The ubiquitous throat Hebecker, A.; March-Russell, J.
Nuclear physics. B,
10/2007, Volume:
781, Issue:
1
Journal Article
Peer reviewed
We attempt to quantify the widely-held belief that large hierarchies induced by strongly-warped geometries are common in the string theory landscape. To this end, we focus on the arguably ...best-understood subset of vacua—type IIB Calabi–Yau orientifolds with non-perturbative Kähler stabilization and a SUSY-breaking uplift (the KKLT setup). Within this framework, vacua with a realistically small cosmological constant are expected to come from Calabi–Yaus with a large number of 3-cycles. For appropriate choices of flux numbers, many of these 3-cycles can, in general, shrink to produce near-conifold geometries. Thus, a simple statistical analysis in the spirit of Denef and Douglas allows us to estimate the expected number and length of Klebanov–Strassler throats in the given set of vacua. We find that throats capable of explaining the electroweak hierarchy are expected to be present in a large fraction of the landscape vacua while shorter throats are essentially unavoidable in a statistical sense.
If the expectation value of the right-handed (rhd) sneutrino comes to dominate the universe, its decay naturally leads to successful leptogenesis, as well as significant dilution of dangerous ...inflationary relics, such as the gravitino. The resulting baryon asymmetry is independent of other cosmological initial conditions. This attractive variant of leptogenesis requires at least one of the rhd neutrinos to have small Yukawa coupling and to have mass ∼106 GeV, much smaller than the grand unified (GUT) scale. We show that these features naturally arise in the context of independently motivated and successful 5d orbifold GUTs with inverse-GUT-scale-sized extra dimensions. Rhd neutrinos are realized as bulk fields Ni with 5d bulk masses, while Yukawa couplings and lepton-number-violating masses for the Ni are localized at the SM boundary. The exponential suppression of the would-be Ni zero-modes leads to the desired small 4d Yukawa couplings and small masses for the rhd neutrino states. The see-saw prediction for the lhd neutrino mass scale is automatically maintained. We show that this realization of rhd neutrinos is nicely accommodated within an attractive orbifold-GUT flavour model, where all flavour hierarchies have a geometrical origin.
Recently, an attractive model of GUT breaking has been proposed in which a 5-dimensional supersymmetric SU(5) gauge theory on an
S
1/(
Z
2×
Z
2′) orbifold is broken down to the 4d MSSM by ...SU(5)-violating boundary conditions. Motivated by this construction and several related realistic models, we investigate the general structure of orbifolds in the effective field theory context, and of this orbifold symmetry breaking mechanism in particular. An analysis of the group theoretic structure of orbifold breaking is performed. This depends upon the existence of appropriate inner and outer automorphisms of the Lie algebra, and we show that a reduction of the rank of the GUT group is possible. Some aspects of larger GUT theories based on SO(10) and E
6 are discussed. We explore the possibilities of defining the theory directly on a space with boundaries and breaking the gauge symmetry by more general consistently chosen boundary conditions for the fields. Furthermore, we derive the relation of orbifold breaking with the familiar mechanism of Wilson line breaking, finding a one-to-one correspondence, both conceptually and technically. Finally, we analyse the consistency of orbifold models in the effective field theory context, emphasizing the necessity for self-adjoint extensions of the Hamiltonian and other conserved operators, and especially the highly restrictive anomaly cancellation conditions that apply if the bulk theory lives in more than 5 dimensions.
To identify approaches to improving methods for assessing tolerability and safety of psychotropic medications in children and adolescents.
Strengths and limitations of current methodology were ...reviewed and possible alternatives examined.
Research on the validity of safety evaluation has been extremely limited. No evidence-based "gold standard" exists. Clinical trials remain the best design to establish causality, but sample size limitations prevent the detection of infrequent, though serious, adverse events. Other designs, such as cohort and case-control studies, and approaches, such as mining of large databases, must be considered.
The current lack of methodological standardization across studies prevents generalizations and meta-analyses. Because the issues relevant to drug safety are diverse, a variety of methodological approaches and instruments are needed. It is, however, possible to adopt standard basic definitions of adverse events, degree of severity, ascertainment methods, and recording procedures, as a common "core," to which more specific assessment instruments can be added. Systematic empirical testing and validation of safety methodology is needed.
We analyse the cosmology of a brane world model where a single brane carrying the standard model fields forms the boundary of a 5-dimensional AdS bulk (the Randall–Sundrum II scenario). We focus on ...the thermal radiation of bulk gravitons, the formation of the bulk black hole, and the holographic AdS/CFT definition of the RSII theory. Our detailed calculation of bulk radiation reduces previous estimates to a phenomenologically acceptable, although potentially visible level. In late cosmology, in which the Friedmann equation depends linearly on the energy density
ρ, only about 1% of energy density is lost to the black hole or, equivalently, to the ‘dark radiation’ (
Ω
d,N≃0.01
at nucleosynthesis). The preceding, unconventional
ρ
2 period can produce up to 10% dark radiation (
Ω
d,N≲0.1
). The AdS/CFT correspondence provides an equivalent description of late RSII cosmology. We show how the AdS/CFT formulation can reproduce the
ρ
2 correction to the standard treatment at low matter density. However, the 4-dimensional effective theory of CFT + gravity breaks down due to higher curvature terms for energy densities where
ρ
2 behaviour in the Friedmann equation is usually predicted. We emphasize that, in going beyond this energy density, the microscopic formulation of the theory becomes essential. For example, the pure AdS
5 and string-motivated AdS
5×S
5 definitions differ in their cosmological implications.
Black holes and sub-millimeter dimensions Argyres, Philip C.; Dimopoulos, Savas; March-Russell, John
Physics letters. B,
11/1998, Volume:
441, Issue:
1
Journal Article
Peer reviewed
Open access
Recently, a new framework for solving the hierarchy problem was proposed which does not rely on low energy supersymmetry or technicolor. The fundamental Planck mass is at a TeV and the observed ...weakness of gravity at long distances is due the existence of new sub-millimeter spatial dimensions. In this letter, we study how the properties of black holes are altered in these theories. Small black holes—with Schwarzschild radii smaller than the size of the new spatial dimensions—are quite different. They are bigger, colder, and longer-lived than a usual (3+1)-dimensional black hole of the same mass. Furthermore, they primarily decay into harmless bulk graviton modes rather than standard-model degrees of freedom. We discuss the interplay of our scenario with the holographic principle. Our results also have implications for the bounds on the spectrum of primordial black holes (PBHs) derived from the photo-dissociation of primordial nucleosynthesis products, distortion of the diffuse gamma-ray spectrum, overclosure of the universe, gravitational lensing, as well as the phenomenology of black hole production. For example, the bound on the spectral index of the primordial spectrum of density perturbations is relaxed from 1.25 to 1.45–1.60 depending on the epoch of the PBH formation. In these scenarios PBHs provide interesting dark matter candidates; for 6 extra dimensions MACHO candidates with mass ∼0.1
M
⊙ can arise. For 2 or 3 extra dimensions PBHs with mass ∼10
4
M
⊙ can occur and may act as both dark matter and seeds for early galaxy and QSO formation.
We modify and extend an earlier proposal by Brown and Teitelboim to relax the effective cosmological term by nucleation of branes coupled to a three-index gauge potential. Microscopic considerations ...from string/M-theory suggest two major innovations in the framework. First, the dependence of brane properties on the compactification of extra dimensions may generate a very small quantized unit for jumps in the effective cosmological term. Second, internal degrees of freedom for multiply coincident branes may enhance tunneling rates by exponentially large density of states factors. These new features essentially alter the relaxation dynamics. By requiring stability on the scale of the lifetime of the universe, rather than absolute stability, we derive a non-trivial relation between the supersymmetry breaking scale and the value of the cosmological term. It is plausibly, though not certainly, satisfied in Nature.
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