1,389 research outputs found
Supersymmetric Grand Unification Model with the Orbifold Symmetry Breaking in the Six Dimensional Supergravity
We construct supersymmetric (SUSY) grand unification (GUT) models in the six
dimensional space-time where the GUT symmetry is broken down to the
standard-model gauge group by a simple orbifolding T^2/Z_4 or T^2/Z_6 and a
pair of massless Higgs doublets in the SUSY standard model are naturally
obtained. Since the background geometry here is simple compared with models
using the Scherk-Schwarz mechanism, one might hope for an approximate gauge
coupling unification in the present models. Here, the presence of the massless
Higgs multiplets in the bulk is quite natural, since the anomaly cancellation
in the six dimensional space-time requires N=2 hyper multiplets in the bulk,
some of which are origins of the Higgs doublets. However, the origin of the
quarks and leptons is still not clear at all.Comment: 9 pages, including 2 figures, UT-96
Higher Dimensional Supersymmetry as an Origin of the Three Families for Quarks and Leptons
In a (0,1) supersymmetric (SUSY) six-dimensional gauge theory, a gauge
fermion gives rise to box anomalies. These anomalies are completely canceled by
assuming a vector multiplet of (1,1) SUSY. With a T^2/Z_3 orbifold
compactification of the extra two dimensional space, the theory provides three
chiral multiplets and three equivalent fixed points. We regard them as the
origin of the three families of quarks and leptons. Quasi anarchy structure in
the SU(5)-5^* representation sector and hence the bimaximal mixing in the
neutrino oscillation are explained quite naturally in this framework. We also
discuss a family symmetry as a remnant of the higher-dimensional R symmetry.Comment: 12 pages, Contribution to "Peccei Fest" January 11-13,2002, a number
of comments and an appendix are adde
Geometric Origin of Large Lepton Mixing in a Higher Dimensional Spacetime
The large mixing in the lepton sector observed in the recent
neutrino-oscillation experiments strongly suggest that nature of left-handed
lepton doublets is very different from that of left-handed quark doublets. This
means that there is a big disparity between the matter multiplets 5^*'s and
10's in the SU(5) unified theory. We show that this big difference can be
explained in a six-dimensional spacetime compactified on the T^2/Z_3 orbifold.
That is, we propose to put three families of 5^*'s on three equivalent fixed
points of the orbifold and the three 10's in the two-dimensional bulk. We
construct an explicit model realizing this situation and show that the
democratic mass structure in the lepton sector is naturally obtained and hence
the model explains the observed bi-large lepton mixing and simultaneously the
required small mixing U_e3. The mass matrices and mixing in the quark sector
are also briefly discussed.Comment: 12 pages, including 1 figure and 1 table, one paragraph and two
references adde
A Solution to the Doublet-Triplet Splitting Problem in the Type IIB Supergravity
The doublet--triplet mass splitting problem is one of the most serious
problems in supersymmetric grand unified theories (GUTs). A class of models
based on a product gauge group, such as the SU(5)_{GUT} times U(3)_H or the
SU(5)_{GUT} times U(2)_H, realize naturally the desired mass splitting that is
protected by an unbroken R symmetry. It has been pointed out that various
features in the models suggest that these product-group unification models are
embedded in a supersymmetric brane world. We show an explicit construction of
those models in the supersymmetric brane world based on the Type IIB
supergravity in ten dimensions. We consider T^6/(Z_{12} times Z_2) orientifold
for the compactified six extra dimensions. We find that all of the particles
needed for the GUT-symmetry-breaking sector are obtained from the D-brane
fluctuations. The three families of quarks and leptons are introduced at an
orbifold singularity, although their origin remains unexplained. This paper
includes extensive discussion on anomaly cancellation in a given orbifold
geometry. Relation to the Type IIB string theory, realization of R symmetry as
a rotation of extra-dimensional space, and effective superpotential at low
energies are also discussed.Comment: 61 page
Higher-Dimensional QCD without the Strong CP Problem
QCD in a five-dimensional sliced bulk with chiral extra-quarks on the
boundaries is generically free from the strong CP problem. Accidental axial
symmetry is naturally present except for suppressed breaking interactions,
which plays a role of the Peccei-Quinn symmetry to make the strong CP phase
sufficiently small.Comment: 7 pages, late
GUT Phase Transition and Hybrid Inflation
The supersymmetric model of hybrid inflation is interesting not only because
of its naturalness but also because of another reason. Its energy scale
determined by the COBE normalization is 10^15 - 10^16 GeV. It happens to be the
energy scale of interest in particle physics, namely, the mass scale of
right-handed neutrinos or the energy scale of the gauge-coupling unification.
It is true that topological defects are produced after the hybrid inflation if
it is related to a U(1)_B-L or a GUT-symmetry breaking, and hence one cannot
naively identify models of particle physics with that of inflation. But those
defects are not necessarily found in modified models. We show in this article
that quite a simple extension of the minimal supersymmetric hybrid inflation
model is free from monopoles or cosmic strings. Moreover, it happens to be
exactly the same as a well-motivated extension of SU(5)-unified theories. The
vacuum energy is dominated by F-term. The \eta-problem is not necessarily
serious when the model is realized by D-branes. Although it has been considered
that a coupling constant has to be very small when the vacuum energy is
dominated by F-term, this constraint is not applied either to the D-brane
model. They are due to a particular form of the Kahler potential and
interaction of the model. Reheating process is also discussed.Comment: 11 pages; references adde
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