2,438 research outputs found
Solving the Decompactification Problem in String Theory
We investigate heterotic ground states in four dimensions in which N=4
supersymmetry is spontaneously broken to N=2. N=4 supersymmetry is restored at
a decompactification limit corresponding to . We calculate the
full moduli dependent threshold corrections and confirm that they are supressed
in the decompactification limit as expected from the restoration
of N=4 supersymmetry. This should be contrasted with the behavior of the
standard N=2 groundstates where the coupling blow up linearly with the volume
of the decompactifying manifold. This mechanism provides a solution to the
decompactification problem for the gauge coupling constants.
We also discuss how the mechanism can be implemented in ground states with
lower supersymmetry.Comment: 14pp, LaTeX some typos correcte
Non-perturbative Supersymmetry Breaking and Finite Temperature Instabilities in N=4 Superstrings
We obtain the non-perturbative effective potential for the dual
five-dimensional N=4 strings in the context of finite-temperature regarded as a
breaking of supersymmetry into four space-time dimensions. Using the properties
of gauged N=4 supergravity we derive the universal thermal effective potential
describing all possible high-temperature instabilities of the known N=4
superstrings. These strings undergo a high-temperature transition to a new
phase in which five-branes condense. This phase is described in detail, using
both the effective supergravity and non-critical string theory in six
dimensions. In the new phase, supersymmetry is perturbatively restored but
broken at the non-perturbative level.Comment: 21 pages. Proceedings of the Corfu Summer Institute on Elementary
Particle Physics, Corfu, Greece, September 199
Gauge couplings in four-dimensional Type I string orbifolds
We compute threshold effects to gauge couplings in four-dimensional
orientifold models of type I strings with and
supersymmetry, and study their dependence on the geometric moduli. We also
compute the tree-level (disk) couplings of the open sector gauge fields to the
twisted closed string moduli of the orbifold in various models and study their
effects and that of the one-loop threshold corrections on gauge coupling
unification. We interpret the results from the (supergravity) effective theory
point of view and comment on the conjectured heterotic-type I dualityComment: 54 pages, LaTeX, comments and references added, final version to
appear in Nucl. Phys.
Magnetized Type I Orbifolds in Four Dimensions
I review the basic features of four dimensional Z_2 x Z_2 (shift)
orientifolds with internal magnetic fields, describing two examples with N=1
supersymmetry. As in the corresponding six-dimensional examples, D9-branes
magnetized along four internal directions can mimic D5-branes, even in presence
of multiplets of image branes localized on different fixed tori. Chiral
low-energy spectra can be obtained if the model also contains D5-branes
parallel to the magnetized directions.Comment: 4 pages, LATEX; misprints correcte
Gauge Unification and Quark Masses in a Pati-Salam Model from Branes
We investigate the phase space of parameters in the Pati-Salam model derived
in the context of D-branes scenarios, requiring low energy string scale. We
find that a non-supersymmetric version complies with a string scale as low as
10 TeV, while in the supersymmetric version the string scale raises up to ~2 x
10^7 TeV. The limited energy region for RGE running demands a large tan(beta)
in order to have experimentally acceptable masses for the top and bottom
quarks.Comment: 11 pages, LaTeX, 7 figures include
Non-Supersymmetric Type I Strings with Zero Vacuum Energy
We study open descendants of non-supersymmetric type IIB asymmetric (freely
acting) orbifolds with zero cosmological constant. A generic feature of these
models is that supersymmetry remains unbroken on the brane at all mass levels,
while it is broken in the bulk in a way that preserves Fermi-Bose degeneracy in
both the massless and massive (closed string) spectrum. This property remains
valid in the heterotic dual of the type II model but only for the massless
excitations. A possible application of these constructions concerns scenarios
of low-energy supersymmetry breaking with large dimensions.Comment: 22 pages, TeX, harvmac. Minor corrections. Final version to appear on
Nucl.Phys.
Physical States of the Quantum Conformal Factor
The conformal factor of the spacetime metric becomes dynamical due to the
trace anomaly of matter fields. Its dynamics is described by an effective
action which we quantize by canonical methods on the Einstein universe . We find an infinite tower of discrete states which satisfy the
constraints of quantum diffeomorphism invariance. These physical states are in
one-to-one correspondence with operators constructed by integrating integer
powers of the Ricci scalar.Comment: PlainTeX File, 34 page
Laboratory Investigation on Gravel and Mixed Beaches During an Oblique Wave Attack
An experimental investigation into the behavior of gravel and mixed (sand and gravel) beaches was carried out at the 3-D Wave Basin located at Franzius-Institute (Marienwerder) of University of Hannover, at a nominal scale of 1:1. The experiment aimed to provide full scale measurements of cross-shore processes on gravel and mixed beaches, during an oblique wave attack, with uniform slope and a trench. Measurements included sediment transport, cross-shore beach profiles and wave-induced currents, for regular and random wave tests, for both types of beaches. Analysis of both cross-shore and long-shore currents shown interesting behaviour for both gravel and mixed beach, especially at the trench. There were morphological differences between the two types of beach concerning the crest and the step formation, the onshore sediment movement, and the erosion below the SWL, concluding the general difference of their mobility
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