The angular distribution of the reaction νˉe+p→e++n\bar{\nu}_e + p \to e^+ + n

The reaction $\bar{\nu}_e + p \to e^+ + n$ is very important for low-energy ($E_\nu \lesssim 60$ MeV) antineutrino experiments. In this paper we calculate the positron angular distribution, which at low energies is slightly backward. We show that weak magnetism and recoil corrections have a large effect on the angular distribution, making it isotropic at about 15 MeV and slightly forward at higher energies. We also show that the behavior of the cross section and the angular distribution can be well-understood analytically for $E_\nu \lesssim 60$ MeV by calculating to ${\cal O}(1/M)$, where $M$ is the nucleon mass. The correct angular distribution is useful for separating $\bar{\nu}_e + p \to e^+ + n$ events from other reactions and detector backgrounds, as well as for possible localization of the source (e.g., a supernova) direction. We comment on how similar corrections appear for the lepton angular distributions in the deuteron breakup reactions $\bar{\nu}_e + d \to e^+ + n + n$ and $\nu_e + d \to e^- + p + p$. Finally, in the reaction $\bar{\nu}_e + p \to e^+ + n$, the angular distribution of the outgoing neutrons is strongly forward-peaked, leading to a measurable separation in positron and neutron detection points, also potentially useful for rejecting backgrounds or locating the source direction.Comment: 10 pages, including 5 figure

Semiclassical evaluation of average nuclear one and two body matrix elements

Thomas-Fermi theory is developed to evaluate nuclear matrix elements averaged on the energy shell, on the basis of independent particle Hamiltonians. One- and two-body matrix elements are compared with the quantal results and it is demonstrated that the semiclassical matrix elements, as function of energy, well pass through the average of the scattered quantum values. For the one-body matrix elements it is shown how the Thomas-Fermi approach can be projected on good parity and also on good angular momentum. For the two-body case the pairing matrix elements are considered explicitly.Comment: 15 pages, REVTeX, 6 ps figures; changed conten

Evidence for SU(3) symmetry breaking from hyperon production

We examine the SU(3) symmetry breaking in hyperon semileptonic decays (HSD) by considering two typical sets of quark contributions to the spin content of the octet baryons: Set-1 with SU(3) flavor symmetry and Set-2 with SU(3) flavor symmetry breaking in HSD. The quark distributions of the octet baryons are calculated with a successful statistical model. Using an approximate relation between the quark fragmentation functions and the quark distributions, we predict polarizations of the octet baryons produced in $e^+e^-$ annihilation and semi-inclusive deeply lepton-nucleon scattering in order to reveal the SU(3) symmetry breaking effect on the spin structure of the octet baryons. We find that the SU(3) symmetry breaking significantly affects the hyperon polarization. The available experimental data on the $\Lambda$ polarization seem to favor the theoretical predictions with SU(3) symmetry breaking. We conclude that there is a possibility to get a collateral evidence for SU(3) symmetry breaking from hyperon production. The theoretical errors for our predictions are discussed.Comment: 3 tables, 14 figure

Local Charge Excesses in Metallic Alloys: a Local Field Coherent Potential Approximation Theory

Electronic structure calculations performed on very large supercells have shown that the local charge excesses in metallic alloys are related through simple linear relations to the local electrostatic field resulting from distribution of charges in the whole crystal. By including local external fields in the single site Coherent Potential Approximation theory, we develop a novel theoretical scheme in which the local charge excesses for random alloys can be obtained as the responses to local external fields. Our model maintains all the computational advantages of a single site theory but allows for full charge relaxation at the impurity sites. Through applications to CuPd and CuZn alloys, we find that, as a general rule, non linear charge rearrangements occur at the impurity site as a consequence of the complex phenomena related with the electronic screening of the external potential. This nothwithstanding, we observe that linear relations hold between charge excesses and external potentials, in quantitative agreement with the mentioned supercell calculations, and well beyond the limits of linearity for any other site property.Comment: 11 pages, 1 table, 7 figure

Indirect search for dark matter: prospects for GLAST

Possible indirect detection of neutralino, through its gamma-ray annihilation product, by the forthcoming GLAST satellite from our galactic halo, M31, M87 and the dwarf galaxies Draco and Sagittarius is studied. Gamma-ray fluxes are evaluated for the two representative energy thresholds, 0.1 GeV and 1.0 GeV, at which the spatial resolution of GLAST varies considerably. Apart from dwarfs which are described either by a modified Plummer profile or by a tidally-truncated King profiles, fluxes are compared for halos with central cusps and cores. It is demonstrated that substructures, irrespective of their profiles, enhance the gamma-ray emission only marginally. The expected gamma-ray intensity above 1 GeV at high galactic latitudes is consistent with the residual emission derived from EGRET data if the density profile has a central core and the neutralino mass is less than 50 GeV, whereas for a central cusp only a substantial enhancement would explain the observations. From M31, the flux can be detected above 0.1 GeV and 1.0 GeV by GLAST only if the neutralino mass is below 300 GeV and if the density profile has a central cusp, case in which a significant boost in the gamma-ray emission is produced by the central black hole. For Sagittarius, the flux above 0.1 GeV is detectable by GLAST provided the neutralino mass is below 50 GeV. From M87 and Draco the fluxes are always below the sensitivity limit of GLAST.Comment: 14 Pages, 7 Figures, 3 Tables, version to appear on Physical Review

Can forest management based on natural disturbances maintain ecological resilience?

Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance

Accretion and ejection in black-hole X-ray transients

Aims: We summarize the current observational picture of the outbursts of black-hole X-ray transients (BHTs), based on the evolution traced in a hardness-luminosity diagram (HLD), and we offer a physical interpretation. Methods: The basic ingredient in our interpretation is the Poynting-Robertson Cosmic Battery (PRCB, Contopoulos & Kazanas 1998), which provides locally the poloidal magnetic field needed for the ejection of the jet. In addition, we make two assumptions, easily justifiable. The first is that the mass-accretion rate to the black hole in a BHT outburst has a generic bell-shaped form. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state. The second assumption is that at low accretion rates the accretion flow is geometrically thick, ADAF-like, while at high accretion rates it is geometrically thin. Results: Both, at the beginning and the end of an outburst, the PRCB establishes a strong poloidal magnetic field in the ADAF-like part of the accretion flow, and this explains naturally why a jet is always present in the right part of the HLD. In the left part of the HLD, the accretion flow is in the form of a thin disk, and such a disk cannot sustain a strong poloidal magnetic filed. Thus, no jet is expected in this part of the HLD. The counterclockwise traversal of the HLD is explained as follows: the poloidal magnetic field in the ADAF forces the flow to remain ADAF and the source to move upwards in the HLD rather than to turn left. Thus, the history of the system determines the counterclockwise traversal of the HLD. As a result, no BHT is expected to ever traverse the entire HLD curve in the clockwise direction. Conclusions: We offer a physical interpretation of accretion and ejection in BHTs with only one parameter, the mass transfer rate.Comment: Accepted for publication in A&

Avalanche Dynamics in Evolution, Growth, and Depinning Models

The dynamics of complex systems in nature often occurs in terms of punctuations, or avalanches, rather than following a smooth, gradual path. A comprehensive theory of avalanche dynamics in models of growth, interface depinning, and evolution is presented. Specifically, we include the Bak-Sneppen evolution model, the Sneppen interface depinning model, the Zaitsev flux creep model, invasion percolation, and several other depinning models into a unified treatment encompassing a large class of far from equilibrium processes. The formation of fractal structures, the appearance of $1/f$ noise, diffusion with anomalous Hurst exponents, Levy flights, and punctuated equilibria can all be related to the same underlying avalanche dynamics. This dynamics can be represented as a fractal in $d$ spatial plus one temporal dimension. We develop a scaling theory that relates many of the critical exponents in this broad category of extremal models, representing different universality classes, to two basic exponents characterizing the fractal attractor. The exact equations and the derived set of scaling relations are consistent with numerical simulations of the above mentioned models.Comment: 27 pages in revtex, no figures included. Figures or hard copy of the manuscript supplied on reques

Seasonal size variation in the predatory cladoceran Bythotrephes cederstroemii in Lake Michigan

1.  Dry weight, body length and spine length were measured for the exotic cladoceran Bythotrephes cederstroemii collected from offshore and inshore stations in southeastern Lake Michigan. Average dry weight of each developmental stage exhibited seasonal variation by a factor of more than 5. 2.  Mean dry weight of Bythotrephes was closely correlated with water temperature. Contrary to the inverse relationship between water temperature and body size frequently observed for other invertebrates, the dry weight of Bythotrephes increased at higher ambient temperatures. 3.  No significant correlation was observed between abundances of major zooplankton taxa and the dry weight of Bythotrephes . An indirect effect of temperature on prey consumption may cause seasonal variation in dry weight of Bythotrephes in Lake Michigan. 4.  Distances between adjacent pairs of barbs, added to the caudal spine with each moult, are significantly shorter in Bythotrephes which produce resting eggs. Less material investment in the exoskeleton of sexually reproducing females was observed in favour of growth and reproduction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74641/1/j.1365-2427.1994.tb00842.x.pd

Why does fertilization reduce plant species diversity? Testing three competition-based hypotheses

1 Plant species diversity drops when fertilizer is added or productivity increases. To explain this, the total competition hypothesis predicts that competition above ground and below ground both become more important, leading to more competitive exclusion, whereas the light competition hypothesis predicts that a shift from below-ground to above-ground competition has a similar effect. The density hypothesis predicts that more above-ground competition leads to mortality of small individuals of all species, and thus a random loss of species from plots. 2 Fertilizer was added to old field plots to manipulate both below-ground and above-ground resources, while shadecloth was used to manipulate above-ground resources alone in tests of these hypotheses. 3 Fertilizer decreased both ramet density and species diversity, and the effect remained significant when density was added as a covariate. Density effects explained only a small part of the drop in diversity with fertilizer. 4 Shadecloth and fertilizer reduced light by the same amount, but only fertilizer reduced diversity. Light alone did not control diversity, as the light competition hypothesis would have predicted, but the combination of above-ground and below-ground competition caused competitive exclusion, consistent with the total competition hypothesis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75695/1/j.1365-2745.2001.00662.x.pd