1,021 research outputs found
Supergravity Inspired Warped Compactifications and Effective Cosmological Constants
We propose a supergravity inspired derivation of a Randall-Sundrum's type
action as an effective description of the dynamics of a brane coupled to the
bulk through gravity only. The cosmological constants in the bulk and on the
brane appear at the classical level when solving the equations of motion
describing the bosonic sector of supergravities in ten and eleven dimensions
coupled to the brane. They are related to physical quantities like the brane
electric charge and thus inherit some of their physical properties. The most
appealing property is their quantization: in d_\perp extra dimensions,
Lambda_brane goes like N and Lambda_bulk like N^{2/(2-d_perp)}. This dynamical
origin also explains the apparent fine-tuning required in the Randall-Sundrum
scenario. In our approach, the cosmological constants are derived parameters
and cannot be chosen arbitrarily; instead they are determined by the underlying
Lagrangian. Some of the branes we construct that support cosmological constant
in the bulk have supersymmetric properties: D3-branes of type IIB superstring
theory provide an explicit example.Comment: 17 pages, LaTeX, 1 figure. v2: references added and a comment about
D-8 brane of massive IIA sugra included v3: improved argument on the
effective cosmological constants quantization and clarified discussion on the
supersymmetric issue of the solutions constructed. Final version to appear in
NP
Inflating Intersecting Branes and Remarks on the Hierarchy Problem
We generalize solutions of Einstein's equations for intersecting branes in
higher dimensional spacetimes to the nonstatic case, modeling an expanding
universe. The relation between the Hubble rate, the brane tensions, and the
bulk cosmological constant is similar to the case of a single 3-brane in a
5-dimensional spacetime. However, because the bulk inflates as well as the
branes, this class of solutions suffers from Newton's constant tending toward
zero on the TeV brane, where the Randall-Sundrum mechanism should solve the
weak scale hierarchy problem. The strength of gravity remains constant on the
Planck brane, however.Comment: 10 pages, LaTeX. v2:Misprint in eq. (23) corrected; citations fixed
and clarified relationship of our work to hep-th/9909053 and hep-th/9909076
v3: final version to appear in PLB. Corrected discussion of the time
dependance of the 4-D Planck mass on the TeV brane. Some references added to
earlier works on warped Kaluza-Klein compactification
First-Order Electroweak Phase Transition in the Standard Model with a Low Cutoff
We study the possibility of a first-order electroweak phase transition (EWPT)
due to a dimension-six operator in the effective Higgs potential. In contrast
with previous attempts to make the EWPT strongly first-order as required by
electroweak baryogenesis, we do not rely on large one-loop thermally generated
cubic Higgs interactions. Instead, we augment the Standard Model (SM) effective
theory with a dimension-six Higgs operator. This addition enables a strong
first-order phase transition to develop even with a Higgs boson mass well above
the current direct limit of 114 GeV. The phi^6 term can be generated for
instance by strong dynamics at the TeV scale or by integrating out heavy
particles like an additional singlet scalar field. We discuss conditions to
comply with electroweak precision constraints, and point out how future
experimental measurements of the Higgs self couplings could test the idea.Comment: 5 pages, 4 figures. v2: corrected typos, improved discussion of the
case lambda<0 and added references. To be published in PR
En 2010, les PME bénéficient de la reprise mais diffèrent leurs investissements.
En 2010, la reprise de l’activité permet aux PME d’améliorer leur rentabilité et de consolider leurs capitaux propres, même si de fortes disparités demeurent. Le renforcement de la trésorerie et la faiblesse de l’investissement demeurent caractéristiques d’un comportement attentiste.PME, activité, rentabilité, endettement, investissement.
La dernière transition glaciaire/interglaciaire des Andes tropicales sud (Bolivie) d'après l'étude des variations des niveaux lacustres et des fluctuations glaciaires
Une comparaison entre les variations des niveaux lacustres dans le bassin de Uyuni-Coipasa et les fluctuations glaciaires dans la Cordillère Orientale montre que les lacs et les glaciers ont simultanément enregistré les changements des précipitations. Un intense maximum lacustre (environ 13-12 14C ka BP) a eu lieu aux latitudes tropicales sud de la Bolivie, 4 000 à 6 000 ans avant le maximum lacustre de l'Afrique nord-équatoriale. (Résumé d'auteur
Variations du régime et de la nature des précipitations au cours des 15 derniers millénaires dans les Andes de Bolivie
La morphologie des versants et l'évolution de la sédimentation fluviatile des Andes boliviennes témoignent de l'existence de deux régimes bien distincts de précipitations au cours des 15 derniers millénaires. Des périodes à averses pluvieuses orageuses et estivales ont alterné avec des périodes à précipitations neigeuses assez bien distribuées au long de l'année. Les datations par la méthode du 14C indiquent que ces changements du régime climatique s'opèrent à une échelle de temps de 2 a 3 millénaires au maximum. (Résumé d'auteur)
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