3,589 research outputs found
Anomaly Cancelations in Orientifolds with Quantized B Flux
We consider anomaly cancelations in Type IIB orientifolds on T^4/Z_N with
quantized NS-NS sector background B-flux. For a rank b B-flux on T^4 (b is
always even) and when N is even, the cancelation requires a 2^{b/2}
multiplicity of states in the 59-open string sector. We identify the twisted
sector R-R scalars and tensor multiplets which are involved in the
Green-Schwarz mechanism. We give more details of the construction of these
models and argue that consistency with the 2^{b/2} multiplicity of 59-sector
states requires a modification of the relation between the open string 1-loop
channel modulus and the closed string tree channel modulus in the 59-cylinder
amplitudes.Comment: Revtex 3.0, 34 pages, 2 figures, references adde
Phenomenology of 3-Family Grand Unified String Models
In the 3-family grand unified string models constructed so far, there is only
one adjoint (and no higher dimensional representation) Higgs field in the grand
unified gauge group. In this preliminary analysis, we address the proton-decay
problem in the 3-family E_6 and related SO(10) string models. In particular, we
analyze the doublet-triplet splitting (within certain assumptions about
non-perturbative dynamics). It appears that generically some fine-tuning is
necessary to arrange for a pair of Higgs doublets to be light, while having
color Higgs triplets superheavy. We also discuss charge-2/3 quark mass matrix
that generically also seems to require some fine-tuning to have rank 1.Comment: 12 pages, Revtex 3.0. Minor corrections mad
Studies of new media radiation induced laser
Various lasants were investigated especially, 2-iodohepafluoropropane (i-C3F7I) for the direct solar pumped lasers. Optical pumping of iodine laser was achieved using a small flashlamp. Using i-C3F7I as a laser gain medium, threshold inversion density, small signal gain, and laser performance at the elevated temperature were measured. The experimental results and analysis are presented. The iodine laser kinetics of the C3F7I and IBr system were numerically simulated. The concept of a direct solar-pumped laser amplifier using (i-C3F7I) as the laser material was evaluated and several kinetic coefficients for i-C3F7I laser system were reexamined. The results are discussed
Nitrate and ammonium lead to distinct global dynamic phosphorylation patterns when resupplied to nitrogen-starved Arabidopsis seedlings
Nitrogen is an essential macronutrient for plant growth and development. Inorganic nitrogen and its assimilation products control various metabolic, physiological and developmental processes. Although the transcriptional responses induced by nitrogen have been extensively studied in the past, our work here focused on the discovery of candidate proteins for regulatory events that are complementary to transcriptional changes. Most signaling pathways involve modulation of protein abundance and/or activity by protein phosphorylation. Therefore, we analyzed the dynamic changes in protein phosphorylation in membrane and soluble proteins from plants exposed to rapid changes in nutrient availability over a time course of 30 min. Plants were starved of nitrogen and subsequently resupplied with nitrogen in the form of nitrate or ammonium. Proteins with maximum change in their phosphorylation level at up to 5 min after nitrogen resupply (fast responses) included GPI-anchored proteins, receptor kinases and transcription factors, while proteins with maximum change in their phosphorylation level after 10 min of nitrogen resupply (late responses) included proteins involved in protein synthesis and degradation, as well as proteins with functions in central metabolism and hormone metabolism. Resupply of nitrogen in the form of nitrate or ammonium resulted in distinct phosphorylation patterns, mainly of proteins with signaling functions, transcription factors and transporters
Asymmetric Non-Abelian Orbifolds and Model Building
The rules for the free fermionic string model construction are extended to
include general non-abelian orbifold constructions that go beyond the real
fermionic approach. This generalization is also applied to the asymmetric
orbifold rules recently introduced. These non-abelian orbifold rules are quite
easy to use. Examples are given to illustrate their applications.Comment: 30 pages, Revtex 3.
Rapid Tunneling and Percolation in the Landscape
Motivated by the possibility of a string landscape, we reexamine tunneling of
a scalar field across single/multiple barriers. Recent investigations have
suggested modifications to the usual picture of false vacuum decay that lead to
efficient and rapid tunneling in the landscape when certain conditions are met.
This can be due to stringy effects (e.g. tunneling via the DBI action), or by
effects arising due to the presence of multiple vacua (e.g. resonance
tunneling). In this paper we discuss both DBI tunneling and resonance
tunneling. We provide a QFT treatment of resonance tunneling using the
Schr\"odinger functional approach. We also show how DBI tunneling for
supercritical barriers can naturally lead to conditions suitable for resonance
tunneling. We argue using basic ideas from percolation theory that tunneling
can be rapid in a landscape where a typical vacuum has multiple decay channels
and discuss various cosmological implications. This rapidity vacuum decay can
happen even if there are no resonance/DBI tunneling enhancements, solely due to
the presence of a large number of decay channels. Finally, we consider various
ways of circumventing a recent no-go theorem for resonance tunneling in quantum
field theory.Comment: 47 pages, 16 figures. Acknowledgements adde
A Review of Three-Family Grand Unified String Models
We review the construction and classification of three-family grand unified
models within the framework of asymmetric orbifolds in perturbative heterotic
superstring. We give a detailed survey of all such models which is organized to
aid analysis of their phenomenological properties. We compute tree-level
superpotentials for these models. These superpotentials are used to analyze the
issues of proton stability (doublet-triplet splitting and R-parity violating
terms) and Yukawa mass matrices. To have agreement with phenomenological data
all these models seem to require certain degree of fine-tuning. We also analyze
the possible patterns of supersymmetry breaking in these models.
We find that the supersymmetry breaking scale comes out either too high to
explain the electroweak hierarchy problem, or below the electroweak scale
unless some degree of fine-tuning is involved. Thus, none of the models at hand
seem to be phenomenologically flawless.Comment: 49 pages, Revtex 3.0; one ps figure included. To appear in the Review
section of Int.J.Mod.Phy
- …