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The properties of nuclear matter are studied in the framework of quantum hadrodynamics. Assuming an ω-meson field, periodic in space, a self-consistent set of equations is derived in the mean-field approximation for the description of nucleons interacting via σ-meson and ω-meson fields. Solutions of these self-consistent equations have been found: The baryon density is constant in space, however, the baryon current density is periodic. This high density phase of nuclear matter can be produced by anisotropic external pressure, occurring, e.g., in relativistic heavy ion reactions. The self-consistent fields developing beyond the instability limit have a special screw symmetry. In the presence of such an ω field, the energy spectrum of the relativistic nucleons exhibits allowed and forbidden bands, similar to the energy spectrum of the electrons in solids.
We investigate the production of heavy quarks in continuum and bound states in nuclear collisions. Creation rates for free bb and tt quark pairs and for bottomonium and toponium in the ground state are computed at energies of the BNL Relativistic Heavy Ion Collider, CERN Large Hadron Collider (LHC), and Superconducting Super Collider. Central and peripheral heavy-ion collisions are discussed. For top-quark creation we assumed a mass range of 90≤mt≤250 GeV. The creation rate for top quarks in peripheral collisions is estimated to be by a factor 40 to 130 smaller compared with corresponding central collisions. For mt=130 GeV we calculated a creation rate of about 4760 top-quark pairs per day at the LHC (3.5 TeV/nucleon) for Pb-Pb collisions.
We discuss the multiplicity distribution of electron-positron pairs created in the strong electromagnetic fields of ultrarelativistic heavy-ion transits. Based on nonperturbative expressions for the N-pair creation amplitudes, the Poisson distribution is derived by neglecting interference terms. The source of unitarity violation is identified in the vacuum-to-vacuum amplitude, and a perturbative expression for the mean number of pairs is given.
Relativistic heavy ion collisions constitute a prolific source of hyperons: tens of hyperons per event are predicted at energies E≥10 GeV/nucleon, providing a scenario for the formation of metastable exotic multihypernuclear objects. They may exhibit exceptional properties: bound neutral (e.g., 4M2Λ2n, 10M2Λ8n, pure Λ droplets, 8Λ) and even negatively charged composites objects with positive baryon number (e.g., 4M2Σ-2n, 6M2Λ2Ξ-2n) could be formed in rare events. Such negative nuclei can easily be identified in a magnetic spectrometer. They could be considerably more abundant than antinuclei of the same A. We use the relativistic meson-baryon field theory—which gives an excellent description of normal nuclear and single-Λ hypernuclear properties—to calculate the rich spectrum of such exotic objects, their stability, and their structure. We also find solutions for a large variety of bound short-lived nuclei (e.g., 8M2Λ,2Σ-2p2n), which may decay strongly via formation of cascade (Ξ) particles. Multi-Ξ hypernuclei are also evaluated. A variety of potential candidates for such metastable exotic nuclei is presented. It turns out that the properties of such exotic multihypernuclear objects reveal quite similar features as the strangelet proposed as a unique signature for quark-gluon plasma formation in heavy ion collisions.
A quasiclassical Pauli potential is used to simulate the Fermi motion of nucleons in a molecular dynamical simulation of heavy ion collisions. The thermostatic properties of a Fermi gas with and without interactions are presented. The inclusion of this Pauli potential into the quantum molecular dynamics (QMD) approach yields a model with well defined fermionic ground states, which is therefore also able to give the excitation energies of the emitted fragments. The deexcitation mechanisms (particle evaporation and multifragmentation) of the new model are investigated. The dynamics of the QMD with Pauli potential is tested by a wide range of comparisons of calculated and experimental double-differential cross sections for inclusive p-induced reactions at incident energies of 80 to 160 MeV. Results at 256 and 800 MeV incident proton energy are presented as predictions for completed experiments which are as yet unpublished.
Stopping power and thermalization in relativistic heavy ion collisions is investigated employing the quantum molecular dynamics approach. For heavy systems stopping of the incoming nuclei is predicted, independent of the energy. The influence of the quantum effects and their increasing importance at low energies, is demonstrated by inspection of the mean free path of the nucleons and the n-n collision number. Classical models, which neglect these effects, overestimate the stopping and the thermalization as well as the collective flow and squeeze out. The sensitivity of the transverse and longitudinal momentum transfer to the in-medium cross section and to the pressure is investigated.
Experimental results are presented on the charge, velocity, and angular distributions of intermediate mass fragments (IMFs) for the reaction Fe+Au at bombarding energies of 50 and 100 MeV/nucleon. Results are compared to the quantum molecular dynamics (QMD) model and a modified QMD which includes a Pauli potential and follows the subsequent statistical decay of excited reaction products. The more complete model gives a good representation of the data and suggests that the major source of IMFs at large angles is due to multifragmentation of the target residue.
Strong correlations between baryon stopping in the projectile rapidity hemisphere and target excitation have been found in the light-ion-induced reactions at the BNL Alternating Gradient Synchrotron (AGS) (E814 group). Results in the framework of the relativistic molecular dynamics approach (RQMD) describe recent E814 data quite well. We discuss the RQMD results together with proton and pion data from the E802 group near midrapidity. They have raised the question of whether partial transparency could be seen in these experiments. The RQMD results indicate strong transverse baryon flow in central Si+Au collisions after the projectile has been stopped in the target.
We present a calculation of antiproton yields in Si+Al and Si+Au collisions at 14.5A GeV in the framework of the relativistic quantum molecular dynamics approach (RQMD). Multistep processes lead to the formation of high-mass flux tubes. Their decay dominates the initial antibaryon yield. However, the subsequent annihilation in the surrounding baryon-rich matter suppresses the antiproton yield considerably: Two-thirds of all antibaryons are annihilated even for the light Si+Al system. Comparisons with preliminary data of the E802 experiment support this analysis.
By replacing the irises in an electron linac by a slit one gets a structure capable of focussing/defocussing an electron beam (rf-quadrupoles). Therefore one can think of a combination of rf- and conventional magnetic quadrupoles for transversal focussing in linear-colliders. Furthermore they can meet the demands of BNS-damping without initial energy spread. Considering multibunch-operation of a collider, the long-range wake behaviour of this kind of structure has to be investigated. A three-cell structure has been built and investigated for dipole-type transversal long-range wakes. The experimental results are compared to numerical simulations done with MAFIA.
Determination of field strength and quality factor of heavily HOM damped accelerator cavities
(1992)
Two methods of of measuring field strength in accelerator cavities, heavily damped with respect to higher order modes (HOM), are presented. From the results of the field measurements the coupling (damping) factor and thus the quality factor of the damped resonator can be derived. Measurements of a pillbox resonator with heavily damped TM110-mode (Q < 20) demonstrate the usefulness of the techniques presented, even in this extreme range.
The energy dependence of rapidity distributions and flow effects was studied in central Ar+Pb collisions at 400, 800, and 1800 MeV/nucleon using a streamer chamber. Rapidity distributions for proton and pions are found to have a Gaussian shape whereas those for deuterons exhibit a two-peak structure at the two higher energies. The average in-plane transverse momentum per/nucleon and per/event shows saturation of flow around 800 MeV/nucleon for this asymmetric system. The aspect ratio of the sphericity tensor is closely correlated with the flow angle. This correlation appears to be independent of beam energy. The number of participating nucleons in central collisions varies from 213 at 400 to 135 at 1800 MeV/nucleon indicating that at the lowest energy almost the entire target nucleus participates in the collision.
The general subset sum problem is NP-complete. However, there are two algorithms, one due to Brickell and the other to Lagarias and Odlyzko, which in polynomial time solve almost all subset sum problems of sufficiently low density. Both methods rely on basis reduction algorithms to find short nonzero vectors in special lattices. The Lagarias-Odlyzko algorithm would solve almost all subset sum problems of density < 0.6463 . . . in polynomial time if it could invoke a polynomial-time algorithm for finding the shortest non-zero vector in a lattice. This paper presents two modifications of that algorithm, either one of which would solve almost all problems of density < 0.9408 . . . if it could find shortest non-zero vectors in lattices. These modifications also yield dramatic improvements in practice when they are combined with known lattice basis reduction algorithms.
Other than in Belgium, German banks may hold even controlling equity participations in industrial firms (and such firms may own banks) and do so to a large extent. Vis-a-vis the European development this leads to two questions: From the perspective of the (Belgian and other) competitors of these banks, whether their own domestic System might be disadvantageous to them. And from a public interest perspective, which advantages and drawbacks are connected with the different regulations in Europe. The article first informs about the legal framework and some statistical facts. Then the various and different reasons why banks acquire and hold shares on own account are analyzed. The following Parts deal with the various public policy arguments whether equity links between banks and industrial firms should be prohibited or not (safety and soundness of banking; autonomie de Ia fonction bancaire ; abuse of confidential information and conflicts of interest; antitrust considerations; negative and positive impacts on the respective firm). In its last part the article deals with recent proposals in the German political debate to limit stockholdings of banks. The article argues that a step-by-step approach to the Single Problems and issues (conflict of interests; anticompetitive effects etc.) should be preferred to a general limitation of stock ownership of banks.
The corporate governance Systems in the U.K. and in Germany differ markedly. German large firms have a two-board structure, they are subject to employee codetermination, their managements are not confronted with public hostile takeover bids, and banks play a major role in corporate governance, through equity stakes, through proxies given to them by small investors, and through bankers positions on the supervisory boards of these firms. One of the main issues of corporate governance in large firms, the Problem of shareholders passivity in monitoring management in Berle-Means type corporations, is thus addressed by an institutional Provision, the role of the banks, rather than by a market-oriented Solution as we find it in the U.K. with its market for corporate control through the threat of hostile takeovers. These two different approaches to corporate governance have been compared several times recently, and it was argued that a bank-based or institutional Solution has clear advantages and should be preferred. Cosh, Hughes and Singh, for example, argue at the conclusion of their discussion of takeovers and short-termism in the U.K. that the institutional shareholder [in the UK] should take a much more active and vigorous part in the internal governance of corporations. . . . In Order for such a proposal to be effective both in disciplining inefficient managements and promoting long-term investments, far reaching changes in the internal workings and behaviour of the financial institutions would be required. The financial institutions would need to pool their resources together, set up specialised departments for promoting investment and innovations - in other words behave like German banks. The following remarks seek to continue this discussion from the German perspective. The article will first attempt to evaluate the monitoring potential of our domestic bank or institution-oriented corporate governance System and then, in a further patt, compare it with that of a market-oriented Solution. lt will be argued that both Systems focus on different Problems and have specific advantages and drawbacks, and that there are still quite a few puzzles to be solved until all pros and cons of each of these monitoring devices tan be assessed. The perception that both Systems focus on different Problems suggests combining institutional monitoring with a market for corporate control rather than considering them to be contrasting and incompatible approaches. The article is organized as follows. Section II will describe the legal structure of the large corporation in Germany in more detail. Section Ill explains why a market for corporate control by the threat of public hostile takeover bids does not exist in Germany. Section IV then Shows how corporate governance in publicly held corporations with small investors is organized instead, and deals with the role of banks in corporate governance in these firms. Section V of the atticle then will try to compare the monitoring potential of a marketoriented and our bank or institution-oriented corporate governance System. Concluding remarks follow.