Supersymmetry and Rb_{b} in the light of LEP 1.5

We re-examine the possible magnitude of the supersymmetric contribution to R_b\equiv\Gamma(Z^0\to\bar b b)/\Gamma(Z^0\to{\rm all}) in the light of the constraints imposed by the absence of light charginos at LEP~1.5, implementing also other available phenomenological constraints. We find the supersymmetric contribution to be R^{\rm susy}_b < 0.0017, and discuss the extent to which this upper bound could be strengthened by future constraints on the chargino and top-squark masses. Such values of R^{\rm susy}_b tend to disfavor a supersymmetry explanation of the apparent R_b discrepancy

Report by the ESA-ESO Working Group on Fundamental Cosmology

ESO and ESA agreed to establish a number of Working Groups to explore possible synergies between these two major European astronomical institutions. This Working Group's mandate was to concentrate on fundamental questions in cosmology, and the scope for tackling these in Europe over the next ~15 years. One major resulting recommendation concerns the provision of new generations of imaging survey, where the image quality and near-IR sensitivity that can be attained only in space are naturally matched by ground-based imaging and spectroscopy to yield massive datasets with well-understood photometric redshifts (photo-z's). Such information is essential for a range of new cosmological tests using gravitational lensing, large-scale structure, clusters of galaxies, and supernovae. Great scope in future cosmology also exists for ELT studies of the intergalactic medium and space-based studies of the CMB and gravitational waves; here the synergy is less direct, but these areas will remain of the highest mutual interest to the agencies. All these recommended facilities will produce vast datasets of general applicability, which will have a tremendous impact on broad areas of astronomy.Comment: ESA-ESO Working Groups Report No. 3, 125 pages, 28 figures. A PDF version including the cover is available from http://www.stecf.org/coordination/esa_eso/cosmology/report_cover.pdf and a printed version (A5 booklet) is available in limited numbers from the Space Telescope-European Coordinating Facility (ST-ECF): [email protected]

Graph Separation and Search Number

We relate two concepts in graph theory and algorithmic complexity, namely the search number and the vertex separation of a graph. Lengauer has previously related vertex separation to progressive black/white pebble demand. Let a (G) denote the search number and vs(G) denote the vertex separation of a connected, undirected graph G. We show that vs(G) \u3c s(G) \u3c vs(G) +2 and we give a simple transformation from G to G^1 such that vs(G^1) = s(G). We give algorithm that, for any tree T, compute vs(T) in linear time and compute an optimal layout with respect to vertex separation in time O(n log n). We characterize those trees having a given vertex separation and describe the smallest such trees. We give an algorithm which, for all fixed k\u3e1, decides the problem: Is vs(G

CP Violation in the Minimal Supersymmetric Seesaw Model

We study CP violation in the lepton sector of the supersymmetric extension of the Standard Model with three generations of massive singlet neutrinos with Yukawa couplings $Y_\nu$ to lepton doublets, in a minimal seesaw model for light neutrino masses and mixing. This model contains six physical CP-violating parameters, namely the phase $\delta$ observable in oscillations between light neutrino species, two Majorana phases $\phi_{1,2}$ that affect $\beta \beta_{0 \nu}$ decays, and three independent phases appearing in ${Y_\nu}{Y_\nu}^\dagger$, that control the rate of leptogenesis. Renormalization of the soft supersymmetry-breaking parameters induces observable CP violation at low energies, including T-odd asymmetries in polarized $\mu\to eee$ and $\tau \to \ell \ell \ell$ decays, as well as lepton electric dipole moments. In the leading-logarithmic approximation in which the massive singlet neutrinos are treated as degenerate, these low-energy observables are sensitive via ${Y_\nu}^\dagger{Y_\nu}$ to just one combination of the leptogenesis and light-neutrino phases. We present numerical results for the T-odd asymmetry in polarized $\mu\to eee$ decay, which may be accessible to experiment, but the lepton electric dipole moments are very small in this approximation. To the extent that the massive singlet neutrinos are not degenerate, low-energy observables become sensitive also to two other combinations of leptogenesis and light-neutrino phases, in this minimal supersymmetric seesaw model

Hydantoin-bridged medium ring scaffolds by migratory insertion of urea-tethered nitrile anions into aromatic C-N bonds

Bicyclic or tricyclic nitrogen-containing heterocyclic scaffolds were constructed rapidly by intramolecular nucleophilic aromatic substitution of metallated nitriles tethered by a urea linkage to a series of electronically unactivated heterocyclic precursors. The substitution reaction constitutes a ring expansion, enabled by the conformationally constrained tether between the nitrile and the heterocycle. Attack of the metallated urea leaving group on the nitrile generates a hydantoin that bridges the polycyclic products. X-ray crystallography reveals ring-dependant strain within the hydantoin

The Stability of an Isotropic Cosmological Singularity in Higher-Order Gravity

We study the stability of the isotropic vacuum Friedmann universe in gravity theories with higher-order curvature terms of the form $(R_{ab}R^{ab})^{n}$ added to the Einstein-Hilbert Lagrangian of general relativity on approach to an initial cosmological singularity. Earlier, we had shown that, when $$, a special isotropic vacuum solution exists which behaves like the radiation-dominated Friedmann universe and is stable to anisotropic and small inhomogeneous perturbations of scalar, vector and tensor type. This is completely different to the situation that holds in general relativity, where an isotropic initial cosmological singularity is unstable in vacuum and under a wide range of non-vacuum conditions. We show that when $n\neq 1$, although a special isotropic vacuum solution found by Clifton and Barrow always exists, it is no longer stable when the initial singularity is approached. We find the particular stability conditions under the influence of tensor, vector, and scalar perturbations for general $n$ for both solution branches. On approach to the initial singularity, the isotropic vacuum solution with scale factor $a(t)=t^{P_{-}/3}$ is found to be stable to tensor perturbations for $0.5<n< 1.1309$ and stable to vector perturbations for $0.861425 < n \leq 1$, but is unstable as $t \to 0$ otherwise. The solution with scale factor $a(t)=t^{P_{+}/3}$ is not relevant to the case of an initial singularity for $n>1$ and is unstable as $t \to 0$ for all $n$ for each type of perturbation.Comment: 25 page

On the thermal regeneration rate for light gravitinos in the early universe

We investigate the light gravitino regeneration rate in the early Universe in models based on N=1 supergravity. Motivated by a recent claim by Fischler, we evaluate finite-temperature effects on the gravitino regeneration rate due to the hot primordial plasma for a wide range of the supersymmetry-breaking scale F. We find that thermal corrections to the gravitino pole mass and to the Goldstino coupling are negligible for a wide range of temperatures, thereby justifying the extension of the equivalence theorem for the helicity-1/2 gravitino and Goldstino to a hot primordial plasma background. Utilizing the Braaten-Pisarski resummation method, the helicity-1/2 gravitino regeneration rate is found to be 0.25 \alpha_s(T) \log(1/\alpha_s(T))|{m_{\rm soft}/F}|^2 T^3(1 + \alpha_s(T) \log(1/\alpha_s(T)) + T^2 / |F|) up to a calculable, model-dependent {\cal O}(1) numerical factor. We review the implications of this regeneration rate for supergravity cosmology, focusing in particular on scenaria for baryogenesis

Kaluza-Klein Dark Matter

We propose that cold dark matter is made of Kaluza-Klein particles and explore avenues for its detection. The lightest Kaluza-Klein state is an excellent dark matter candidate if standard model particles propagate in extra dimensions and Kaluza-Klein parity is conserved. We consider Kaluza-Klein gauge bosons. In sharp contrast to the case of supersymmetric dark matter, these annihilate to hard positrons, neutrinos and photons with unsuppressed rates. Direct detection signals are also promising. These conclusions are generic to bosonic dark matter candidates.Comment: 4 pages, 3 figures, discussion of spin-independent cross section clarified, references added, published versio

Bino Dark Matter and Big Bang Nucleosynthesis in the Constrained E6SSM with Massless Inert Singlinos

We discuss a new variant of the E6 inspired supersymmetric standard model (E6SSM) in which the two inert singlinos are exactly massless and the dark matter candidate has a dominant bino component. A successful relic density is achieved via a novel mechanism in which the bino scatters inelastically into heavier inert Higgsinos during the time of thermal freeze-out. The two massless inert singlinos contribute to the effective number of neutrino species at the time of Big Bang Nucleosynthesis, where the precise contribution depends on the mass of the Z' which keeps them in equilibrium. For example for mZ' > 1300 GeV we find Neff \approx 3.2, where the smallness of the additional contribution is due to entropy dilution. We study a few benchmark points in the constrained E6SSM with massless inert singlinos to illustrate this new scenario.Comment: 24 pages, revised for publication in JHE

Directionality in protein fold prediction

<p>Abstract</p> <p>Background</p> <p>Ever since the ground-breaking work of Anfinsen et al. in which a denatured protein was found to refold to its native state, it has been frequently stated by the protein fold prediction community that all the information required for protein folding lies in the amino acid sequence. Recent in vitro experiments and in silico computational studies, however, have shown that cotranslation may affect the folding pathway of some proteins, especially those of ancient folds. In this paper aspects of cotranslational folding have been incorporated into a protein structure prediction algorithm by adapting the Rosetta program to fold proteins as the nascent chain elongates. This makes it possible to conduct a pairwise comparison of folding accuracy, by comparing folds created sequentially from each end of the protein.</p> <p>Results</p> <p>A single main result emerged: in 94% of proteins analyzed, following the sense of translation, from N-terminus to C-terminus, produced better predictions than following the reverse sense of translation, from the C-terminus to N-terminus. Two secondary results emerged. First, this superiority of N-terminus to C-terminus folding was more marked for proteins showing stronger evidence of cotranslation and second, an algorithm following the sense of translation produced predictions comparable to, and occasionally better than, Rosetta.</p> <p>Conclusions</p> <p>There is a directionality effect in protein fold prediction. At present, prediction methods appear to be too noisy to take advantage of this effect; as techniques refine, it may be possible to draw benefit from a sequential approach to protein fold prediction.</p