Will relativistic heavy-ion colliders destroy our planet?

Experiments at the Brookhaven National Laboratory will study collisions between gold nuclei at unprecedented energies. The concern has been voiced that ``strangelets''-hypothetical products of these collisions - may trigger the destruction of our planet. We show how naturally occurring heavy-ion collisions can be used to derive a safe and stringent upper bound on the risk incurred in running these experiments.Comment: LaTeX, no figure

Positron production in crossed beams of bare uranium nuclei

Positron creation in crossed-beam collisions of high-energy, fully stripped heavy ions is investigated within the coupled-channel formalism. In comparison with fixed-target collisions of highly stripped heavy-ion projectiles positron production probabilities are enhanced by more than one order of magnitude. The increase results from the possibility to excite electrons from the negative energy continuum into all bound states. The positron spectrum is shifted towards higher energies because of the absence of electron screening. Rutherford scattering as well as nuclear collisions with time delay are investigated. We also discuss the filling of empty bound states by electrons from pair-production processes

Comment on ``Kepler problem in Dirac theory for a particle with position-dependent mass''

Based on easy-to-follow considerations it is not difficult to be vehemently opposed not only the solutions found in that paper but also the conclusions manifested there.Comment: 4 page

Continuum percolation of wireless ad hoc communication networks

Wireless multi-hop ad hoc communication networks represent an infrastructure-less and self-organized generalization of todays wireless cellular networks. Connectivity within such a network is an important issue. Continuum percolation and technology-driven mutations thereof allow to address this issue in the static limit and to construct a simple distributed protocol, guaranteeing strong connectivity almost surely and independently of various typical uncorrelated and correlated random spatial patterns of participating ad hoc nodes.Comment: 30 pages, to be published in Physica

A Physical Realization of the Generalized PT-, C-, and CPT-Symmetries and the Position Operator for Klein-Gordon Fields

Generalized parity (P), time-reversal (T), and charge-conjugation (C)operators were initially definedin the study of the pseudo-Hermitian Hamiltonians. We construct a concrete realization of these operators for Klein-Gordon fields and show that in this realization PT and C operators respectively correspond to the ordinary time-reversal and charge-grading operations. Furthermore, we present a complete description of the quantum mechanics of Klein-Gordon fields that is based on the construction of a Hilbert space with a relativistically invariant, positive-definite, and conserved inner product. In particular we offer a natural construction of a position operator and the corresponding localized and coherent states. The restriction of this position operator to the positive-frequency fields coincides with the Newton-Wigner operator. Our approach does not rely on the conventional restriction to positive-frequency fields. Yet it provides a consistent quantum mechanical description of Klein-Gordon fields with a genuine probabilistic interpretation.Comment: 20 pages, published versio

Electron emission and positron production in deep inelastic heavy-ion reactions

Atomic excitations are used to obtain information on the course of a nuclear reaction. Employing a semiclassical picture we calculate the emission of δ electrons and positrons in deep inelastic nuclear reactions for the example of U+U collisions incorporating nuclear trajectories resulting from two different nuclear friction models. The emission spectra exhibit characteristic deviations from those expected for elastic Coulomb scattering. The theoretical probabilities are compared with recent experimental data by Backe et al. A simple model is used to estimate the influence of a threebody breakup of the compound system upon atomic excitations

Literacy abilities of children and adolescents with Williams Syndrome.

In this dissertation I report findings from two studies of the literacy abilities of children with Williams syndrome (WS), a genetic disorder associated with intellectual disability. I had two overarching goals: 1) evaluate the applicability of theories of reading and spelling acquisition developed based on typically developing children to children with WS; and 2) provide results that would inform the development of targeted assessments and interventions. In Study 1, individual differences in phonological awareness, visual spatial perception, vocabulary, overall intellectual abilities, and reading instruction approach (systematic phonics vs. other approaches) for sixty-nine 6 – 7-year-olds (most of whom were in kindergarten) were used to predict word-reading abilities three years later. Multiple regression analyses indicated that Time 1 reading instruction method, phonological awareness, and visual-spatial perception each explained significant unique variance in word reading at Time 2. A systematic phonics approach was associated with significantly better performance than other reading-instruction approaches. Results of a simple mediation analysis indicated that vocabulary at Time 1 indirectly influenced Time 2 word reading through its effect on Time 1 phonological awareness. In Study 2, relations between spelling, word reading, and vocabulary abilities and method of reading instruction were investigated for eighty 9 – 17-year-olds. Spelling and reading abilities were highly correlated. Students taught to read using systematic phonics instruction had significantly higher spelling scores than those taught to read using other approaches. Spelling ability contributed significant unique variance to word-reading ability, beyond the effects of phonological awareness, vocabulary, and reading instruction method. Overall, the results from Study 1 and 2 indicated that the word-reading and spelling abilities of students with WS vary widely but on average are well below the mean for same-aged children in the general population. Variations in overall intellectual ability did not play a central role in accounting for individual differences in word reading and spelling. Findings from this dissertation provide support for the universality of theoretical models of reading development developed based on typically developing children and suggest that educational approaches known to be effective for children who are having difficulty learning to read are likely to be appropriate for children with WS

Spin and Pseudospin symmetries in the Dirac equation with central Coulomb potentials

We analyze in detail the analytical solutions of the Dirac equation with scalar S and vector V Coulomb radial potentials near the limit of spin and pseudospin symmetries, i.e., when those potentials have the same magnitude and either the same sign or opposite signs, respectively. By performing an expansion of the relevant coefficients we also assess the perturbative nature of both symmetries and their relations the (pseudo)spin-orbit coupling. The former analysis is made for both positive and negative energy solutions and we reproduce the relations between spin and pseudospin symmetries found before for nuclear mean-field potentials. We discuss the node structure of the radial functions and the quantum numbers of the solutions when there is spin or pseudospin symmetry, which we find to be similar to the well-known solutions of hydrogenic atoms.Comment: 9 pages, 2 figures, uses revte

Influence of trapping potentials on the phase diagram of bosonic atoms in optical lattices

We study the effect of external trapping potentials on the phase diagram of bosonic atoms in optical lattices. We introduce a generalized Bose-Hubbard Hamiltonian that includes the structure of the energy levels of the trapping potential, and show that these levels are in general populated both at finite and zero temperature. We characterize the properties of the superfluid transition for this situation and compare them with those of the standard Bose-Hubbard description. We briefly discuss similar behaviors for fermionic systems.Comment: 4 pages, 3 figures; final version, to be published in Phys. Rev.

The effects of medium on nuclear properties in multifragmentation

In multifragmentation of hot nuclear matter, properties of fragments embedded in a soup of nucleonic gas and other fragments should be modified as compared with isolated nuclei. Such modifications are studied within a simple model where only nucleons and one kind of heavy nuclei are considered. The interaction between different species is described with a momentum-dependent two-body potential whose parameters are fitted to reproduce properties of cold isolated nuclei. The internal energy of heavy fragments is parametrized according to a liquid-drop model with density and temperature dependent parameters. Calculations are carried out for several subnuclear densities and moderate temperatures, for isospin-symmetric and asymmetric systems. We find that the fragments get stretched due to interactions with the medium and their binding energies decrease with increasing temperature and density of nuclear matter.Comment: 12 pages, 11 figures, Phys. Rev.C (in press