Traces of the theta+ pentaquark in K+ -nucleus dynamics

Long-standing anomalies in K(+)-nucleus integral cross sections could be resolved by extending the impulse-approximation trho optical-potential framework to incorporate K(+) absorption on pairs of nucleons. Substantially improved fits to the data at p(lab) approximately 500-700 MeV/c are obtained. An upper bound on the absorption cross section per nucleon is derived, sigmaabs((K(+))/A approximately 3.5 mb. We conjecture that the underlying microscopic absorption process is K(+) nN--> Theta+ N, where Theta(+) (1540) is the newly discovered exotic Y = 2, I = 0, Z = 1 pentaquark baryon, and estimate that sigma(K(+) d--> Theta(+) p) is a fraction of millibarn. Comments are made on Theta(+) production reactions on nuclei.     

Stabilization of Yang-Mills chaos in non-Abelian Born-Infeld theory

We investigate dynamics of the homogeneous time-dependent SU(2) Yang-Mills fields governed by the non-Abelian Born-Infeld Lagrangian, which arises in superstring theory as a result of summation of all orders in the string slope parameter α′. It is shown that, generically, the Born-Infeld dynamics are less chaotic than those in the ordinary Yang-Mills theory, and at a high enough field strength the Yang-Mills chaos is stabilized. More generally, a smothering effect of the string nonlocality on the behavior of classical fields is conjectured.     

Time independent description of rapidly oscillating potentials

The classical and quantum dynamics in a high frequency field are found to be described by an effective time independent Hamiltonian. It is calculated in a systematic expansion in the inverse of the frequency (omega) to order omega(-4). The work is an extension of the classical result for the Kapitza pendulum, which was calculated in the past to order omega(-2). The analysis makes use of an implementation of the method of separation of time scales and of a quantum gauge transformation in the framework of Floquet theory. The effective time independent Hamiltonian enables one to explore the dynamics in the presence of rapidly oscillating fields, in the framework of theories that were developed for systems with time independent Hamiltonians. The results are relevant, in particular, for exploring the dynamics of cold atoms.     

The effect of two electron-withdrawing groups on remote tertiary hydrogens susceptible to electrophilic fluorination using F2

The direct electrophilic fluorination of unactivated remote tertiary hydrogens is effected by the number, nature and distance of electron-withdrawing groups. The effect of these groups on reducing electron density of the tertiary CH bond seems to be additive. The more remote these groups are from the reaction center, the higher the yield of the fluorination.     

Estimates of the spectral radius of graphs

We give lower and upper bounds on the spectral radius of directed Eulerian multigraphs and undirected multigraphs.     

Electronic structure of Zn in oxide spinels

Using⁶⁷Zn Mössbauer absorption and emission spectroscopy, we have investigated the electronic structure at the A and B sites in the normal spinels (Zn)[Al₂]O₄, (Zn)[Fe₂]O₄ and (Zn)[Ga₂]O₄. Within each system, the center shift S_c at the A site is more positive. In all systems investigated, the electric field gradientV _u at the B site is negative. The values for S_C andV _U scale with oxygen nearest-neighbour distance to Zn. In the Fe spinel, a transferred magnetic hyperfine field is observed at the Zn site below the antiferromagnetic ordering temperatureT _N=10 K. For a more detailed discussion of the chemical bond, we have performed ab initio Hartree-Fock cluster calculations for the Al and Fe spinels. Our experimental and theoretical results show that all hyperfine parameters are essentially determined by covalency effects. Our data on the Ga spinel raise the question of a partially inverse structure.     

Linear programming method for rational approximation

Development of a new algorithm, based on linear programming, for the computation of the best rational approximation of a continuous function.     

High-resolution67Zn-Mössbauer study of oxide spinels

We report on first experiments to investigate the electronic structure in the normal spinels ZnAl₂O₄ and ZnFe₂O₄ and in the inverse spinels Zn₂SnO₄ and Zn₂TiO₄ using the high-resolution⁶⁷Zn-Mössbauer spectroscopy. The electric field gradient for⁶⁷Zn at the B (octahedral) site in ZnAl₂O₄ is negative, whereas the A (tetrahedral) site remains essentially cubic, however, with a more positive center shift. ZnFe₂O₄ orders antiferromagnetically at ≈10K. Due to superexchange a magnetic field is observed at⁶⁷Zn. In the inverse spinels short range order leads to only few (Zn,Sn) and (Zn,Ti) configurations at the octahedral sites. The s-electron densities at the⁶⁷Zn sites are distinct and cover a surprisingly broad range for Zn₂TiO₄. This strongly suggests that d-electrons of Ti play an essential role in the chemical bond of this compound.     

Extraction of prussian blue into chloroform in the presence of ajatin

Extraction of Prussian Blue into chloroform in the presence of ajatin (dimethyl-lauryl-benzylammonium bromide) is proposed for the concentration and determination of ferrocyanide ion. Optimum concentrations of sulphuric acid and of ajatin have been established and the selectivity of the extraction has been investigated. Ways of eliminating some interferences are discussed.