14 research outputs found
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
A perfect match of MSSM-like orbifold and resolution models via anomalies
Compactification of the heterotic string on toroidal orbifolds is a promising
set-up for the construction of realistic unified models of particle physics.
The target space dynamics of such models, however, drives them slightly away
from the orbifold point in moduli space. This resolves curvature singularities,
but makes the string computations very difficult. On these smooth manifolds we
have to rely on an effective supergravity approximation in the large volume
limit. By comparing an orbifold example with its blow-up version, we try to
transfer the computational power of the orbifold to the smooth manifold. Using
local properties, we establish a perfect map of the the chiral spectra as well
as the (local) anomalies of these models. A key element in this discussion is
the Green-Schwarz anomaly polynomial. It allows us to identify those
redefinitions of chiral fields and localized axions in the blow-up process
which are relevant for the interactions (such as Yukawa-couplings) in the model
on the smooth space.Comment: 2+35 pages, 1 figur
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
Mass Hierarchies from MSSM Orientifold Compactifications
We investigate phenomenologically viable four- and five-stack MSSM D-brane
quivers which exhibit realistic fermion mass hierarchies. In our analysis, the
mass hierarchies arise either from higher order terms containing the VEV's of
SM singlets or from D-instanton effects, where the latter utilizes either
family splitting or a factorizable Yukawa matrix. Extending the systematic
bottom-up analysis of arXiv:0905.3379, we present the only four-stack quiver
with a semi-realistic Yukawa texture. Investigation of five-stack MSSM models
reveals many more quivers with analogous Yukawa textures, as well as a few
examples which exhibit three different mass scales for the up-quarks,
down-quarks, and electrons. Potential problems in this class of quivers are the
presence of U(1) instantons, which might lead to undesired effects, such as
R-parity violating couplings, and the presence of dimension 5 operators that
could lead to rapid proton decay. We present a five-stack setup which overcomes
all of these problems and exhibits three different mass scales for the
up-quarks, down-quarks and electrons.Comment: 31 page
On the interpretation of a possible ∼ 750 GeV particle decaying into γγ
We consider interpretations of the recent ∼ 3σ reports by the CMS and ATLAS collaborations of a possible X (∼750 GeV) state decaying into γγ final states. We focus on the possibilities that this is a scalar or pseudoscalar electroweak isoscalar state produced by gluon-gluon fusion mediated by loops of heavy fermions. We consider several models for these fermions, including a single vector-like charge 2/3 T quark, a doublet of vector-like quarks (T, B), and a vector-like generation of quarks, with or without leptons that also contribute to the X → γγ decay amplitude. We also consider the possibility that X (750) is a dark matter mediator, with a neutral vector-like dark matter particle. These scenarios are compatible with the present and prospective direct limits on vector-like fermions from LHC Runs 1 and 2, as well as indirect constraints from electroweak precision measurements, and we show that the required Yukawa-like couplings between the X particle and the heavy vector-like fermions are small enough to be perturbative so long as the X particle has dominant decay modes into gg and γγ. The decays X → ZZ, Zγ and W+W− are interesting prospective signatures that may help distinguish between different vector-like fermion scenarios
A realistic intersecting D6-brane model after the first LHC run
With the Higgs boson mass around 125 GeV and the LHC supersymmetry search
constraints, we revisit a three-family Pati-Salam model from intersecting
D6-branes in Type IIA string theory on the
orientifold which has a realistic phenomenology. We systematically scan the
parameter space for , and find that the gravitino mass is
generically heavier than about 2 TeV for both cases due to the Higgs mass low
bound 123 GeV. In particular, we identify a region of parameter space with the
electroweak fine-tuning as small as 24-32 (3-4). In the
viable parameter space which is consistent with all the current constraints,
the mass ranges for gluino, the first two-generation squarks and sleptons are
respectively TeV, TeV, and TeV. For the
third-generation sfermions, the light stop satisfying WMAP bounds via
neutralino-stop coannihilation has mass from 0.5 to 1.2 TeV, and the light stau
can be as light as 800 GeV. We also show various coannihilation and resonance
scenarios through which the observed dark matter relic density is achieved.
Interestingly, the certain portions of parameter space has excellent
-- and - Yukawa coupling unification. Three regions of
parameter space are highlighted as well where the dominant component of the
lightest neutralino is a bino, wino or higgsino. We discuss various scenarios
in which such solutions may avoid recent astrophysical bounds in case if they
satisfy or above observed relic density bounds. Prospects of finding
higgsino-like neutralino in direct and indirect searches are also studied. And
we display six tables of benchmark points depicting various interesting
features of our model.Comment: 41 pages, 12 figures, 6 table