189 research outputs found
TeV Scale Leptoquarks as a Signature of Standard--like Superstring Models
We show that there can be TeV scale scalar and fermionic leptoquarks with
very weak Yukawa couplings in a generic standard--like superstring model.
Leptoquark--(down--like) quark mixing though present, is not large enough to
violate the unitarity bounds on the CKM matrix. The constraints on leptoquark
masses and couplings from FCNCs are easily satisfied whereas those from baryon
number violation may cause problems. The leptoquarks of the model are compared
to the ones in Calabi--Yau and flipped models.Comment: 14 pages, uses phyzzx.tex, WIS-93/114/DEC-PH. (Section 2 is
shortened, comparison to the flipped SU(5)XU(1) models added in Section 5 and
six references added.
Proton stability in superstring derived models
I discuss the problem of proton decay, from dimension four, five and six
operators, in superstring derived standard--like models. I classify the sectors
that produce color triplet superfields which may generate proton decay from
dimension five and six operators. I show that for two of these sectors there
exist a unique superstring doublet--triplet splitting mechanism that projects
out the color triplets by means of the GSO projection, while leaving the
electroweak doublets in the physical spectrum. I investigate possible proton
decay due to additional color triplets in the massless spectrum and due to
nonrenormalizable terms. I show that there exist models in which the dimension
four, five and six operators, that lead to proton decay, are forbidden by the
symmetries of the string models, to all orders of nonrenormalizable terms.Comment: IASSNS--HEP--94/18, 19 pages, uses PHYZZ
Cabibbo Mixing in Superstring Derived Standard--like Models
We examine the problem of generation mixing in realistic superstring derived
standard--like models, constructed in the free fermionic formulation. We study
the possible sources of family mixing in these models . In a specific model we
estimate the Cabibbo angle. We argue that a Cabibbo angle of the correct order
of magnitude can be obtained in these models.Comment: WIS--92/3/JAN--PH, 15 page
Construction of Realistic superstring Standard--like Models
I discuss the construction of realistic superstring standard--like models in
the four dimensional free fermionic formulation. I discuss proton lifetime
constraints on superstring models. I discuss the massless spectrum of the
superstring standard--like models, the texture of fermion mass matrices in
these models and argue that the realistic features of these models are due to
the underlying orbifold compactification. (talk presented at
the TEXAS\\PASCHOS conference, Berkeley CA, December 13--18 1992)Comment: WIS--92/103/DEC--PH, 8 page
Gauge and Matter Condensates in Realistic String Models
We examine the inter-relationship of the superpotential containing hidden and
observable matter fields and the ensuing condensates in free fermionic string
models. These gauge and matter condensates of the strongly interacting hidden
gauge groups play a crucial role in the determination of the physical
parameters of the observable sector. Supplementing the above information with
the requirement of modular invariance, we find that a generic model with only
trilinear superpotential allows for a degenerate (and sometimes pathological)
set of vacua. This degeneracy may be lifted by higher order terms in the
superpotential. We also point out some other subtle points that may arise in
calculations of this nature. We exemplify our observations by computing
explicitly the modular invariant gaugino and matter condensates in the flipped
SU(5) string model with hidden gauge group .Comment: 11 pages (7 if reduced
Μέθοδοι της θεωρίας πινάκων σε στοχαστικά μοντέλα
Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Εφαρμοσμένες Μαθηματικές Επιστήμες
The Search for a realistic flipped SU(5) string model
We present an extensive search for a general class of flipped models
built within the free fermionic formulation of the heterotic string. We
describe a set of algorithms which constitute the basis for a computer program
capable of generating systematically the massless spectrum and the
superpotential of all possible models within the class we consider. Our search
through the huge parameter space to be explored is simplified considerably by
the constraint of spacetime supersymmetry and the need for extra representations beyond the standard ones in order to possibly achieve string
gauge coupling unification at scales of {\cal O}(10^{18}\GeV). Our results
are remarkably simple and evidence the large degree of redundancy in this kind
of constructions. We find one model with gauge group SU(5)\times
U(1)_\ty\times SO(10)_h\times SU(4)_h\times U(1)^5 and fairly acceptable
phenomenological properties. We study the - and -flatness constraints and
the symmetry breaking pattern in this model and conclude that string gauge
coupling unification is quite possible.Comment: 44 page
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
Symmetries at ultrahigh energies and searches for neutrino oscillations
Motivated by the possibility that new (gauge) symmetries which are broken at
the grand- (string-) unification scale give rise to texture zeros in the
fermion mass matrices which are at the origin of the hierarchy of masses and
mixings we explore the effect of such zeros on the neutrino spectrum of
SUSY-GUT models. We find that the quadratic-seesaw spectrum on which most
expectations are focused is neither the only nor the most interesting
possibility. Cases of strong or
mixing are present for a specific texture structure of the Yukawa matrices and
experimental evidence can thus throw some light on the latter. In contrast if
the quadratic-seesaw scenario should be confirmed very little could be said
about the symmetries of the Yukawa sector.Comment: 13 pages of LATE
Supersymmetry and Light Quark Masses in a Realistic Superstring Model
We examine the light quark masses in a standard--like superstring model in
the four dimensional free fermionic formulation. We find that the supersymmetry
constraints in the observable and hidden sectors eliminate all large
contributions to and and force them to be much smaller than the
other quark masses. The requirement for an acceptable Higgs doublet spectrum
results in . In these models a realistic can always be obtained
whereas is at most . For particular choices of flat
directions or vacua can be as small as but cannot vanish.Comment: WIS-93/98/OCT-PH, 22 pages and 2 tables, uses phyzzx.tex and
tables.te
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