298 research outputs found
Symmetry Scheme for Amino Acid Codons
Group theoretical concepts are invoked in a specific model to explain how
only twenty amino acids occur in nature out of a possible sixty four. The
methods we use enable us to justify the occurrence of the recently discovered
twenty first amino acid selenocysteine, and also enables us to predict the
possible existence of two more, as yet undiscovered amino acids.Comment: 18 pages which include 4 figures & 3 table
Z', new fermions and flavor changing processes, constraints on E models from --> eee
We study a new class of flavor changing interactions, which can arise in
models based on extended gauge groups (rank 4) when new charged fermions are
present together with a new neutral gauge boson. We discuss the cases in which
the flavor changing couplings in the new neutral current coupled to the
are theoretically expected to be large, implying that the observed
suppression of neutral flavor changing transitions must be provided by heavy
masses together with small - mixing angles.
Concentrating on E models, we show how the tight experimental limit on implies serious constraints on the mass and mixing
angle. We conclude that if the value of the flavor changing parameters is
assumed to lie in a theoretically natural range, in most cases the presence of
a much lighter than 1 TeV is unlikely.Comment: plain tex, 22 pages + 2 pages figures in PostScript (appended after
`\bye'), UM-TH 92-1
Baryon and Lepton Number Assignment in Models
In models there are new particles whose baryon number is not uniquely
assigned. We point out that the baryon and lepton number assignment to these
particles can change the baryogenesis scenario significantly. We consider
left-right symmetric extension of the standard model in which quantum
number is gauged. The identification of with a generator of is
used to define the baryon and lepton numbers for the exotic particles in a way
that the electroweak baryon and lepton number anomaly corresponding to the
group vanishes, {\it i.e.}, there is no non-perturbative baryon or
lepton number violation during the electroweak phase transition. We study some
consequences of the new assignment.Comment: 14 pages, LaTeX file, 1 submitted Figure file(.eps
Implications of LEP Results for SO(10) Grandunification with Two Intermediate Stages
We consider the breaking of the grand unification group to the
standard model gauge group through several chains containing two intermediate
stages. Using the values of the gauge coupling constants at scale derived
from recent LEP data, we determine the range of their intermediate and
unification scales. In particular, we identify those chains that permit new
gauge structure at relatively low energy .Comment: (LATEX, 9 pages + 3 pages of figures not included) OITS-48
Maximal atmospheric neutrino mixing and the small ratio of muon to tau mass
We discuss the problem of the small ratio of muon mass to tau mass in a class
of seesaw models where maximal atmospheric neutrino mixing is enforced through
a -- interchange symmetry. We introduce into those models an
additional symmetry such that in the case of exact
invariance. The symmetry may be softly broken in the Higgs potential, and
one thus achieves in a technically natural way. We speculate
on a wider applicability of this mechanism.Comment: 10 pages, plain LaTeX, no figures, minor changes, final version for
J. Phys.
Possible Origin of Fermion Chirality and Gut Structure From Extra Dimensions
The fundamental chiral nature of the observed quarks and leptons and the
emergence of the gauge group itself are most puzzling aspects of the standard
model. Starting from general considerations of topological properties of gauge
field configurations in higher space-time dimensions, it is shown that the
existence of non-trivial structures in ten dimensions would determine a class
of models corresponding to a grand unified GUT structure with complex fermion
representations with respect to . The
discussion is carried out within the framework of string theories with
characteristic energy scales below the Planck mass. Avoidance of topological
obstructions upon continuous deformation of field configurations leads to
global chiral symmetry breaking of the underlying fundamental theory, imposes
rigorous restrictions on the structure of the vacuum and space-time itself and
determines uniquely the gauge structure and matter content.Comment: final version to appear in Phys. Rev.
Spontaneous parity violation and minimal Higgs models
In this paper we present a model for the spontaneous breaking of parity with
two Higgs doublets and two neutral Higgs singlets which are even and odd under
D-parity. The condition can be satisfied without introducing
bidoublets and it is induced by the breaking of D-parity through the vacuum
expectation value of the odd Higgs singlet. Examples of left-right symmetric
and mirror fermions models in grand unified theories are presented.Comment: Revised version. Accepted in Eur. Phys. Journal
On Composite Two Higgs Doublet Models
We investigate composite two Higgs doublet models realized as pseudo
Goldstone modes, generated through the spontaneous breaking of a global
symmetry due to strong dynamic at the TeV scale. A detailed comparative survey
of two possible symmetry breaking patterns, SU(5) -> SU(4) x U(1) and SU(5) x
SU(4), is made. We point out choices for the Standard Model fermion
representations that can alleviate some phenomenological constraints, with
emphasis towards a simultaneous solution of anomalous Zb\bar{b} coupling and
Higgs mediated Flavor Changing Neutral Currents. We also write down the kinetic
lagrangian for several models leading to Two Higgs Doublets and identify the
anomalous contributions to the T parameter. Moreover, we describe a model based
on the breaking in which there is no tree-level breaking of
custodial symmetry, discussing also the possible embeddings for the fermion
fields.Comment: 17 pages. Mistake corrected, added one section on a T- and flavor
safe model based on SO(9)/SO(8). Matches published versio
New fermions and a vector-like third generation in models
We study two 3-3-1 models with i) five (four) charge 2/3 () quarks and,
ii) four (five) charge 2/3 () quarks and a vector-like third generation.
Possibilities beyond these models are also briefly considered.Comment: 32 pages, Revtex 3.0, no figure
SO(10) Cosmic Strings and Baryon Number Violation
SO(10) cosmic strings formed during the phase transition Spin(10)
SU(5) are studied. Two types of strings ---
one effectively Abelian and one non-Abelian --- are constructed and the string
solutions are calculated numerically. The non-Abelian string can catalyze
baryon number violation via the ``twisting'' of the scalar field which causes
mixing of leptons and quarks in the fermion multiplet. The non-Abelian string
is also found to have the lower energy possibly for the entire range of the
parameters in the theory. Scattering of fermions in the fields of the strings
is analyzed, and the baryon number violation cross section is calculated. The
role of the self-adjoint parameters is discussed and the values are computed.Comment: LaTex (RevTex), 36 pages, 6 figures (available upon request),
MIT-CTP#215
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