1,854 research outputs found
Supersymmetric 4D Orientifolds of Type IIA with D6-branes at Angles
We study a certain class of four-dimensional N=1 supersymmetric orientifolds
for which the world-sheet parity transformation is combined with a complex
conjugation in the compact directions. We investigate in detail the
orientifolds of the Z_3, Z_4, Z_6 and Z_6' toroidal orbifolds finding solutions
to the tadpole cancellation conditions for all models. Generically, all the
massless spectra turn out to be non-chiral.Comment: 28 pages, LaTeX, 6 figures; v2: slight modification of closed string
spectr
Three (Marginal?) Questions Regarding Convergence
This paper focuses on three (marginal?) questions surrounding the analysis of economic convergence and uses Spanish provinces as a means of illustration. The three questions in hand are the following: (i) given that the geographical units of analysis are usually quite different in economic size, is the weighting of economic units relevant in convergence analysis? (ii) the average per capita income of a given region, or country, is the first moment in the distribution of income, but what about the second moment, inequality? Have we converged in inequality? and (iii) an aggregate welfare index must take into account, at least, the evolution of the first two moments of the distribution of income, and so does the adjustment for inequality make important differences in the evolution of average per capita income? The answer to the first two questions is yes, but to the third it is clearly no.Convergence, income distribution, inequality indices and regional analysis
Quantum Gravity via Causal Dynamical Triangulations
"Causal Dynamical Triangulations" (CDT) represent a lattice regularization of
the sum over spacetime histories, providing us with a non-perturbative
formulation of quantum gravity. The ultraviolet fixed points of the lattice
theory can be used to define a continuum quantum field theory, potentially
making contact with quantum gravity defined via asymptotic safety. We describe
the formalism of CDT, its phase diagram, and the quantum geometries emerging
from it. We also argue that the formalism should be able to describe a more
general class of quantum-gravitational models of Horava-Lifshitz type.Comment: To appear in "Handbook of Spacetime", Springer Verlag. 31 page
Noncommutative Compactifications of Type I Strings on Tori with Magnetic Background Flux
We construct six- and four-dimensional toroidal compactifications of the Type
I string with magnetic flux on the D-branes. The open strings in this
background probe a noncommutative internal geometry. Phenomenologically
appealing features such as chiral fermions and supersymmetry breaking in the
gauge sector are naturally realized by these vacua. We investigate the spectra
of such noncommutative string compactifications and in a bottom-up approach
discuss the possibility to obtain the standard or some GUT like model.Comment: TeX harvmac, 25 pages, 5 figure
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