16 research outputs found
Attractions to radiation-like eras in superstring cosmologies
We review the cosmology induced by finite temperature and quantum effects on
non-supersymmetric string models. We show the evolution is attracted to
radiation-like solutions after the Hagedorn era and before the electroweak
phase transition. This mechanism generates a hierarchy between the Planck mass
and the supersymmetry breaking scale. A dynamical change of space-time
dimension can take place.Comment: 8 page
Super no-scale models in string theory
We consider "super no-scale models" in the framework of the heterotic string,
where the N=4,2,1 --> 0 spontaneous breaking of supersymmetry is induced by
geometrical fluxes realizing a stringy Scherk-Schwarz perturbative mechanism.
Classically, these backgrounds are characterized by a boson/fermion degeneracy
at the massless level, even if supersymmetry is broken. At the 1-loop level,
the vacuum energy is exponentially suppressed, provided the supersymmetry
breaking scale is small, m_{3/2} << M_{string}. We show that the "super
no-scale string models" under consideration are free of Hagedorn-like tachyonic
singularities, even when the supersymmetry breaking scale is large, m_{3/2} ~
M_{string}. The vacuum energy decreases monotonically and converges
exponentially to zero, when m_{3/2} varies from M_{string} to 0. We also show
that all Wilson lines associated to asymptotically free gauge symmetries are
dynamically stabilized by the 1-loop effective potential, while those
corresponding to non-asymtotically free gauge groups lead to instabilities and
condense. The Wilson lines of the conformal gauge symmetries remain massless.
When stable, the stringy super no-scale models admit low energy effective
actions, where decoupling gravity yields theories in flat spacetime, with
softly broken supersymmetry.Comment: 1+46 pages, 1 figur
Large volume susy breaking with a solution to the decompactification problem
We study heterotic ground states in which supersymmetry is broken by coupling
the momentum and winding charges of two large extra dimensions to the R-charges
of the supersymmetry generators. The large dimensions give rise to towers of
heavy string thresholds that contribute to the running of the gauge couplings.
In the general case, these contributions are proportional to the volume of the
two large dimensions and invalidate the perturbative string expansion. The
problem is evaded if the susy breaking sectors arise as a spontaneously broken
phase of N=4 -> N=2 -> N=0 supersymmetry, provided that N=4 supersymmetry is
restored on the boundary of the moduli space. We discuss the mechanism in the
case of Z_2 x Z_2 orbifolds, which requires that the twisted sector that
contains the large extra dimensions has no fixed points. We analyse the full
string partition function and show that the twisted sectors distribute
themselves in non-aligned N=2 orbits, hence preserving the solution to the
string decompactification problem. Remarkably, we find that the contribution to
the vacuum energy from the N=2 -> N=0 sectors is suppressed, and the only
substantial contribution arises from the breaking of the N=4 sector to N=0.Comment: 1+63 page
Stability and Vacuum Energy in Open String Models with Broken Supersymmetry
We construct type I string models with supersymmetry broken by compactifi- cation that are non-tachyonic and have exponentially small effective potential at one-loop. All open string moduli can be stabilized, while the closed string moduli remain massless at one-loop. The backgrounds of interest have rigid Wilson lines by the use of stacked branes, and some models should have heterotic duals. We also present non-tachyonic backgrounds with positive potentials of runaway type at one-loop. This class of models could be used to test various swampland conjectures
On the equivalence of N=1 brane worlds and geometric singularities with flux
We consider Kaluza Klein reductions of M-theory on the Z_N orbifold of the
spin bundle over S^3 along two different U(1) isometries. The first one gives
rise to the familiar ``large N duality'' of the N=1 SU(N) gauge theory in which
the UV is realized as the world-volume theory of N D6-branes wrapped on S^3,
whereas the IR involves N units of RR flux through an S^2. The second reduction
gives an equivalent version of this duality in which the UV is realized
geometrically in terms of an S^2 of A_{N-1} singularities, with one unit of RR
flux through the S^2. The IR is reached via a geometric transition and involves
a single D6 brane on a lens space S^3/Z_N or, alternatively, a singular
background (S^2\times R^4)/Z_N, with one unit of RR flux through S^2 and,
localized at the singularities, an action of their stabilizer group in the U(1)
RR gauge bundle, so that no massless twisted states occur. We also consider
linear sigma-model descriptions of these backgrounds.Comment: 25 pages, LaTeX; v2: one reference added, published versio
Quantum stability in open string theory with broken supersymmetry
International audienceWe consider the 1-loop effective potential in type I string theory compactified on a torus, with supersymmetry broken by the Scherk-Schwarz mechanism. At fixed supersymmetry breaking scale M, and up to exponentially suppressed terms, we show that the potential admits local minima of arbitrary sign, in dimension d †5. While the open string Wilson lines are massive, the closed string moduli are flat directions. In a T-dual picture, the relevant backgrounds involve isolated œ-branes, whose positions are frozen on orientifold planes, thus decreasing the rank of the gauge group, and introducing massless fermions in fundamental representations
Instanton transition in thermal and moduli deformed de Sitter cosmology
35 pages, 10 eps figuresWe consider the de Sitter cosmology deformed by the presence of a thermal bath of radiation and/or time-dependent moduli fields. Depending on the parameters, either a first or second order phase transition can occur. In the first case, an instanton allows a double analytic continuation. It induces a probability to enter the inflationary evolution by tunnel effect from another cosmological solution. The latter starts with a big bang and, in the case the transition does not occur, ends with a big crunch. A temperature duality exchanges the two cosmological branches. In the limit where the pure de Sitter universe is recovered, the tunnel effect reduces to a "creation from nothing", due to the vanishing of the big bang branch. However, the latter may be viable in some range of the deformation parameter. In the second case, there is a smooth evolution from a big bang to the inflationary phase
super no-scale models and moduli quantum stability
International audienceWe consider a class of heterotic N=2â0 super no-scale Z2 -orbifold models. An appropriate stringy ScherkâSchwarz supersymmetry breaking induces tree level masses to all massless bosons of the twisted hypermultiplets and therefore stabilizes all twisted moduli. At high supersymmetry breaking scale, the tachyons that occur in the N=4â0 parent theories are projected out, and no Hagedorn-like instability takes place in the N=2â0 models (for small enough marginal deformations). At low supersymmetry breaking scale, the stability of the untwisted moduli is studied at the quantum level by taking into account both untwisted and twisted contributions to the 1-loop effective potential. The latter depends on the specific branch of the gauge theory along which the background can be deformed. We derive its expression in terms of all classical marginal deformations in the pure Coulomb phase, and in some mixed Coulomb/Higgs phases. In this class of models, the super no-scale condition requires having at the massless level equal numbers of untwisted bosonic and twisted fermionic degrees of freedom. Finally, we show that N=1â0 super no-scale models are obtained by implementing a second Z2 orbifold twist on N=2â0 super no-scale Z2 -orbifold models