33 research outputs found
F-theory Compactifications for Supersymmetric GUTs
We construct a family of elliptically fibered Calabi-Yau four-folds Y_4 for
F-theory compactifications that realize SU(5) GUTs in the low-energy limit. The
three-fold base X_3 of these fibrations is almost Fano and satisfies the
topological criteria required to ensure that the U(1)_Y gauge boson remains
massless, while allowing a decoupling of GUT and Planck scale physics. We study
generic features of these models and the ability to engineer three chiral
generations of MSSM matter. Finally, we demonstrate that it is relatively easy
to implement the topological conditions required to reproduce certain
successful features of local F-theory models, such as the emergence of flavor
hierarchies.Comment: 55 pages, 10 figure
The Generalized Green-Schwarz Mechanism for Type IIB Orientifolds with D3- and D7-Branes
In this paper, we work out in detail the tadpole cancellation conditions as
well as the generalized Green-Schwarz mechanism for type IIB orientifold
compactifications with D3- and D7-branes. We find that not only the well-known
D3- and D7-tadpole conditions have to be satisfied, but in general also the
vanishing of the induced D5-brane charges leads to a non-trivial constraint. In
fact, for the case the latter condition is important for
the cancellation of chiral anomalies. We also extend our analysis by including
D9- as well as D5-branes and determine the rules for computing the chiral
spectrum of the combined system.Comment: 33+7 pages; 2 figures; v2: references added; v3: published versio
Origin of Life
The evolution of life has been a big enigma despite rapid advancements in the
fields of biochemistry, astrobiology, and astrophysics in recent years. The
answer to this puzzle has been as mind-boggling as the riddle relating to
evolution of Universe itself. Despite the fact that panspermia has gained
considerable support as a viable explanation for origin of life on the Earth
and elsewhere in the Universe, the issue remains far from a tangible solution.
This paper examines the various prevailing hypotheses regarding origin of life
like abiogenesis, RNA World, Iron-sulphur World, and panspermia; and concludes
that delivery of life-bearing organic molecules by the comets in the early
epoch of the Earth alone possibly was not responsible for kick-starting the
process of evolution of life on our planet.Comment: 32 pages, 8 figures,invited review article, minor additio
Metal enrichment processes
There are many processes that can transport gas from the galaxies to their
environment and enrich the environment in this way with metals. These metal
enrichment processes have a large influence on the evolution of both the
galaxies and their environment. Various processes can contribute to the gas
transfer: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy
interactions and others. We review their observational evidence, corresponding
simulations, their efficiencies, and their time scales as far as they are known
to date. It seems that all processes can contribute to the enrichment. There is
not a single process that always dominates the enrichment, because the
efficiencies of the processes vary strongly with galaxy and environmental
properties.Comment: 18 pages, 8 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 17; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
D-brane Deconstructions in IIB Orientifolds
With model building applications in mind, we collect and develop basic
techniques to analyze the landscape of D7-branes in type IIB compact Calabi-Yau
orientifolds, in three different pictures: F-theory, the D7 worldvolume theory
and D9-anti-D9 tachyon condensation. A significant complication is that
consistent D7-branes in the presence of O7^- planes are generically singular,
with singularities locally modeled by the Whitney Umbrella. This invalidates
the standard formulae for charges, moduli space and flux lattice dimensions. We
infer the correct formulae by comparison to F-theory and derive them
independently and more generally from the tachyon picture, and relate these
numbers to the closed string massless spectrum of the orientifold
compactification in an interesting way. We furthermore give concrete recipes to
explicitly and systematically construct nontrivial D-brane worldvolume flux
vacua in arbitrary Calabi-Yau orientifolds, illustrate how to read off D-brane
flux content, enhanced gauge groups and charged matter spectra from tachyon
matrices, and demonstrate how brane recombination in general leads to flux
creation, as required by charge conservation and by equivalence of geometric
and gauge theory moduli spaces.Comment: 49 pages, v2: two references adde
Employing Travel Time to Compare the Value of Competing Cultural Organizations
A number of studies have applied non-market valuation techniques to measure the value of cultural goods. Virtually all of these studies are single case applications and rely mostly on stated preferences, such as contingent valuation techniques. We compare the relative value of multiple, competing goods and show how revealed preferences, in particular travel time, may be used for this. In addition, we account for heterogeneity. Using a unique transaction database with the visiting behavior of 80,821 Museum Cardholders to 108 Dutch museums, we propose a latent class application of a logit model to account for the different distances of museums to the population and for differences in willingness-to-travel.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44648/1/10824_2005_Article_5796.pd
Preliminary dragonfly (Odonata) species list from the Pampa biome in Rio Grande do Sul, Brazil, with ecological notes for 19 new records for the State
Science Parksâ tenants versus out-of-Park firms: who innovates more? A duration model
Science Parks, Survival analysis, Matching sample, Patents, Innovation, L29, O39, C41,