10,337 research outputs found
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
Wild horses in a ‘European wilderness’: imagining sustainable development in the post-Communist countryside
When the Soviet Union and its satellite regimes collapsed, they bequeathed to successor states an
unexpected dual legacy: an outsized, backward agrarian sector on the one hand and a wealth of
undeveloped nature, rich in biological diversity, on the other. Popular perceptions of the region
centre on nightmarish images of environmental devastation, but environmentalists on both sides of
the former ‘Iron Curtain’ are increasingly recognizing the unintended benefits to nature of Communist underdevelopment. Eight of the post-Communist states have now consummated their long-awaited ‘return to Europe’, but as they begin a new era as European Union members, they confront a critical developmental challenge. Faced with declining agricultural prospects and growing Western interest in Eastern nature, what to do with a large and underemployed rural population and an everexpanding area of marginal farmland? This article contributes to a growing literature on the political ecology of post-Communist transformation in the ‘Second World’ through a case study from Latvia. At Lake Pape, the Latvian program office of WWF International has implemented a Western-funded project in ecosystem restoration and eco-tourism promotion involving introduction of wild horses in a remote but ecologically rich coastal wetland area. I explore diverse Latvian responses to the polysemic Western narratives of sustainable rural development and biodiversity conservation that have been borne into Eastern Europe along with Western aid monies. Local mediation of these narratives is shaped by the struggle between competing ‘agrarian’ and ‘internationalist’ understandings of national geography, identity and developmental destiny that have structured cultural and political discourse since the emergence of Latvian nationalism 150 years ago
Numerical study of the transition of the four dimensional Random Field Ising Model
We study numerically the region above the critical temperature of the four
dimensional Random Field Ising Model. Using a cluster dynamic we measure the
connected and disconnected magnetic susceptibility and the connected and
disconnected overlap susceptibility. We use a bimodal distribution of the field
with for all temperatures and a lattice size L=16. Through a
least-square fit we determine the critical exponents and . We find the magnetic susceptibility and the overlap
susceptibility diverge at two different temperatures. This is coherent with the
existence of a glassy phase above . Accordingly with other simulations
we find . In this case we have a scaling theory with
two indipendet critical exponentsComment: 10 pages, 2 figures, Late
Electron-hydrogen scattering in Faddeev-Merkuriev integral equation approach
Electron-hydrogen scattering is studied in the Faddeev-Merkuriev integral
equation approach. The equations are solved by using the Coulomb-Sturmian
separable expansion technique. We present - and -wave scattering and
reactions cross sections up to the threshold.Comment: 2 eps figure
Lysophosphatidic acid signaling promotes proliferation, differentiation, and cell survival in rat growth plate chondrocytes
AbstractGrowth plate cartilage is responsible for long bone growth in children and adolescents and is regulated by vitamin D metabolites in a cell zone-specific manner. Resting zone chondrocytes (RC cells) are regulated by 24,25-dihydroxyvitamin D3 via a phospholipase D-dependent pathway, suggesting downstream phospholipid metabolites are involved. In this study, we showed that 24R,25(OH)2D3 stimulates rat costochondral RC chondrocytes to release lysophosphatidic acid (LPA) and, therefore sought to determine the role of LPA signaling in these cells. RC cells expressed the G-protein coupled receptors LPA1–5 and peroxisome proliferator-activated receptor gamma (PPAR-γ). LPA and the LPA1/3 selective agonist OMPT increased proliferation and two maturation markers, alkaline phosphatase activity and [35S]-sulfate incorporation. LPA and 24R,25(OH)2D3's effects were inhibited by the LPA1/3 selective antagonist VPC32183(S). Furthermore, apoptosis induced by either inorganic phosphate or chelerythrine was attenuated by LPA, based on DNA fragmentation, TUNEL staining, caspase-3 activity, and Bcl-2:Bax protein ratio. LPA prevented apoptotic signaling by decreasing the abundance, nuclear localization, and transcriptional activity of the tumor-suppressor p53. LPA treatment also regulated the expression of the p53-target genes Bcl-2 and Bax to enhance cell survival. Collectively, these data suggest that LPA promotes differentiation and survival in RC chondrocytes, demonstrating a novel physiological function of LPA-signaling
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