Analysis and Assembling of Network Structure in Mutualistic Systems

It has been observed that mutualistic bipartite networks have a nested structure of interactions. In addition, the degree distributions associated with the two guilds involved in such networks (e.g. plants & pollinators or plants & seed dispersers) approximately follow a truncated power law. We show that nestedness and truncated power law distributions are intimately linked, and that any biological reasons for such truncation are superimposed to finite size effects . We further explore the internal organization of bipartite networks by developing a self-organizing network model (SNM) that reproduces empirical observations of pollination systems of widely different sizes. Since the only inputs to the SNM are numbers of plant and animal species, and their interactions (i.e., no data on local abundance of the interacting species are needed), we suggest that the well-known association between species frequency of interaction and species degree is a consequence rather than a cause, of the observed network structure.Comment: J. of. Theor. Biology, in pres

Measurement of the forward-backward asymmetry in low-mass bottom-quark pairs produced in proton-antiproton collisions

We report a measurement of the forward-backward asymmetry, A(FB), in b (b) over bar pairs produced in proton-antiproton collisions and identified by muons from semileptonic b-hadron decays. The event sample is collected at a center-of-mass energy of root s = 1.96 TeV with the CDF II detector and corresponds to 6.9 fb(-1) of integrated luminosity. We obtain an integrated asymmetry of A(FB)(b (b) over bar) = (1.2 +/- 0.7)% at the particle level for b-quark pairs with invariant mass, m(b (b) over bar), down to 40 GeV/c(2) and measure the dependence of A(FB)(b (b) over bar) on m(b (b) over bar). The results are compatible with expectations from the standard model.Peer reviewe

Measurement of the forward-backward asymmetry of top-quark and antiquark pairs using the full CDF Run II data set

We measure the forward-backward asymmetry of the production of top-quark and antiquark pairs in proton-antiproton collisions at center-of-mass energy root s = 1.96 TeV using the full data set collected by the Collider Detector at Fermilab (CDF) in Tevatron Run II corresponding to an integrated luminosity of 9.1 fb(-1). The asymmetry is characterized by the rapidity difference between top quarks and antiquarks (Delta y) and measured in the final state with two charged leptons (electrons and muons). The inclusive asymmetry, corrected to the entire phase space at parton level, is measured to be A(FB)(t (t) over bar) = 0.12 +/- 0.13, consistent with the expectations from the standard model (SM) and previous CDF results in the final state with a single charged lepton. The combination of the CDF measurements of the inclusive A(FB)(t (t) over bar) in both final states yields A(FB)(t (t) over bar) 0.160 +/- 0.045, which is consistent with the SM predictions. We also measure the differential asymmetry as a function of Delta y. A linear fit to A(FB)(t (t) over bar) (vertical bar Delta y vertical bar),assuming zero asymmetry at Delta y = 0, yields a slope of alpha = 0.14 +/- 0.15, consistent with the SM prediction and the previous CDF determination in the final state with a single charged lepton. The combined slope of A(FB)(t (t) over bar) (vertical bar Delta y vertical bar) the two final states is alpha = 0.227 +/- 0.057, which is 2.0 sigma larger than the SM prediction.Peer reviewe

(2+1)-Gravity with Moving Particles in an Instantaneous Gauge

By defining a regular gauge which is conformal-like and provides instantaneous field propagation, we investigate classical solutions of (2+1)-Gravity coupled to arbitrarily moving point-like particles. We show how to separate field equations from self-consistent motion and we provide a solution for the metric and the motion in the two-body case with arbitrary speed, up to second order in the mass parameters.Comment: 16 pages, LaTeX, no figure

Measurement of the WW and WZ production cross section using final states with a charged lepton and heavy-flavor jets in the full CDF Run II data set

We present a measurement of the total WW and WZ production cross sections in p (p) over bar collision at root s = 1.96 TeV, in a final state consistent with leptonic W boson decay and jets originating from heavy-flavor quarks from either a W or a Z boson decay. This analysis uses the full data set collected with the CDF II detector during Run II of the Tevatron collider, corresponding to an integrated luminosity of 9.4 fb(-1). An analysis of the dijet mass spectrum provides 3.7 sigma evidence of the summed production processes of either WW or WZ bosons with a measured total cross section of sigma(WW+WZ) = 13.7 +/- 3.9 pb. Independent measurements of the WW and WZ production cross sections are allowed by the different heavy- flavor decay patterns of the W and Z bosons and by the analysis of secondary- decay vertices reconstructed within heavy- flavor jets. The productions of WW and of WZ dibosons are independently seen with significances of 2.9s and 2.1s, respectively, with total cross sections of sigma(WW) = 9.4 +/- 4.2 pb and sigma(WZ) = 3.7(-2.2)(+2.5) pb. The measurements are consistent with standard- model predictions.Peer reviewe

Search for a low-mass neutral Higgs boson with suppressed couplings to fermions using events with multiphoton final states

A search for a Higgs boson with suppressed couplings to fermions, h(f), assumed to be the neutral, lower-mass partner of the Higgs boson discovered at the Large Hadron Collider, is reported. Such a Higgs boson could exist in extensions of the standard model with two Higgs doublets, and could be produced via p (p) over bar -> H(+/-)h(f) -> W*h(f)h(f) -> 4 gamma + X where H (+/-) is a charged Higgs boson. This analysis uses all events withat least three photons in the final state from proton-antiproton collisions at a center-of-mass energy of 1.96 TeV collected by the Collider Detector at Fermilab, corresponding to an integrated luminosity of 9.2 fb(-1). No evidence of a signal is observed in the data. Values of Higgs-boson masses between 10 and 100 GeV/c(2) are excluded at 95% Bayesian credibility.Peer reviewe

Combined Forward-Backward Asymmetry Measurements in Top-Antitop Quark Production at the Tevatron

The CDF and D0 experiments at the Fermilab Tevatron have measured the asymmetry between yields of forward- and backward-produced top and antitop quarks based on their rapidity difference and the asymmetry between their decay leptons. These measurements use the full data sets collected in proton-antiproton collisions at a center-of-mass energy of root s = 1.96 TeV. We report the results of combinations of the inclusive asymmetries and their differential dependencies on relevant kinematic quantities. The combined inclusive asymmetry is A(FB)(t (t) over bar) = 0.128 +/- 0.025. The combined inclusive and differential asymmetries are consistent with recent standard model predictions.Peer reviewe

The "recoil" correction of order mα6m \alpha^6 to hyperfine splitting of positronium ground state

The "recoil" correction of order $m \alpha^6$ to the hyperfine splitting of positronium ground state was found. The formalism employed is based on the noncovariant perturbation theory in QED. Equation for two-particle component of full (many-body) wave function is used, in which effective Hamiltonian depends on the energy of a system. The effective Hamiltonian is not restricted to the nonrelativistic region, so there is no need in any regularization. To evaluate integrals over loop momenta, they are divided into "hard" and "soft" parts, coming from large and small momenta respectively. Soft contributions were found analytically, and hard ones are evaluated by numerical integration. Some soft terms due to the retardation cancel each other. To calculate the "hard" contributions, a great number of noncovariant graphs is replaced by only a few covariant ones. The hard contribution was found in two ways. The first way is to evaluate contributions of separate graphs, using the Coulomb gauge. The second one is to calculate full hard contribution as a whole using the Feynman gauge. The final result for the "recoil" correction is 0.381(6) m\al^6 and agrees with those of previous papers. Diagram-to-diagram comparison with the revised results of Adkins&Sapirstein was done. All the results agree, so the "recoil" correction is now firmly established. This means a considerable disagreement with the experimental data.Comment: 28 pages, latex including latex figure

ONE LOOP QED VERTEX IN ANY COVARIANT GAUGE: ITS COMPLETE ANALYTIC FORM

The one loop vertex in QED is calculated in arbitrary covariant gauges as an analytic function of its momenta. The vertex is decomposed into a longitudinal part, that is fully responsible for ensuring the Ward and Ward-Takahashi identities are satisfied, and a transverse part. The transverse part is decomposed into 8 independent components each being separately free of kinematic singularities in $\bf any$ covariant gauge in a basis that modifies that proposed by Ball and Chiu. Analytic expressions for all 11 components of the ${O(\alpha)}$ vertex are given explicitly in terms of elementary functions and one Spence function. These results greatly simplify in particular kinematic regimes.Comment: 35 pages, latex, 2 figures, Complete postscript file available from: ftp://cpt1.dur.ac.uk/pub/preprints/dtp95/dtp9506/dtp9406.p

High energy photon-neutrino elastic scattering

The one-loop helicity amplitudes for the elastic scattering process $\gamma\nu\to\gamma\nu$ in the Standard Model are computed at high center of mass energies. A general decomposition of the amplitudes is utilized to investigate the validity of some of the key features of our results. In the center of mass, where $\sqrt{s} = 2\omega$, the cross section grows roughly as $\omega^6$ to near the threshold for $W$-boson production, $\sqrt{s} = m_W$. Although suppressed at low energies, we find that the elastic cross section exceeds the cross section for $\gamma\nu\to\gamma\gamma\nu$ when $\sqrt{s}>13$ GeV. We demonstrate that the scattered photons are circularly polarized and the net value of the polarization is non-zero. Astrophysical implications of high energy photon-neutrino scattering are discussed.Comment: 9 pages, 7 figures, RevTeX