1,093 research outputs found

### Top quark mass coupling and classification of weakly-coupled heterotic superstring vacua

The quest for the Standard Model among the huge number of string vacua is
usually based on a set of phenomenological criteria related to the massless
spectrum of string models. In this work we study criteria associated with
interactions in the effective low energy theory and in particular with the
presence of the coupling that provides mass to the top quark. Working in the
context of the Free Fermionic Formulation of the heterotic superstring, we
demonstrate that, in a big class of phenomenologically promising $Z_2\times
Z_2$ compactifications, these criteria can be expressed entirely in terms of
the generalised GSO projection coefficients entering the definition of the
models. They are shown to be very efficient in identifying phenomenologically
viable vacua, especially in the framework of computer-based search, as they are
met by approximately one every $10^4$ models. We apply our results in the
investigation of a class of supersymmetric Pati-Salam vacua, comprising
$10^{16}$ configurations, and show that when combined with other
phenomenological requirements they lead to a relatively small set of about
$10^7$ Standard Model compatible models that can be fully classified.Comment: 13 pages, 1 figure, minor changes, to be published in The European
Physical Journal

### The "landscape" of Pati--Salam heterotic superstring vacua

The main aspects of a recently developed method for classification of
heterotic superstring vacua in the Free Fermionic Formulation are presented.
Using these techniques we classify a big number of approximately $10^{15}$
heterotic string vacua with Pati--Salam, SU(4)x{SU(2)}_Lx{SU(2)}_R gauge
symmetry with respect to their main phenomenological features as the number of
families, Pati--Salam breaking Higgs, Standard Model Higgs doublets, additional
triplets and exotic charge states. We identify an interesting subclass of these
vacua, approximately one to one million, whose massless spectrum is completely
free of fractionally charge states.Comment: 4 pages, 2 figues, Presented at "School and Workshops on Cosmology
and Strings: Theory-Cosmology-Phenomenology" (CORFU2009), September 6-13,
Corfu, Greec

### Hierarchical Neutrino Masses and Mixing in Flipped-SU(5)

We consider the problem of neutrino masses and mixing in the framework of
flipped SU(5). The right-handed neutrino mass, generated through the operation
of a seesaw mechanism by a sector of gauge singlets, leads naturally, at a
subsequent level, to the standard seesaw mechanism resulting into three light
neutrino states with masses of the desired phenomenological order of magnitude.
In this framework we study simple Ansatze for the singlet couplings for which
hierarchical neutrino masses emerge naturally, parametrized in terms of the
Cabbibo parameter. The resulting neutrino mixing matrices are characterized by
a hierarchical structure, in which theta-(13) is always predicted to be the
smallest. Finally, we discuss a possible factorized parametrization of the
neutrino mass that, in addition to Cabbibo mixing, encodes also mixing due to
the singlet sector.Comment: 10 page

### A D-brane inspired U(3)_CxU(3)_LxU(3)_R model

Motivated by D-brane scenarios, we consider a non-supersymmetric model based
on the gauge symmetry U(3)_CxU(3)_LxU(3)_R$ which is equivalent to the SU(3)^3
``trinification'' model supplemented by three U(1)s. Two U(1) combinations are
anomalous while the third U(1)_Z' is anomaly free and contributes to the
hypercharge generator. This hypercharge embedding correspods to
sin^2\theta_W=6/19 in the case of full gauge coupling unification. The U(3)^3
symmetry is broken down to the Standard Model by vev's of two (1,3,\bar3)-
scalar multiplets supplemented by two Higgs fields in (1,3,1) and (1,1,3)
representations. The latter break U(1)_Z' and provide heavy masses to the extra
lepton doublets. Fermions belong to (3,\bar 3,1)+(\bar 3,1,3)+ (1,3,\bar 3)
representations as in the trinification model. The model predicts a natural
quark-lepton hierarchy, since quark masses are obtained from tree-level
couplings, while charged leptons receive masses from fourth order Yukawa terms,
as a consequence of the extra abelian symmetries. Light Majorana neutrino
masses are obtained through a see-saw type mechanism operative at the SU(3)_R
breaking scale of the order M_R\ge 10^9 GeV.Comment: 10 pages, 2 figure

### A Pati-Salam model from branes

We explore the possibility of embedding the Pati-Salam model in the context
of Type I brane models. We study a generic model with U(4)_C x U(2)_L x U(2)_R
gauge symmetry and matter fields compatible with a Type I brane configuration.
Examining the anomaly cancellation conditions of the surplus abelian symmetries
we find an alternative hypercharge embedding that is compatible with a low
string/brane scale of the order of 5-7 TeV, when the U(4)_C and U(2)_R brane
stack couplings are equal. Proton stability is assured as baryon number is
associated to a global symmetry remnant of the broken abelian factors. It is
also shown that this scenario can accommodate an extra low energy abelian
symmetry that can be associated to lepton number. The issue of fermion and
especially neutrino masses is also discussed.Comment: 14 pages, 1 figure, final version to be published in Phys. Lett.

### Higgs Mass Textures in Flipped SU(5)

We analyze the Higgs doublet-triplet mass splitting problem in the version of
flipped SU(5) derived from string theory. Analyzing non-renormalizable terms up
to tenth order in the superpotential, we identify a pattern of field vev's that
keeps one pair of electroweak Higgs doublets light, while all other Higgs
doublets and all Higgs triplets are kept heavy, with the aid of the economical
missing-doublet mechanism found in the field-theoretical version of flipped
SU(5). The solution predicts that second-generation charge -1/3 quarks and
charged leptons are much lighter than those in the third generation.Comment: 15 pages LaTe

### Classification of the N=2, Z2 X Z2-symmetric type II orbifolds and their type II asymmetric duals

Using free world-sheet fermions, we construct and classify all the N=2, Z2 X
Z2 four-dimensional orbifolds of the type IIA/B strings for which the orbifold
projections act symmetrically on the left and right movers. We study the
deformations of these models out of the fermionic point, deriving the partition
functions at a generic point in the moduli of the internal torus T6=T2 X T2 X
T2. We investigate some of their perturbative and non-perturbative dualities
and construct new dual pairs of type IIA/type II asymmetric orbifolds, which
are related non-perturbatively and allow us to gain insight into some of the
non-perturbative properties of the type IIA/B strings in four dimensions. In
particular, we consider some of the (non-)perturbative gravitational
corrections.Comment: Latex, 47 pages, no figure

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