Top Quark Production Dynamics in QCD

A calculation of the total cross section for top quark production in hadron-hadron collisions is presented based on an all-orders perturbative resummation of initial-state gluon radiative contributions to the basic quantum chromodynamics subprocesses. Principal-value resummation is used to evaluate all relevant large threshold contributions. In this method there are no arbitrary infrared cutoffs, and the perturbative regime of applicability is well defined, two attributes that significantly reduce the estimated uncertainty of the results. For $p\bar{p}$ collisions at center-of-mass energy $\sqrt{s}=1.8$ TeV and a top mass of 175 GeV, we obtain $\sigma(t\bar{t})=5.52^{+0.07}_{-0.45} pb$, in agreement with experiment. Predicted cross sections are provided as a function of top mass in $p\bar{p}$ collisions at $\sqrt{s}=2.0$ TeV and in $p p$ collisions at CERN LHC energies.Comment: 12 pages. Latex file plus 8 ps files of figures submitted altogther in uuencoded form; Invited talk presented by E. L. Berger at the International Symposium on Heavy Flavor and Electroweak Theory, Beijing, August 16 - 19, 199

Pick matrix conditions for sign-definite solutions of the algebraic Riccati equation

We study the existence of positive and negative semidefinite solutions of algebraic Riccati equations (ARE) corresponding to linear quadratic problems with an indefinite cost functional. The problem to formulate reasonable necessary and sufficient conditions for the existence of such solutions is a long-standing open problem. A central role is played by certain two-variable polynomial matrices associated with the ARE. Our main result characterizes all unmixed solutions of the ARE in terms of the Pick matrices associated with these two-variable polynomial matrices. As a corollary of this result we obtain that the signatures of the extremal solutions of the ARE are determined by the signatures of particular Pick matrices

Threshold Resummation and the Total Cross Section for Top Quark Production

We discuss the motivation for resummation of the effects of initial-state soft-gluon radiation, to all orders in the strong coupling strength, for processes in which the near-threshold region in the partonic subenergy is important. We summarize our calculation of the total cross section for top quark production at hadron colliders. Comments are included on the differences between our treatment of subleading logarithmic terms and other methods.Comment: 7 pages, 1 figure, uses aipproc.sty; Paper presented by E. L. Berger, to be published in the Proceedings of DIS'97, Fifth International Workshop on Deep Inelastic Scattering and QCD, Chicago, IL, April 14 - 18, 199

The Perturbative Resummed Series for Top Production

Our calculation of the total cross section for inclusive production of $t\bar{t}$ pairs in hadron collisions is summarized. The principal ingredient of this calculation is resummation of the universal leading-logarithm effects of gluon radiation to all orders in the quantum chromodynamics coupling strength, restricted to the region of phase space that is manifestly perturbative. We present predictions of the physical cross section as a function of top quark mass in proton-antiproton reactions at center-of-mass energies of 1.8 and 2.0 TeV.Comment: 9 pages. Latex file plus 6 ps files of figures submitted togther in uuencoded form; invited talk presented by Harry Contopanagos at the XXXI Rencontres de Moriond on QCD and High Energy Hadronic Interactions, Les Arcs, France, March, 1996. New version to correct problem with unsuccessful posting of figures on May

Investigation of Cathode Kinetics in SOFC: Model Thin Film SrTi_(1-x)Fe_xO_(3-δ) Mixed Conducting Oxides

To understand the kinetics controlling the SOFC cathode processes, a model mixed conducting perovskite materials system, SrTi_(1-x)Fe_xO_(3-δ), was selected, offering the ability to systematically control both the levels of electronic and ionic electrical conductivity as well as the energy band structure. This, in combination with considerably simplified electrode geometry, served to demonstrate that the rate of oxygen exchange at the surface of SrTi_(1-x)Fe_xO_(3-δ) is only weakly correlated with either high electronic or ionic conductivity, in apparent contradiction with common expectations. On the other hand, evidence was found suggesting the importance of minority electronic species in determining the rate of oxygen exchange. Furthermore, the enrichment of Sr to the surface of the electrodes was found to reduce the oxygen exchange rate constant; this effect becoming more evident with increasing values of x. The observed trends are discussed in relation to the cathodic behavior of MIEC electrodes

Model Reduction for Controllable Systems

In the papers [1], [7] a new scheme for passivity-preserving model reduction has been proposed. We have shown in [2] that the approach can also be interpreted from a dissipativity theory point of view, and we put forward two procedures in order to compute a driving variable or output nulling representation of a reduced order model for a given behavior. In this paper we illustrate improved versions of both algorithms, which produce a controllable reduced-order model. The new algorithms are based on several original results of independent interest

Influence of friction on granular segregation

Vertical shaking of a mixture of small and large beads can lead to segregation where the large beads either accumulate at the top of the sample, the so called Brazil Nut effect (BNE), or at the bottom, the Reverse Brazil Nut effect (RBNE). Here we demonstrate experimentally a sharp transition from the RBNE to the BNE when the particle coefficient of friction increases due to aging of the particles. This result can be explained by the two competing mechanisms of buoyancy and sidewall-driven convection, where the latter is assumed to grow in strength with increasing friction.Comment: 3 pages, 2 figure