17 research outputs found
D-branes at Toric Singularities: Model Building, Yukawa Couplings and Flavour Physics
We discuss general properties of D-brane model building at toric
singularities. Using dimer techniques to obtain the gauge theory from the
structure of the singularity, we extract results on the matter sector and
superpotential of the corresponding gauge theory. We show that the number of
families in toric phases is always less than or equal to three, with a unique
exception being the zeroth Hirzebruch surface. With the physical input of three
generations we find that the lightest family of quarks is massless and the
masses of the other two can be hierarchically separated. We compute the CKM
matrix for explicit models in this setting and find the singularities possess
sufficient structure to allow for realistic mixing between generations and CP
violation.Comment: 55 pages, v2: typos corrected, minor comments adde
The gravitational dynamics of warped throats
We investigate the time evolution due to gravitational dynamics of a particular spacetime commonly used in brane-cosmology and string compactifications, namely the Klebanov-Strassler geometry, which is achieved by adding a perturbation to the momentum of the static solution. We observe the effects this has on the spacetime and look for evidence of black hole formation or collapsing cycles which could lead to singular geometry. The cycles are seen to commonly re-expand after reaching a minimum value, showing the stability of the solution against perturbations which would change its size. However black holes are observed to form for certain perturbations, which could impede common uses of the throat's stable tip
D-brane Instantons as Gauge Instantons in Orientifolds of Chiral Quiver Theories
Systems of D3-branes at orientifold singularities can receive
non-perturbative D-brane instanton corrections, inducing field theory operators
in the 4d effective theory. In certain non-chiral examples, these systems have
been realized as the infrared endpoint of a Seiberg duality cascade, in which
the D-brane instanton effects arise from strong gauge theory dynamics. We
present the first UV duality cascade completion of chiral D3-brane theories, in
which the D-brane instantons arise from gauge theory dynamics. Chiral examples
are interesting because the instanton fermion zero mode sector is topologically
protected, and therefore lead to more robust setups. As an application of our
results, we provide a UV completion of certain D-brane orientifold systems
recently claimed to produce conformal field theories with conformal invariance
broken only by D-brane instantons.Comment: 50 pages, 32 figures. v2: version published in JHEP with references
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Stable Non-Supersymmetric Throats in String Theory
We construct a large class of non-supersymmetric AdS-like throat geometries
in string theory by taking non-supersymmetric orbifolds of supersymmetric
backgrounds. The scale of SUSY breaking is the AdS radius, and the dual field
theory has explicitly broken supersymmetry. The large hierarchy of energy
scales in these geometries is stable. We establish this by showing that the
dual gauge theories do not have any relevant operators which are singlets under
the global symmetries. When the geometries are embedded in a compact internal
space, a large enough discrete subgroup of the global symmetries can still
survive to prevent any singlet relevant operators from arising. We illustrate
this by embedding one case in a non-supersymmetric orbifold of a Calabi-Yau
manifold. These examples can serve as a starting point for obtaining
Randall-Sundrum models in string theory, and more generally for constructing
composite Higgs or technicolor-like models where strongly coupled dynamics
leads to the breaking of electro-weak symmetry. Towards the end of the paper,
we briefly discuss how bulk gauge fields can be incorporated by introducing
D7-branes in the bulk, and also show how the strongly coupled dynamics can lead
to an emergent weakly coupled gauge theory in the IR with matter fields
including scalars.Comment: 28 pages,2 figure
Radiative Fermion Masses in Local D-Brane Models
In the context of D-brane model building, we present a realistic framework
for generating fermion masses that are forbidden by global symmetries. We show
that the string theoretical Large volume scenario circumvents the standard lore
that fermion masses generated by loop effects are too small in generic gravity
mediated scenarios. We argue that the fact that in toric singularity models,
the up quark masses have always a zero eigenvalue, corresponding to the
lightest generation, is due to the presence of approximate global symmetries
that we explicitly identify in del Pezzo singularities. These symmetries are
broken by global effects and therefore proportional to inverse powers of the
volume. We estimate the generic size of radiative corrections to fermion masses
in different phenomenological manifestations of the Large volume scenario.
Concrete realizations in terms of flavor violating soft-terms are estimated and
contrasted with current bounds on flavour changing neutral currents.
Contributions from generic extra Higgs-like fields set bounds on their masses
close to the GUT scale to produce realistic fermion masses.Comment: 21 pages. v2: minor rewording and reference adde
Towards a Systematic Construction of Realistic D-brane Models on a del Pezzo Singularity
A systematic approach is followed in order to identify realistic D-brane
models at toric del Pezzo singularities. Requiring quark and lepton spectrum
and Yukawas from D3 branes and massless hypercharge, we are led to Pati-Salam
extensions of the Standard Model. Hierarchies of masses, flavour mixings and
control of couplings select higher order del Pezzo singularities, minimising
the Higgs sector prefers toric del Pezzos with dP3 providing the most
successful compromise. Then a supersymmetric local string model is presented
with the following properties at low energies: (i) the MSSM spectrum plus a
local B-L gauge field or additional Higgs fields depending on the breaking
pattern, (ii) a realistic hierarchy of quark and lepton masses and (iii)
realistic flavour mixing between quark and lepton families with computable CKM
and PMNS matrices, and CP violation consistent with observations. In this
construction, kinetic terms are diagonal and under calculational control
suppressing standard FCNC contributions. Proton decay operators of dimension 4,
5, 6 are suppressed, and gauge couplings can unify depending on the breaking
scales from string scales at energies in the range 10^{12}-10^{16} GeV,
consistent with TeV soft-masses from moduli mediated supersymmetry breaking.
The GUT scale model corresponds to D3 branes at dP3 with two copies of the
Pati-Salam gauge symmetry SU(4)\timesSU(2)R\timesSU(2)L. D-brane instantons
generate a non-vanishing mu-term. Right handed sneutrinos can break the B-L
symmetry and induce a see-saw mechanism of neutrino masses and R-parity
violating operators with observable low-energy implications.Comment: 27 pages plus 5 appendices (42 pages total), 9 figures. v3: equation
refs and citation correcte