Z_2 x Z_2 Heterotic Orbifold Models of Non Factorisable Six Dimensional Toroidal Manifolds

We discuss heterotic strings on Z_2 x Z_2 orbifolds of non factorisable six-tori. Although the number of fixed tori is reduced as compared to the factorisable case, Wilson lines are still needed for the construction of three generation models. An essential new feature is the straightforward appearance of three generation models with one generation per twisted sector. We illustrate our general arguments for the occurrence of that property by an explicit example. Our findings give further support for the conjecture that four dimensional heterotic strings formulated at the free fermionic point are related to Z_2 x Z_2 orbifolds.Comment: 33 pages, LaTeX; discussion of modular invariance added in section four; added references; to be published in JHE

Yukawa Hierarchy Transfer Based on Superconformal Dynamics and Geometrical Realization in String Models

We propose a scenario that leads to hierarchical Yukawa couplings and degenerate sfermion masses at the same time, in the context of extra-dimensional models, which can be naturally embedded in a wide class of string models. The hierarchy of Yukawa couplings and degeneracy of sfermion masses can be realized thanks to superconformal gauge dynamics. The sfermion mass degeneracy is guaranteed by taking the superconformal fixed point to be family independent. In our scenario, the origin of Yukawa hierarchy is attributed to geometry of compactified dimensions and the consequent volume dependence of gauge couplings in the superconformal sectors. The difference in these gauge couplings is dynamically transferred to the hierarchy of the Yukawa couplings. Thus, our scenario combines a new dynamical approach and the conventional geometrical approach to the supersymmetric flavor problem.Comment: 12 pages, latex, no figur

Effect of staking and non-staking systems on disease severity, yield and quality attributes of yams (Dioscorea alata)

Aims: The aim of this study was to evaluate the effect of staking and non-staking systems on disease severity, yield and quality attributes of yams. Methodology: High costs and lack of planting materials, labour, staking and inappropriate knowledge on production techniques are major constraints of yam production in Sierra Leone. A total of seven promising hybrid genotypes of yams from International Institute of Tropical Agriculture and one local cultivar, Pulli, were evaluated for yield, reaction to local pest and disease and desirable market traits during 2011 and 2012 in three agro-ecological zones of Sierra Leone. The experiment was laid out in randomized complete block replicated thrice at the experimental sites of the Njala Agricultural Research Centre. Results: Results revealed higher disease pressure in non-staked plots compared to staked plots. Fresh tuber yields were significantly higher in staked plots than the non-staked plots. Five genotypes with yields ranging between 11.8 and 14.7 t.ha-1 significantly out-yielded Pulli (9.1 t.ha-1) in the staked plots, while only genotype TDa 02/00012 (11.9 t.ha-1) significantly out-yielded Pulli (7.1 t.ha-1) in the non-staked plots. Farmers’ preferences for all genotypes were similar to that of the local cultivar. Staking contributed 30.5% mean yield increase compared to non-staking. Makeni had the highest percent mean yield increase due to staking (38.5%) compared to Njala (29.7%) and Kenema (26.4%). Conclusion: Staking is beneficial in yam production contributing an average of 28.2% more fresh tuber yields than non-staking. Genotypes in staking system were more tolerant to in-field local diseases, thereby significantly out-yielding those in non-staking system. Genotypes TDa 98/01174, TDa 98/01176, TDa 02/00012, TDa 98/01168 and TDa 00/00194 had stable resistance to in-field diseases in the staking system and desired food quality traits compared to the check variety, Pulli. Findings have good implications for multiple disease resistance breeding for various production systems as the different genes controlling these traits could be pyramided into an ideotype. Similar technique could be used to breed for yield and other desired food quality traits

A Space-Time Orbifold: A Toy Model for a Cosmological Singularity

We explore bosonic strings and Type II superstrings in the simplest time dependent backgrounds, namely orbifolds of Minkowski space by time reversal and some spatial reflections. We show that there are no negative norm physical excitations. However, the contributions of negative norm virtual states to quantum loops do not cancel, showing that a ghost-free gauge cannot be chosen. The spectrum includes a twisted sector, with strings confined to a ``conical'' singularity which is localized in time. Since these localized strings are not visible to asymptotic observers, interesting issues arise regarding unitarity of the S-matrix for scattering of propagating states. The partition function of our model is modular invariant, and for the superstring, the zero momentum dilaton tadpole vanishes. Many of the issues we study will be generic to time-dependent cosmological backgrounds with singularities localized in time, and we derive some general lessons about quantizing strings on such spaces.Comment: 21 pages, 2 figure

Grand Unification with Three Generations in Free Fermionic String Models

We examine the problem of constructing three generation free fermionic string models with grand unified gauge groups. We attempt the construction of $G\times G$ models, where $G$ is a grand unified group realized at level 1. This structure allows those Higgs representations to appear which are necessary to break the symmetry down to the standard model gauge group. For $G=SO(10)$, we find only models with an even number of generations. However, for $G=SU(5)$ we find a number of 3 generation models.Comment: 22 pages, latex. References added to original versio

Prime ideals in nilpotent Iwasawa algebras

Let G be a nilpotent complete p-valued group of finite rank and let k be a field of characteristic p. We prove that every faithful prime ideal of the Iwasawa algebra kG is controlled by the centre of G, and use this to show that the prime spectrum of kG is a disjoint union of commutative strata. We also show that every prime ideal of kG is completely prime. The key ingredient in the proof is the construction of a non-commutative valuation on certain filtered simple Artinian rings

Associations of Irritability With Functional Connectivity of Amygdala and Nucleus Accumbens in Adolescents and Young Adults With ADHD

OBJECTIVE: Irritability is a common characteristic in ADHD. We examined whether dysfunction in neural connections supporting threat and reward processing was related to irritability in adolescents and young adults with ADHD. METHOD: We used resting-state fMRI to assess connectivity of amygdala and nucleus accumbens seeds in those with ADHD (n = 34) and an age- and gender-matched typically-developing comparison group (n = 34). RESULTS: In those with ADHD, irritability was associated with atypical functional connectivity of both seed regions. Amygdala seeds showed greater connectivity with right inferior frontal gyrus and caudate/putamen, and less connectivity with precuneus. Nucleus accumbens seeds showed altered connectivity with middle temporal gyrus and precuneus. CONCLUSION: The irritability-ADHD presentation is associated with atypical functional connectivity of reward and threat processing regions with cognitive control and emotion processing regions. These patterns provide novel evidence for irritability-associated neural underpinnings in adolescents and young adults with ADHD. The findings suggest cognitive and behavioral treatments that address response to reward, including omission of an expected reward and irritability, may be beneficial for ADHD

Searching singlet extensions of the supersymmetric standard model in Z6−II Z_{6-II} orbifold compactification of heterotic string

We search for supersymmetric standard model realizations with extra singlets and extra $U(1)$ using the heterotic string compactification on the $Z_{6-II}$ orbifold with two Wilson lines. We analyze the vacuum restabilization mechanism for three representative Pati-Salam string models obtained in the literature and present detailed results for the effective superpotential compatible with the string selection rules. An automated selection of semi-realistic vacua along flat directions in the non-Abelian singlet modes field space is performed by requiring the presence of massless pairs of electroweak Higgs bosons having trilinear superpotential couplings with massless singlet modes and the decoupling of color triplet exotic modes needed to suppress $B$ and $L$ number violating processes.Comment: revtex4 format, 21 pages, 7 tables, shortened version added reference

Theory-Motivated Benchmark Models and Superpartners at the Tevatron

Recently published benchmark models have contained rather heavy superpartners. To test the robustness of this result, several benchmark models have been constructed based on theoretically well-motivated approaches, particularly string-based ones. These include variations on anomaly and gauge-mediated models, as well as gravity mediation. The resulting spectra often have light gauginos that are produced in significant quantities at the Tevatron collider, or will be at a 500 GeV linear collider. The signatures also provide interesting challenges for the LHC. In addition, these models usually account for electroweak symmetry breaking with relatively less fine-tuning than previous benchmark models.Comment: 44 pages, 4 figures; some typos corrected. Revisions reflect published versio