Cosmic Rays From Cosmic Strings

It has been speculated that cosmic string networks could produce ultra-high energy cosmic rays as a by-product of their evolution. By making use of recent work on the evolution of such networks, it will be shown that the flux of cosmic rays from cosmologically useful, that is GUT scale strings, is too small to be used as a test for strings with any foreseeable technology.Comment: 11, Imperial/TP/93-94/2

Dripping Faucet Dynamics Clarified by an Improved Mass-Spring Model

An improved mass-spring model for a dripping faucet is presented. The model is constructed based on the numerical results which we recently obtained from fluid dynamical calculations. Both the fluid dynamical calculations and the present mass-spring model exhibit a variety of complex behavior including transition to chaos in good agreement with experiments. Further, the mass-spring model reveals fundamental dynamics inherent in the dripping faucet system.Comment: 17 pages, 17 figure

Electronic structure investigation of CoO by means of soft X-ray scattering

The electronic structure of CoO is studied by resonant inelastic soft X-ray scattering spectroscopy using photon energies across the Co 2p absorption edges. The different spectral contributions from the energy-loss structures are identified as Raman scattering due to d-d and charge-transfer excitations. For excitation energies close to the L3 resonance, the spectral features are dominated by quartet-quartet and quartet-doublet transitions of the 3d7 configuration. At excitation energies corresponding to the satellites in the Co 2p X-ray absorption spectrum of CoO, the emission features are instead dominated by charge-transfer transitions to the 3d8L-1 final state. The spectra are interpreted and discussed with the support of simulations within the single impurity Anderson model with full multiplet effects which are found to yield consistent spectral functions to the experimental data.Comment: 8 pages, 2 figures, 2 tables, http://link.aps.org/doi/10.1103/PhysRevB.65.20510

Dilatonic current-carrying cosmic strings

We investigate the nature of ordinary cosmic vortices in some scalar-tensor extensions of gravity. We find solutions for which the dilaton field condenses inside the vortex core. These solutions can be interpreted as raising the degeneracy between the eigenvalues of the effective stress-energy tensor, namely the energy per unit length U and the tension T, by picking a privileged spacelike or timelike coordinate direction; in the latter case, a phase frequency threshold occurs that is similar to what is found in ordinary neutral current-carrying cosmic strings. We find that the dilaton contribution for the equation of state, once averaged along the string worldsheet, vanishes, leading to an effective Nambu-Goto behavior of such a string network in cosmology, i.e. on very large scales. It is found also that on small scales, the energy per unit length and tension depend on the string internal coordinates in such a way as to permit the existence of centrifugally supported equilibrium configuration, also known as vortons, whose stability, depending on the very short distance (unknown) physics, can lead to catastrophic consequences on the evolution of the Universe.Comment: 10 pages, ReVTeX, 2 figures, minor typos corrected. This version to appear in Phys. Rev.

HELIUM PHOTODISINTEGRATION AND NUCLEOSYNTHESIS: IMPLICATIONS FOR TOPOLOGICAL DEFECTS, HIGH ENERGY COSMIC RAYS, AND MASSIVE BLACK HOLES

We consider the production of $^3$He and $^2$H by $^4$He photodisintegration initiated by non-thermal energy releases during early cosmic epochs. We find that this process cannot be the predominant source of primordial $^2$H since it would result in anomalously high $^3$He/D ratios in conflict with standard chemical evolution assumptions. We apply this fact to constrain topological defect models of highest energy cosmic ray (HECR) production. Such models have been proposed as possible sources of ultrahigh energy particles and gamma-rays with energies above $10^{20}$eV. The constraints on these models derived from $^4$He-photodisintegration are compared to corresponding limits from spectral distortions of the cosmic microwave background radiation (CMBR) and from the observed diffuse gamma-ray background. It is shown that for reasonable primary particle injection spectra superconducting cosmic strings, unlike ordinary strings or annihilating monopoles, cannot produce the HECR flux at the present epoch without violating at least the $^4$He-photodisintegration bound. The constraint from the diffuse gamma-ray background rules out the dominant production of HECR by the decay of Grand Unification particles in models with cosmological evolution assuming standard fragmentation functions. Constraints on massive black hole induced photodisintegration are also discussed.Comment: 20 latex pages, 1 figure added via figures comman

Enhanced etoposide sensitivity following adenovirus-mediated human topoisomerase II α gene transfer is independent of topoisomerase II β

The roles that the α and β isoforms of topoisomerase II (topo II) play in anticancer drug action were determined using MDA-VP etoposide-resistant human breast cancer cells and a newly constructed adenoviral vector containing the topo IIα gene (Ad-topo IIα). MDA-VP cells were more resistant to etoposide than to amsacrine and had more resistance to etoposide than did MDA-parental cells. MDA-VP cells also expressed lower topo IIα RNA and protein levels than parental cells but had comparable topo IIβ levels. After infection with Ad-topo IIα, topo IIα, RNA and protein levels increased significantly, as did the cells' sensitivity to etoposide. In contrast, topo IIβ levels remained constant with little alteration in the cells' sensitivity to amsacrine. Band-depletion immunoblotting assays indicated that topo IIα was depleted in etoposide-treated, Ad-topo IIα-transduced MDA-VP cells but not in amsacrine-treated cells. Topo IIβ was depleted in amsacrine-treated, Ad-topo IIα-MDA-VP cells, with little change in the topo IIα levels. These results suggest that topo IIα gene transfer does not alter topo IIβ expression and that enhanced sensitivity to etoposide is therefore secondary to change in topo IIα levels. These studies support the theory that etoposide preferentially targets topo IIα, while amsacrine targets topo IIβ. © 2001 Cancer Research Campaign http://www.bjcancer.co

Regulatory control and the costs and benefits of biochemical noise

Experiments in recent years have vividly demonstrated that gene expression can be highly stochastic. How protein concentration fluctuations affect the growth rate of a population of cells, is, however, a wide open question. We present a mathematical model that makes it possible to quantify the effect of protein concentration fluctuations on the growth rate of a population of genetically identical cells. The model predicts that the population's growth rate depends on how the growth rate of a single cell varies with protein concentration, the variance of the protein concentration fluctuations, and the correlation time of these fluctuations. The model also predicts that when the average concentration of a protein is close to the value that maximizes the growth rate, fluctuations in its concentration always reduce the growth rate. However, when the average protein concentration deviates sufficiently from the optimal level, fluctuations can enhance the growth rate of the population, even when the growth rate of a cell depends linearly on the protein concentration. The model also shows that the ensemble or population average of a quantity, such as the average protein expression level or its variance, is in general not equal to its time average as obtained from tracing a single cell and its descendants. We apply our model to perform a cost-benefit analysis of gene regulatory control. Our analysis predicts that the optimal expression level of a gene regulatory protein is determined by the trade-off between the cost of synthesizing the regulatory protein and the benefit of minimizing the fluctuations in the expression of its target gene. We discuss possible experiments that could test our predictions.Comment: Revised manuscript;35 pages, 4 figures, REVTeX4; to appear in PLoS Computational Biolog

Long-lived photoexcited states in polydiacetylenes with different molecular and supramolecular organization

With the aim of determining the importance of the molecular and supramolecular organization on the excited states of polydiacetylenes, we have studied the photoinduced absorption spectra of the red form of poly[1,6-bis(3,6-didodecyl-N-carbazolyl)-2,4-hexadiyne] (polyDCHD-S) and the results compared with those of the blue form of the same polymer. An interpretation of the data is given in terms of both the conjugation length and the interbackbone separation also in relation to the photoinduced absorption spectra of both blue and red forms of poly[1,6-bis(N-carbazolyl)-2,4-hexadiyne] (polyDCHD), which does not carry the alkyl substituents on the carbazolyl side groups. Information on the above properties is derived from the analysis of the absorption and Raman spectra of this class of polydiacetylenes

Application of kernel functions for accurate similarity search in large chemical databases

Background Similaritysearch in chemical structure databases is an important problem with many applications in chemical genomics, drug design, and efficient chemical probe screening among others. It is widely believed that structure based methods provide an efficient way to do the query. Recently various graph kernel functions have been designed to capture the intrinsic similarity of graphs. Though successful in constructing accurate predictive and classification models, graph kernel functions can not be applied to large chemical compound database due to the high computational complexity and the difficulties in indexing similarity search for large databases. Results To bridge graph kernel function and similarity search in chemical databases, we applied a novel kernel-based similarity measurement, developed in our team, to measure similarity of graph represented chemicals. In our method, we utilize a hash table to support new graph kernel function definition, efficient storage and fast search. We have applied our method, named G-hash, to large chemical databases. Our results show that the G-hash method achieves state-of-the-art performance for k-nearest neighbor (k-NN) classification. Moreover, the similarity measurement and the index structure is scalable to large chemical databases with smaller indexing size, and faster query processing time as compared to state-of-the-art indexing methods such as Daylight fingerprints, C-tree and GraphGrep. Conclusions Efficient similarity query processing method for large chemical databases is challenging since we need to balance running time efficiency and similarity search accuracy. Our previous similarity search method, G-hash, provides a new way to perform similarity search in chemical databases. Experimental study validates the utility of G-hash in chemical databases