146 research outputs found
A Solution to the Decompactification Problem in Chiral Heterotic Strings
We present a solution to the decompactification problem of gauge thresholds
in chiral heterotic string theories with two large extra dimensions, where
supersymmetry is spontaneously broken by the Scherk-Schwarz mechanism. Whenever
the Kaluza-Klein scale is much lower than the string scale, the infinite towers
of heavy states contribute non-trivially to the renormalisation of gauge
couplings, which typically grow linearly with the large volume of the internal
space and invalidate perturbation theory. We trace the origin of the
decompactification problem to properties of the six dimensional theory obtained
in the infinite volume limit and show that thresholds may instead exhibit
logarithmic volume dependence and we provide the conditions for this to occur.
We illustrate this mechanism with explicit string constructions where the
decompactification problem does not occur.Comment: 26 pages, 1 figur
Spinor-vector duality and light Z' in heterotic string vacua
We discuss the construction of heterotic--string models that allow for the
existence of an extra at low scales. One of the main difficulties
encountered is that the desired symmetries tend to be anomalous in the
prevailing three generation constructions. The reason is that these models
utilise the symmetry breaking pattern
by GGSO projections. Consequently, becomes anomalous. The
spinor--vector duality that was observed in the fermionic
orbifold compactifications is used to construct a phenomenological three
generation Pati--Salam heterotic--string model in which is anomaly
free and therefore can be a component of a low scale . The model
implies existence of matter states at the breaking scale, which are
required for anomaly cancelation. Moreover, the string model gives rise to
exotic states, which are singlets but carry exotic
charges. These states arise due to the breaking of by discrete Wilson
lines and provide natural dark matter candidates. Initial indications suggest
that the existence of additional gauge symmetries at the TeV scale may be
confirmed in run II of the LHC experiment.Comment: To appear in the proceedings of PLANCK 2015 international conference,
25-29 May 2015, Ioannnina, Greece. 17 pages. Standard LaTex. 2 figure
A Light Z' Heterotic-String Derived Model
The existence of an extra Z' inspired from heterotic-string theory at
accessible energy scales attracted considerable interest in the particle
physics literature. Surprisingly, however, the construction of
heterotic--string derived models that allow for an extra Z' to remain unbroken
down to low scales has proven to be very difficult. The main reason being that
the U(1) symmetries that are typically discussed in the literature are either
anomalous or have to be broken at a high scale to generate light neutrino
masses. In this paper we use for that purpose the self duality property under
the spinor vector duality, which was discovered in free fermionic
heterotic-string models. The chiral massless states in the self--dual models
fill complete 27 representations of E6. The anomaly free gauge symmetry in the
effective low energy field theory of our string model is , where is the family
universal symmetry that descends from E6, and is typically anomalous in
other free fermionic heterotic-string models. Our model therefore allows for
the existence of a low scale Z', which is a combination of , and
. The string model is free of exotic fractionally charged states in
the massless spectrum. It contains exotic SO(10) singlet states that carry
fractional, non--E6 charge, with respect to . These non-E6 states
arise in the string model due to the breaking of the E6 symmetry by discrete
Wilson lines. They represent a distinct signature of the string vacua and
cannot arise in E6 Grand Unified Theories. They may provide viable dark matter
candidates.Comment: 18 pages. Standard LaTeX. 3 table
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 Flipped SU(5) Heterotic-String Vacua
We extend the classification of the free fermionic heterotic-string vacua to
models in which the SO(10) GUT symmetry at the string scale is broken to the
flipped SU(5) subgroup. In our classification method, the set of basis vectors
defined by the boundary conditions which are assigned to the free fermions is
fixed and the enumeration of the string vacua is obtained in terms of the
Generalised GSO (GGSO) projection coefficients entering the one-loop partition
function. We derive algebraic expressions for the GGSO projections for all the
physical states appearing in the sectors generated by the set of basis vectors.
This enables the analysis of the entire string spectrum to be programmed in to
a computer code therefore, we performed a statistical sampling in the space of
2^{44} (approximately 10^{13}) flipped vacua and scanned up to 10^{12}
GGSO configurations. For that purpose, two independent codes were developed
based on JAVA and FORTRAN95. All the results presented here are confirmed by
the two independent routines. Contrary to the corresponding Pati-Salam
classification, we do not find exophobic flipped SU(5) vacua with an odd number
of generations. We study the structure of exotic states appearing in the three
generation models that additionally contain a viable Higgs spectrum. Moreover,
we demonstrate the existence of models in which all the exotic states are
confined by a hidden sector non-Abelian gauge symmetry as well as models that
may admit the racetrack mechanism.Comment: Minor changes (Version 2) - 51 pages - 3 figures - Added
acknowledgement
Top Quark Mass in Exophobic Pati--Salam Heterotic String Model
We analyse the phenomenology of an exemplary exophobic Pati-Salam heterotic
string vacuum, in which no exotic fractionally charged states exist in the
massless string spectrum. Our model also contains the Higgs representations
that are needed to break the gauge symmetry to that of the Standard Model and
to generate fermion masses at the electroweak scale. We show that the
requirement of a leading mass term for the heavy generation, which is not
degenerate with the mass terms of the lighter generations, places an additional
strong constraint on the viability of the models. In many models a top quark
Yukawa may not exist at all, whereas in others two or more generations may
obtain a mass term at leading order. In our exemplary model a mass term at
leading order exist only for one family. Additionally, we demonstrate the
existence of supersymmetric F- and D-flat directions that give heavy mass to
all the colour triplets beyond those of the Standard Model and leave one pair
of electroweak Higgs doublets light. Hence, below the Pati-Salam breaking
scale, the matter states in our model that are charged under the observable
gauge symmetries, consist solely of those of the Minimal Supersymmetric
Standard Model.Comment: 16 pages. 2 figures. Minor typos correcte
Implications of Anomalous U(1) Symmetry in Unified Models: the Flipped SU(5) x U(1) Paradigm
A generic feature of string-derived models is the appearance of an anomalous
Abelian U(1)_A symmetry which, among other properties, constrains the Yukawa
couplings and distinguishes the three families from each other. In this paper,
we discuss in a model-independent way the general constraints imposed by such a
U(1)_A symmetry on fermion masses, R-violating couplings and proton-decay
operators in a generic flipped SU(5) x U(1)' model. We construct all possible
viable fermion mass textures and give various examples of effective low-energy
models which are distinguished from each other by their different predictions
for B-, L- and R-violating effects. We pay particular attention to predictions
for neutrino masses, in the light of the recent Super-Kamiokande data.Comment: 28 pages, reference adde
Technical assistance-doubts and hopes
[Δε διατίθεται περίληψη][No abstract available
The 750 GeV di-photon LHC excess and extra Z's in heterotic-string derived models
The ATLAS and CMS collaborations recently recorded possible di-photon excess
at 750 GeV and a less significant di-boson excess around 1.9 TeV. Such excesses
may be produced in heterotic-string derived Z' models, where the di-photon
excess may be connected with the Standard Model singlet scalar responsible for
the Z' symmetry breaking, whereas the di-boson excess arises from production of
the extra vector boson. Additional vector-like states in the string Z' model
are instrumental to explain the relatively large width of the di-photon events
and mandated by anomaly cancellation to be in the vicinity of the Z' breaking
scale. Wilson line breaking of the non-Abelian gauge symmetries in the string
models naturally gives rise to dark matter candidates. Future collider
experiments will discriminate between the high-scale heterotic-string models,
which preserve the perturbative unification paradigm indicated by the Standard
Model data, versus the low scale string models. We also discuss the possibility
for the production of the di-photon events with high scale
breaking.Comment: 17 pages. 1 figure. Minor revisions. References added. Published
versio
String Model Building on Quantum Annealers
For the first time the direct construction of string models on quantum annealers has been explored and has been investigated their efficiency and effectiveness in the model discovery process. Through a thorough comparison with traditional methods such as simulated annealing, random scans, and genetic algorithms, it is highlighted the potential advantages offered by quantum annealers, which in this study promised to be roughly 50 times faster than random scans and genetic algorithm and approximately four times faster than simulated annealing
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