1,004 research outputs found
Large radii and string unification
We study strong coupling effects in four-dimensional heterotic string models
where supersymmetry is spontaneously broken with large internal dimensions,
consistently with perturbative unification of gauge couplings. These effects
give rise to thresholds associated to the dual theories: type I superstring or
M-theory. In the case of one large dimension, we find that these thresholds
appear close to the field-theoretical unification scale GeV,
offering an appealing scenario for unification of gravitational and gauge
interactions. We also identify the inverse size of the eleventh dimension of
M-theory with the energy at which four-fermion effective operators become
important.Comment: 12 pages, LateX, no figures. Version to appear in Physics Letters B.
One reference adde
A New Gauge Mediation Theory
We propose a class of models with gauge mediation of supersymmetry breaking,
inspired by simple brane constructions, where R-symmetry is very weakly broken.
The gauge sector has an extended N=2 supersymmetry and the two electroweak
Higgses form an N=2 hypermultiplet, while quarks and leptons remain in N=1
chiral multiplets. Supersymmetry is broken via the D-term expectation value of
a secluded U(1) and it is transmitted to the Standard Model via gauge
interactions of messengers in N=2 hypermultiplets: gauginos thus receive Dirac
masses. The model has several distinct experimental signatures with respect to
ordinary models of gauge or gravity mediation realizations of the Minimal
Supersymmetric Standard Model (MSSM). First, it predicts extra states as a
third chargino that can be observed at collider experiments. Second, the
absence of a D-flat direction in the Higgs sector implies a lightest Higgs
behaving exactly as the Standard Model one and thus a reduction of the `little'
fine-tuning in the low tan(beta) region. This breaking of supersymmetry can be
easily implemented in string theory modelsComment: 23 pages, 3 figures, uses axodraw.sty. v2: a mistake in the radiative
generation of the scalar masses is corrected. The main conclusions are
unchange
On boundary terms and conformal transformations in curved space-times
We intend to clarify the interplay between boundary terms and conformal
transformations in scalar-tensor theories of gravity. We first consider the
action for pure gravity in five dimensions and show that, on compactifing a la
Kaluza-Klein to four dimensions, one obtains the correct boundary terms in the
Jordan (or String) Frame form of the Brans-Dicke action. Further, we analyze
how the boundary terms change under the conformal transformations which lead to
the Pauli (or Einstein) frame and to the non-minimally coupled massless scalar
field. In particular, we study the behaviour of the total energy in
asymptotically flat space-times as it results from surface terms in the
Hamiltonian formalism.Comment: LaTeX 2e, 12 pages, no figure
Brans-Dicke-type theories and avoidance of the cosmological singularity
We tudy flat Friedmann-Robertson-Walker cosmology in Brans-Dicke-type
theories of gravitation with minimal coupling between the scalar field and the
matter fields in the Einstein frame (general relativity with an extra scalar
field) for arbitrary values of the Brans-Dicke parameter . It is
shown that the cosmological singularity occuring in the Einstein frame
formulation of this theory is removed in the Jordan frame in the range
. This result is interpreted in the ligth of a
viewpoint (first presented in reference gr-qc/9905071) asserting that both
Jordan frame and Einstein frame formulations of general relativity are
physically equivalent. The implications of the obtained result for string
theory are outlined.Comment: 9 pages, LaTeX, no figures. Improved version accepted for publication
in PR
Evolution of density perturbations in double exponential quintessence models
In this work we investigate the evolution of matter density perturbations for
quintessence models with a self-interaction potential that is a combination of
exponentials. One of the models is based on the Einstein theory of gravity,
while the other is based on the Brans-Dicke scalar tensor theory. We constrain
the parameter space of the models using the determinations for the growth rate
of perturbations derived from data of the 2-degree Field Galaxy Redshift
Survey.Comment: 5 pages, 3 eps figure
Higgs Mediated EDMs in the Next-to-MSSM: An Application to Electroweak Baryogenesis
We perform a study on the predictions of electric-dipole moments (EDMs) of
neutron, Mercury (Hg), Thallium (Tl), deuteron, and Radium (Ra) in the
framework of next-to-minimal supersymmetric standard model (NMSSM) with
CP-violating parameters in the superpotential and soft-supersymmetry-breaking
sector. We confine to the case in which only the physical tree-level CP phase
, associated with the couplings of the singlet
terms in the superpotential and with the vacuum-expectation-values (VEVs),
takes on a nonzero value. We found that the one-loop contributions from
neutralinos are mostly small while the two-loop Higgs-mediated contributions of
the Barr-Zee (BZ) type diagrams dominate. We emphasize a scenario motivated by
electroweak baryogenesis.Comment: 36 pages, 9 figures, to appear in PR
Higgs Boson Sector of the Next-to-MSSM with CP Violation
We perform a comprehensive study of the Higgs sector in the framework of the
next-to-minimal supersymmetric standard model with CP-violating parameters in
the superpotential and in the soft-supersymmetry-breaking sector. Since the CP
is no longer a good symmetry, the two CP-odd and the three CP-even Higgs bosons
of the next-to-minimal supersymmetric standard model in the CP-conserving limit
will mix. We show explicitly how the mass spectrum and couplings to gauge
bosons of the various Higgs bosons change when the CP-violating phases take on
nonzero values. We include full one-loop and the logarithmically enhanced
two-loop effects employing the renormalization-group (RG) improved approach. In
addition, the LEP limits, the global minimum condition, and the positivity of
the square of the Higgs-boson mass have been imposed. We demonstrate the
effects on the Higgs-mass spectrum and the couplings to gauge bosons with and
without the RG-improved corrections. Substantial modifications to the allowed
parameter space happen because of the changes to the Higgs-boson spectrum and
their couplings with the RG-improved corrections. Finally, we calculate the
mass spectrum and couplings of the few selected scenarios and compare to the
previous results in literature where possible; in particular, we illustrate a
scenario motivated by electroweak baryogenesis.Comment: 40 pages, 49 figures; v2: typos corrected and references added; v3:
some clarification and new figures added, version published in PR
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