187 research outputs found
Multipole particle in relativity
We discuss the motion of extended objects in a spacetime by considering a
gravitational field created by these objects. We define multipole moments of
the objects as a classification by Lie group SO(3). Then, we construct an
energy-momentum tensor for the objects and derive equations of motion from it.
As a result, we reproduce the Papapetrou equations for a spinning particle.
Furthermore, we will show that we can obtain more simple equations than the
Papapetrou equations by changing the center-of-mass.Comment: 22 pages, 2 figures. Accepted for publication in Phys. Rev.
Building GUTs from strings
We study in detail the structure of Grand Unified Theories derived as the
low-energy limit of orbifold four-dimensional strings. To this aim, new
techniques for building level-two symmetric orbifold theories are presented.
New classes of GUTs in the context of symmetric orbifolds are then constructed.
The method of permutation modding is further explored and SO(10) GUTs with both
or -plets are obtained. SU(5) models are also found through this
method. It is shown that, in the context of symmetric orbifold GUTs,
only a single GUT-Higgs, either a or a , can be present and it always
resides in an order-two untwisted sector. Very restrictive results also hold in
the case of . General properties and selection rules for string GUTs are
described. Some of these selection rules forbid the presence of some particular
GUT-Higgs couplings which are sometimes used in SUSY-GUT model building. Some
semi-realistic string GUT examples are presented and their properties briefly
discussed.Comment: 40 pages, no figures, Late
Gauge Coupling Unification from Unified Theories in Higher Dimensions
Higher dimensional grand unified theories, with gauge symmetry breaking by
orbifold compactification, possess SU(5) breaking at fixed points, and do not
automatically lead to tree-level gauge coupling unification. A new framework is
introduced that guarantees precise unification -- even the leading loop
threshold corrections are predicted, although they are model dependent. Precise
agreement with the experimental result, \alpha_s^{exp} = 0.117 \pm 0.002,
occurs only for a unique theory, and gives \alpha_s^{KK} = 0.118 \pm 0.004 \pm
0.003. Remarkably, this unique theory is also the simplest, with SU(5) gauge
interactions and two Higgs hypermultiplets propagating in a single extra
dimension. This result is more successful and precise than that obtained from
conventional supersymmetric grand unification, \alpha_s^{SGUT} = 0.130 \pm
0.004 \pm \Delta_{SGUT}. There is a simultaneous solution to the three
outstanding problems of 4D supersymmetric grand unified theories: a large mass
splitting between Higgs doublets and their color triplet partners is forced,
proton decay via dimension five operators is automatically forbidden, and the
absence of fermion mass relations amongst light quarks and leptons is
guaranteed, while preserving the successful m_b/m_\tau relation. The theory
necessarily has a strongly coupled top quark located on a fixed point and part
of the lightest generation propagating in the bulk. The string and
compactification scales are determined to be around 10^{17} GeV and 10^{15}
GeV, respectively.Comment: 29 pages, LaTe
Threshold Corrections and Gauge Symmetry in Twisted Superstring Models
Threshold corrections to the running of gauge couplings are calculated for
superstring models with free complex world sheet fermions. For two N=1
models, the threshold corrections lead to a small increase
in the unification scale. Examples are given to illustrate how a given particle
spectrum can be described by models with different boundary conditions on the
internal fermions. We also discuss how complex twisted fermions can enhance the
symmetry group of an N=4 model to the gauge group
. It is then shown how a mixing angle analogous
to the Weinberg angle depends on the boundary conditions of the internal
fermions.Comment: easier to Tex version, figures to be sent separatel
Strongly Coupled Grand Unification in Higher Dimensions
We consider the scenario where all the couplings in the theory are strong at
the cut-off scale, in the context of higher dimensional grand unified field
theories where the unified gauge symmetry is broken by an orbifold
compactification. In this scenario, the non-calculable correction to gauge
unification from unknown ultraviolet physics is naturally suppressed by the
large volume of the extra dimension, and the threshold correction is dominated
by a calculable contribution from Kaluza-Klein towers that gives the values for
\sin^2\theta_w and \alpha_s in good agreement with low-energy data. The
threshold correction is reliably estimated despite the fact that the theory is
strongly coupled at the cut-off scale. A realistic 5d supersymmetric SU(5)
model is presented as an example, where rapid d=6 proton decay is avoided by
putting the first generation matter in the 5d bulk.Comment: 17 pages, latex, to appear in Phys. Rev.
Chains of N=2, D=4 heterotic/type II duals
We report on a search for heterotic strings that are dual candidates of
type II compactifications on Calabi-Yau threefolds described as
fibrations. We find many new heterotic duals by using standard orbifold
techniques. The associated type II compactifications fall into chains in which
the proposed duals are heterotic compactifications related one another by a
sequential Higgs mechanism. This breaking in the heterotic side typically
involves the sequence , while in the type II side the weights of the complex hypersurfaces and the
structure of the quotient singularities also follow specific patterns.Comment: Latex, 21 pages, 2 table
Fermion Masses and Mixing in Intersecting Branes Scenarios
We study the structure of Yukawa couplings in intersecting D6-branes wrapping
a factorizable 6-torus compact space T^6. Models with MSSM-like spectrum are
analyzed and found to fail in predicting the quark mass spectrum because of the
way in which the family structure for the left-handed, right-handed quarks and,
eventually, the Higgses is `factorized' among the different tori. In order to
circumvent this, we present a model with three supersymmetric Higgs doublets
which satisfies the anomaly cancellation condition in a more natural way than
the previous models, where quarks were not treated universally regarding their
branes assignments, or some particular branes were singled out being invariant
under orientifold projection. In our model, the family structures for the left,
right quarks, left leptons and the Higgses arise from one of the tori and can
naturally lead to universal strength Yukawa couplings which accommodate the
quark mass hierarchy and the mixing angles.Comment: 21 pages, latex, matches the Phys. Rev. D versio
Family Unification in Five and Six Dimensions
In family unification models, all three families of quarks and leptons are
grouped together into an irreducible representation of a simple gauge group,
thus unifying the Standard Model gauge symmetries and a gauged family symmetry.
Large orthogonal groups, and the exceptional groups and have been
much studied for family unification. The main theoretical difficulty of family
unification is the existence of mirror families at the weak scale. It is shown
here that family unification without mirror families can be realized in simple
five-dimensional and six-dimensional orbifold models similar to those recently
proposed for SU(5) and SO(10) grand unification. It is noted that a family
unification group that survived to near the weak scale and whose coupling
extrapolated to high scales unified with those of the Standard model would be
evidence accessible in principle at low energy of the existence of small
(Planckian or GUT-scale) extra dimensions.Comment: 13 pages, 2 figures, minor corrections, references adde
Heterotic/Heterotic Duality in D=6,4
We consider heterotic compactifications on and . The idea of heterotic/heterotic duality in has difficulties for
generic compactifications since for large dilaton values some gauge groups
acquire negative kinetic terms. Recently Duff, Minasian and Witten (DMW)
suggested a solution to this problem which only works if the compactification
is performed assuming the presence of symmetric gauge embeddings on both
's. We consider an alternative in which asymmetric embeddings are possible
and the wrong sign of kinetic terms for large dilaton value is a signal of
spontaneous symmetry breaking. Upon further toroidal compactification to ,
we find that the duals in the DMW case correspond to models in which the
-function of the different group factors verify ,
whereas the asymmetric solutions that we propose have . We
check the consistency of these dualities by studying the different large
limits of the gauge kinetic function. Dual , models can also be
obtained by the operation of appropriate freely acting twists, as shown in
specific examples.Comment: 15 pages, late
Gauge Unification in Supersymmetric Intersecting Brane Worlds
We show that contrary to first expectations realistic three generation
supersymmetric intersecting brane world models give rise to phenomenologically
interesting predictions about gauge coupling unification. Assuming the most
economical way of realizing the matter content of the MSSM via intersecting
branes we obtain a model independent relation among the three gauge coupling
constants at the string scale. In order to correctly reproduce the
experimentally known values of sin^2[theta_W(M_z)] and alpha_s(M_z) this
relation leads to natural gauge coupling unification at a string scale close to
the standard GUT scale 2 x 10^16 GeV. Additional vector-like matter can push
the unification scale up to the Planck scale.Comment: 18 pages, harvmac & 3 figures; v2: one ref. adde
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