-anomalous U(1)'s and the generic
existence of a new hyperweak force under which leptons and/or quarks could be
charged. We propose that such a gauge boson could be responsible for the ghost
muon anomaly recently found at the Tevatron's CDF detector.Comment: 40 pages, 10 figure
Unification and fermion mass relations in low string scale D-brane models
In this talk, gauge coupling evolution is analyzed in D-brane inspired models
with two Higgs doublets and a U(3)xU(2)xU(1)^N gauge symmetry. In particular,
we focus on D-brane configurations with two or three abelian factors. We find
that the correct hypercharge assignment of the Standard Model particles is
reproduced for six viable models distinguished by different brane
configurations. We also investigate the bottom tau quark mass relation and find
that the correct low energy m_b / m_\tau ratio is obtained for equal b-\tau
Yukawa couplings at a string scale around 10^3 TeV.Comment: 10 pages, 3 figures. Talk presented at the ``Corfu Summer
Institute'', Corfu-Greece, September 4-14, 2005. To appear in the proceedings
of RTN workshop on the Quest for Unification Theory Confronts Experimen
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