Momentum Distributions in ttˉt\bar t

We apply the Green function formalism for $t-\bar t$ production and decay near threshold in a study of the effects due to the momentum dependent width for such a system. We point out that these effects are likely to be much smaller than expected from the reduction of the available phase space. The Lippmann--Schwinger equation for the QCD chromostatic potential is solved numerically for $S$ partial wave. We compare the results on the total cross section, top quark intrinsic momentum distributions and on the energy spectra of $W$ bosons from top quark decays with those obtained for the constant width.Comment: 12 pages (without figures) (11 (sub)figures available on request), Karlsruhe preprint TTP93-11, hep-ph/yymmnn

Ratios of BB and DD Meson Decay Constants in Relativistic Quark Model

We calculate the ratios of $B$ and $D$ meson decay constants by applying the variational method to the relativistic hamiltonian of the heavy meson. We adopt the Gaussian and hydrogen-type trial wave functions, and use six different potentials of the potential model. We obtain reliable results for the ratios, which are similar for different trial wave functions and different potentials. The obtained ratios show the deviation from the nonrelativistic scaling law, and they are in a pretty good agreement with the results of the Lattice calculations.Comment: 13 pages, 1 Postscript figur

Improved lower bounds for the ground-state energy of many-body systems

New lower bounds for the binding energy of a quantum-mechanical system of interacting particles are presented. The new bounds are expressed in terms of two-particle quantities and improve the conventional bounds of the Hall-Post type. They are constructed by considering not only the energy in the two-particle system, but also the structure of the pair wave function. We apply the formal results to various numerical examples, and show that in some cases dramatic improvement over the existing bounds is reached.Comment: 29 pages, 5 figures, to be published in Phys. Rev.

Statistical mechanics of typical set decoding

The performance of ``typical set (pairs) decoding'' for ensembles of Gallager's linear code is investigated using statistical physics. In this decoding, error happens when the information transmission is corrupted by an untypical noise or two or more typical sequences satisfy the parity check equation provided by the received codeword for which a typical noise is added. We show that the average error rate for the latter case over a given code ensemble can be tightly evaluated using the replica method, including the sensitivity to the message length. Our approach generally improves the existing analysis known in information theory community, which was reintroduced by MacKay (1999) and believed as most accurate to date.Comment: 7 page

Hadronic Total Cross-sections Through Soft Gluon Summation in Impact Parameter Space

The Bloch-Nordsieck model for the parton distribution of hadrons in impact parameter space, constructed using soft gluon summation, is investigated in detail. Its dependence upon the infrared structure of the strong coupling constant $\alpha_s$ is discussed, both for finite as well as singular, but integrable, $\alpha_s$. The formalism is applied to the prediction of total proton-proton and proton-antiproton cross-sections, where screening, due to soft gluon emission from the initial valence quarks, becomes evident.Comment: 20 pages, Latex2e, input FEYNMAN,12 postscipt figures. Submitted to PR

Lattice calculation of 1−+1^{-+} hybrid mesons with improved Kogut-Susskind fermions

We report on a lattice determination of the mass of the exotic $1^{-+}$ hybrid meson using an improved Kogut-Susskind action. Results from both quenched and dynamical quark simulations are presented. We also compare with earlier results using Wilson quarks at heavier quark masses. The results on lattices with three flavors of dynamical quarks show effects of sea quarks on the hybrid propagators which probably result from coupling to two meson states. We extrapolate the quenched results to the physical light quark mass to allow comparison with experimental candidates for the $1^{-+}$ hybrid meson. The lattice result remains somewhat heavier than the experimental result, although it may be consistent with the $\pi_1(1600)$.Comment: 24 pages, 12 figures. Replaced to match published versio

The (LATTICE) QCD Potential and Running Coupling: How to Accurately Interpolate between Multi-Loop QCD and the String Picture

We present a simple parameterization of a running coupling constant, defined via the static potential, that interpolates between 2-loop QCD in the UV and the string prediction in the IR. Besides the usual \Lam-parameter and the string tension, the coupling depends on one dimensionless parameter, determining how fast the crossover from UV to IR behavior occurs (in principle we know how to take into account any number of loops by adding more parameters). Using a new Ansatz for the LATTICE potential in terms of the continuum coupling, we can fit quenched and unquenched Monte Carlo results for the potential down to ONE lattice spacing, and at the same time extract the running coupling to high precision. We compare our Ansatz with 1-loop results for the lattice potential, and use the coupling from our fits to quantitatively check the accuracy of 2-loop evolution, compare with the Lepage-Mackenzie estimate of the coupling extracted from the plaquette, and determine Sommer's scale $r_0$ much more accurately than previously possible. For pure SU(3) we find that the coupling scales on the percent level for $\beta\geq 6$.Comment: 47 pages, incl. 4 figures in LaTeX [Added remarks on correlated vs. uncorrelated fits in sect. 4; corrected misprints; updated references.

Developmental course of psychopathology in youths with and without intellectual disabilities

Background: We aimed to describe similarities and differences in the developmental course of psychopathology between children with and without intellectual disabilities (ID). Method: Multilevel growth curve analysis was used to analyse the developmental course of psychopathology, using the Child Behavior Checklist (CBCL), in two longitudinal multiple-birth-cohort samples of 6- to 18-year-old children with ID (N=978) and without ID (N=2,047) using three repeated measurements across a 6-year period. Results: Children with ID showed a higher level of problem behaviours across all ages compared to children without ID. A significant difference between the samples in the developmental courses was found for Aggressive Behaviour and Attention Problems, where children with ID showed a significantly larger decrease. Gender differences in the development of psychopathology were similar in both samples, except for Social Problems where males with ID showed a larger decrease in problem behaviour across time than females with ID and males and females without ID. Conclusion: Results indicate that children with ID continue to show a greater risk for psychopathology compared to typically developing children, although this higher risk is less pronounced at age 18 than it is at age 6 for Aggressive Behaviour. Contrary to our expectations, the developmental course of psychopathology in children with ID was quite similar from age 6 to 18 compared to children without ID. The normative developmental trajectories of psychopathology in children with ID, presented here, can serve as a yardstick against which development of childhood psychopathology can be detected as deviant. © 2007 The Authors Journal compilation © 2007 Association for Child and Adolescent Mental Health